View Full Version : bore and stroke- the 'square engine'

03-04-03, 10:13 AM
i was reading zoom magazine last night no. 54 and they had an interesting article on bore and stroke and how it efects the characteristics of the engine.

they speak of the 3 litre supra engine and how it is perfectly square with a bore and stroke of 86mm. This (i think) allows it to make good low down power but still produce up top.

they also spoke of undersquare and over square engines the former being 'torque monsters' down low but can't produce good power up top (they used their turboed ford 4 litre as an eg) and the latter being a high revving motor.

My 2.6 has a bore/stroke ratio of about .92 (bore divided by stroke) hence is undersquare, will make torque down low and won't rev real hard, which it doesn't. I have heard that the 2 litre crank will go into a 2.6 effecting making it a 2.4 but i caculated that the bore/stroke ratio (with the help of my workshop manual) will now be 1.01, basically that magical square number.

does everyone think this is a worth while thing to do or will the drop in torque out weigh the extra revvability ive gained?

any comments on the subject are welcome and if ive missed something or am wrong point it out, cheers ;)

03-04-03, 11:17 AM
doing this is pointless.

more displacement is always better than having a slightly undersquare engine. All your going to get from this is a higher red line, with less power and torque.

Boring out the engine would be a much better way of getting more power, because you get a squarer engine with more revs and increased displacement.

But, I am not sure to what extent you can bore out that engine, and if it would be worth the effort (you coud need a set of new pistons=expensive)

03-04-03, 12:22 PM
It's really more of an engine design issue than an engine modding isuue.

If an enginneer is asked to design an engine for a mid sized hatchback, at about 2 litres capacity, he is likely to use a reasonably long stroke as the expected rpm range (1000-6000) means he can sacrifice some cylinder filling at high revs for more torque (from the longer stroke) at lower revs.

If the same guy is asked to design a 2 litre F1 engine (that doesn't have to be fitted to a passenger car), he is absolutely going to give it miniscule stroke so that the piston speed at 20,000 revs will be acceptable.

On an existing engine design to be used on the street, I suggest that more benefit can be had by keeping the extra capacity, rather than having to rev a smaller capacity engine harder to get the same sort of power.

If it was to be an all out racing engine, with custom crank, rods, etc, then the short stroke may be of benefit.

03-04-03, 12:25 PM
All things being equal, the the oversquare engine will always make more torque (and hence power) than an undersquare.
If for no other reason, it's because of the lower piston speed (lower frictional losses) and less shrouding of the valves. It also allows you to use bigger valves with less problems.

* Edit - GTSboy - smartarse! But thanks ... :)

03-04-03, 01:31 PM
Originally posted by Bill Sherwood
All things being equal, the the oversquare engine will always make more torque (and hence power) than an oversquare.
If for no other reason, it's because of the lower piston speed (lower frictional losses) and less shrouding of the valves. It also allows you to use bigger valves with less problems.

I agree with Bill......an OVERSQUARE engine will always be better than an OVERSQUARE

:):):):):):):):) :p

03-04-03, 04:15 PM
There are really two main factors.

Power is a function of airflow through the engine. Bore and stroke will have little or no effect on maximum airflow. A lot of people find this difficult to visualise. But increasing bore and stroke, and changing nothing else means you will end up with similar maximum peak airflow and power, but developed at a lower RPM.

The second factor is displacement. If you know engine displacement you can pretty well guess what the maximum torque is going to be, regardless of the actual bore, stroke, and number of cylinders.

I can understand why someone would bore or stroke an engine, to gain more low end and mid range torque. But the same engine will also run out of puff fairly quickly at the top end. Even though the top end will feel less strong, power will be about the same as before.

If the original stroke is very long, reducing it might be an advantage. Years ago I fitted a 3.6 crank to a 4.1 Litre Ford crossflow motor, this ended up with bore and stroke being about the same. This made for a nice revvy sporty engine.

03-04-03, 04:56 PM
Originally posted by Warpspeed
There are really two main factors.

Power is a function of airflow through the engine. Bore and stroke will have little or no effect on maximum airflow.

You've contradicted yourself. The former is correct. Airflow=power.

Boring and stroking increases the size of the engine, thus it pumps more air and makes more power.

03-04-03, 05:14 PM

He didn't contradict himself at all. He said that boring/stroking increases torque at low to mid rpm. Hence more torque, and not necessarily more power.

The basis of his argument there is that the head/manifolds etc are the limiting factor on what the engine will flow. So if you increase the size of the pump under the head, you will pull more air through right up until you reach the flow limit of the head, which should happen at lower rpm than before it was bored/stroked.

Of course, in the real world, an engine's airflow is not necessarily limited by the head. The engine might have better breathing than the bottom end really needs and so boring and stroking will give you the extra pump to use some or all of that extra breathing. But even without this, no-one would do internal mods to the bottom end of an engine without doing something to the top end as well, even if it is only a better exhaust.

So there will usually be gains all round from increased capacity.

03-04-03, 08:00 PM
Thanks GTSBoy, you beat me to it.

Sometimes boring and stroking can increase power because of secondary effects. The most common secondary effect is increased compression ratio.

You have to ask yourself what limits the engines airflow in the first place and sets the peak in the power curve.

03-04-03, 11:19 PM
for the sake of not making the post any longer, I have not diiscussed the fact that for a completely objective comparison, running identical spcs (cam exhaust, you name it) in different size engines of the same family, well - technically, they would requirre a slightly diffrent cam, exhaust, port work etc. but even with all these optimised, the comparisons hold true.

how about looking at it like this ;

if you are limited to a certain capacity, then generally a larger bore and shorter stroke will be the path to the highest power and torque output _per litre_ . so if you are in a racing category where the engine size is limited, and a number of bore/stroke combinations are possible, then the larger bore shorter stroke is the pick.

a larger bore allows alrger valve sizes without them being shrouded by the bore etc. there's a bunch more. for one thing, if it fits the same block, either the pistons are shorter, or the connecting rod is shorter. even with a short piston and the same rod length, the longer stroke cranked engine would have a worse rod: stroke ratio. It's no etched in stone, but generally the optimum r:s ratio is governed by the rpm range, cam specs, and a hell of a lot more. outside these optimal levels, as a rule of thumb if the r:s ratio is less than 1.6-1.7: you really start to lose out. apart from the engine being difficult to make power up higher, and the need for a different cam to work well at some higher rpm point, well the powerband gets very narrow as well. Aside from that though, a shorter rod has another issue - it will cause a lot more wear/fatigue. the bores wear faster, and it's much more lkely to break a piston or throw a rod. generally though, most factory enignes have a tolerable sitaution here - even with the longer stroke variants. possibly the best example of this is the 250 ford. hard to make power past some point (from memory around 6000 - people have gotten power higher, but not with fantastic reliability, or power gains. in this case the rod is a big obstacle. It's little wonder that people pursued using the longer 200 rod in a 250. It's clearly of merit here, considering there are a few pistons from ACL made specifically fro fitting a 200 rod to a 250 - made with a short comrpession height to fit it all there without the piston coming up and out of the block.

the same situation exists with holden 6 ford crank stroker buildups. the stroke is increased from 3 inches (it uses the 186 block - which had a 3 inch stroke crank, the 202 has a 3.25 stroke - both use the same rods) -to 3.6 inches, (or maybe 3.68 - but it requires a lot of clearancing work with a die grinder - beyond the block work of the std stroker - on the rods as well) these motors on the st produce massive torque - and a wide torque spread with the right cam etc.. but they really don't like to rev. It's very hard to make decent and wide ranging torque and power above 5500rpm - 6 tops. On a race engine, a 202 or 186 would produc a lot more bhp/litre but they won't get near as much outright torque. in this rare example, the fulyl modified group nc race 202 - by virtue of being able to rev a lot higher, might match the outright power (strokers afaik not legal, but I'm using it as an example.) even if it matched the outright power, it would achieve it at a lot higher rpm, and it wouldn't ever be able to match the torque output of the stroker.

here is an example. the 302 chev was produced around 1970 for a couple of years. they made it so they had an engine to run in the 5 litre trans-am race class. it was a 350 chev (4 inch) bore with a 3 inch stroke. it had the same stroke as the 283 chev (different crank though - stronger and different journal sizes) it was a very successful engine - a very good bore stroke combination.

the 307 chev - which made it into a few hk-t-g holdens is a different combination again. basically is was a 283 block (3.875 in bore) wih a 327 - 3.25inch crank. all else being equal, a 302 engine would slaughter the 307 with identical upgrades. not even worth considering - chev 350 used - suit reco shortblocks are cheap enough. No point wasting time with a 305..

now as good as the 302 chevs potential bhp per litre is, a simmilar spec 350 would have close to 50 ft lbs more torque on a mild streeter. that's a huge difference.

just for reference, the ford 302 windsor and cleveland engines also share a 4 inch bore, 3" stroke. the holden V8 - same bore size, and something like a 3.0625 stroke - roughly - I'm not sure exactly, nor am I sure of the exact stroke of the later 304 ones (slightly shorter stroke being used from - vk onwards, or a fraction earlier. - peter brock and others had input and it was done to bring it just under the capacity limit for the ??-5 litre. otherwise it would have meant the touring cars at the time had to carry a minimum weight of the largest category, whilst having less cubes than other cars in that category. by dropping the couple of cubes, the car could be raced in the next lower class. a negligible loss of cubes, but hey presto being able to run with a much lighter vehicle mass as per the rules.

now that covers racing in a specific class.

for the street, it's a different story. for any motor with a given cam head and induction/exhuast spec, the larger the motor the more power. I.E s 2 litre shorter stroke engine will make more power per litre than a 2.6, but the outright power of the 2.6 would without question be a fair bit higher.

Within budget restraints, one of the best 'tricks' to do is to go to the largest cpaacity engine you could source within a givne family.
this is the best starting point. In terms of $ per hp or ft/lb starting with the biggest engine will return the best bang for the buck..

possibly the only exception would be if the long stroke crank/engine had some peculiar inherent flaws and was not capable of delivering acceptable reliability.

to give you an ide of the relative gains per cube etc - comparing same bore size, different crank stroke, but identical rods. the mini big bore engine. probably 99% of them are 1275. there were small numbers of 970c and 1071cc prduced as well. same bore as th 175, but shorter stroke, they were made to suit the <1 litre and (forget the class the 1071s race withracing class - with a 20thou bore, they are 999cc - just under. the 970 has a massively short stoke..

there were guys in the mid 60s getting over 120bhp per litre with them - 121bhp from 999cc in one case I know of. The guy still has the car. it revved to around 11,000rpm during the races

the 1275 has the same bore, but a much longer stroke (a trait of practially every other mini engine) - the bore wa smaller than the stroke. the same cam.heads etc in a 1275 and it wion't make usable power past 9,000 - peak power coming in around 8200. the 1275 outputs back then? mid 130s. with the 5 port head, people have squeezed as much as 140. with 8 port heads etc, yes, but iwth a fully ported 5 port (as on thew 999cc on) 140 is about it.

the 1275 produces more outright power, but nowhere near the bhp/litre,. It also has more torque across a wide range. bottom line, similarly speced 1275s may not make the same power/litre, but they would absolutely aslaughter a 999cc powered mini

bottom line for it all - unless prohibited by the racing rules without question put together the biggest engine you can inside the given block. on the street it's academic. run the bigger engine. he other thing with shorter stroke smaller capacity engine option, is while it might lbe smoother to rev higher, such rpm levels are of questionable benefit - esp ith regard to longevity. go as big as you can without having to fork out big amount of money.

one last thing - I'm takling about increased capacity by stroke length, and possibly, from selecting the biggest bore factory blocks. Don't try and find power through boring the engine out. the gains from boring it out increase capacity - but by bugger all - your talking less than 1/10 of an inch compared to the usualy quarter of an inch or more the stroke can be increased by. the reason is, although boring can increase capacity, it also leaves you with thinner bore walls. if they are too thin, the engine may appear ok at idle etc, but too thin and under hard yakka, the combustion pressures are enough that the bore sort of 'balloons out' and the ring seal is compromised - result - the rings aren't sealing and energy that should be being transmittd to the crank isn't, it's escaping past the rings and into the crankcase. some engines actually have out of control wall thickness - some can be safely bore over 0.25" (yep a 1/4 of an inch) - but they are the exception. others are thin to start with, and the bore should be strictly limited to the smallest oversize needed to be machined to clean it up.

realworld example of importance of bore wall thickness? in the HQ race series, the engine is very modest - stock head, carb, even air cleaner. permissable modifications are limite. one of them is that a 202 block (duh) is the basis. one of the only free choices was the bore. it was allowed to be run with a std bore, and all small steps in between right up to 0.060" over. the thing is - and remember these are very mild engines - even with their low combustion pressure, and generally modest specs, afaik almost everyone runs a 0.040" oversize. it seems that even on a low buck low output engine, thinner bore walls are a detriment. Imagine comparing it to a high output engine - we'd want much thicker bore walls on that compared to the hq 202 motor

general rule here? find out about the engine in dpeth. ask people who know your engine, and _if_ it can be safetly bored to the next oversize any beyond. If you are still keen, the other option is to have the bores sonic checkd. its a simple prroceedure. they use a device similar to that which finds studs behind plaster walls. they can then verify the bore wall integrity/thickness and advise you on if boring is possible or futile, and further just how much it could be safely bored.

John McKenzie

04-04-03, 12:38 AM
Nice thread.
Can you guys agree that a greater stroke will be more fuel efficient?

I've got an old racetech mag with a story on the Corvette C5R. It has a really good story on the development of the LS1 for racing applications. Initially they destroked the engine by ~0.5mm and bored it by ~10mm to give ~5.8 litres. Eventually they stroked it to 101.6 (4 inches) to give 7 litres, but the engine is still oversquare.
Quite a good article actually, wish I could scan it!

04-04-03, 08:23 AM
Superb post jmac.

04-04-03, 08:31 AM
good info in here! well done guys! :D

But in regards to the original application, with the astron motor you are always going to be hampered by those poxy balance shafts and general harmonics of these motors. it's hard to get an astron to rev no matter what you do with it.

04-04-03, 08:34 AM
Originally posted by Warpspeed
Superb post jmac.


04-04-03, 10:33 AM
I largely agree with jmac, however the engine is part of a larger picture too. Focusing on only the engine is too focused a view if there are competition plans for the car (mind you the rules always dictate what you can do). I would prefer a good close ratio box matched to the engine to an extra 5-10kW in a race situation. The other aspect is torque spread/ toque matching to gearing vs weight of the car. It is quite possible to significantly increase the 'power' of an engine, yet make the car generally slower and much harder to dive. I have personally done this with a 142 volvo. I was young and stupid and thought that what I read was true and that bigger was better (that spam mail about the pills was not around then either!) and hence I ended up with a 2.4lt OHC volvo engine with 51mm intake valves an ex repco-brabham rally/race cam with >12mm lift a big 4-1 exhaust and a car that was a bastard to drive! There was no torque below 3000 and lots from 5000+ in a relatively heavy (say around 1100kg stripped). I had 135kW at 6200 on the dyno (not bad for an old 8V on webers) but when I changed it back to a much more mild tune and 'lost' 20kW it was in reality much 'faster' for a street car (and I didn't get 12mpg!).
Now I had the big valves, big cam and had changed the bore/stroke ratio significantly...all supposedly 'good' things without looking at the bigger picture.

04-04-03, 11:12 AM
Very true nero. And even more so for a road car, where you cannot keep it flat out in the power band ALL the time. Not and keep your license anyway.

04-04-03, 12:14 PM
Spot on Jmac

04-04-03, 01:19 PM
hey heres something funny that I just remembered reading. I"m bringing it up since it actually shows that whilst there is a general trend, rahther than etched in stone rules with engine buildups. whilst getting a 'nice'm rod to stroke ratio is of benefit, and generally speaking a low rod to stroke ratio will make it hard to make power up higher, and also increase chances of piston and rod failure due to increased side thrust and more aggressive acceleration away from top dead centre, one of the engines which has an absolutely atrocious rod: stroke ratio is actually pretty damn impressive - it produces massive power per litre for a production line engine, and is pretty happy at rpms that other engines would lock themselves in the closet to hide from. It's the honda v-tec engines.

I remember reading a us site ( http://www.theoldone.net - the archives there are possibly trhe single most thought provoking and school of thought challenging car related site out there. apparently they've recently reopened the forum there too. ) where they actually cut a flat piece of alloy and then welded it onto the block deck. the reason the rods are shorter is that the pistons are about as short in distance from pin to crown as is feasible - so there isn't room. the extra deck height was the only way to do it on that particular variant.

Larry Widmer is probably someone whose greatest talents will never be realised - as there aren't enough hours in the day. the guy is remarkable. he's not without critics, but he has put the score on the board in very wide and varied auto related topics.

There were people before him that lead the way - one of whom died a year ago - Smokey Yunick - if anyone wants to make a shift from hack to innovator, I'd seriously suggest grabbing anything written by him. He's not always right, nor is Widmer - that's the nature of the physics of automobiles - the understanding is pretty advanced these days, but it's by no means fully understood - not even close.

Nero - no kidding after I left the forums i read aus.cars and I swear I posted stuff basically saying exactly what you have said there - it was in response to a question about which is mroe important - torque or power. I said that technically it's power (it's a question of semantics really more than anything else) or else we don't knbow the rate of torque production, and can't then factor in gearing . I went on to say though, that if one didn't intend to change the gear ratio, then increasing the torque output was vital. increasing the power on a non turbo engine is achieved by a small increase (usually less than 20% for mild cases) in torque, but also by moving that torque peak up teh rev range). now there is more torque avauilable through different gearing, but if gearing is not altered, then the gains will mostly not be realised.... that's the short version of what i wrote.

actually just a little extra widmer comment nero. he mentioned that a few times he had built race winning motors, because that is what he was paid to do. the customer got it on the dyno and it made less power than the front runner. he said that he could build it to make the most power on the dyno but that wasn't gouing to win the race. apart from area under the curve stuff with torque spread and stuff, there's another issue. dynos load up and engine and let the revs come up at a certain rate, and that's not quite what race engines do or need to do. instead of finding the highest peak power, he would set the dyno at a set resistance, and configure the whole engine specs so that it could accelerate from idle or some predetermined rpm point, through to redline in the quickest time. the motor that could get to redline first against this fixed resistance - and the specific load would be particularly critical - but i have no idea how you would work out what the figure might be.... it think he even did it and got the dyno to hit the anchor so to speak at redline, and pull it instantly down to the baseline rpm - i dont' know if he did it via the dyno or via a transmission between engine and dyno brake, but basically it was to simulate running up through the gears. he spent a lot of time on that, and the peak power figures were practically guaranteed to be lower, but the engines were contributing to lap records, race wins and in one particular example - the complete domination of pro stock racing for an entire season - and nobody could get near him (the guy running the heads and cam pacakage he developed - but in particular the heads)

Secoh - from what I've heard about them i agreee that basically the 2600 and 2000 motors are about the same reliability wise - that is to say - both are arguably equally not all that crash hot. as such it would suggest it's better keep the rpm down to a modest level. without question the bigger capacity and 2600 will be of advantage. the turbo models were 2 litre weren't they? I'm actually thinking the reasoning behind that was either because it was a way they could run low enough compression using a combination of existing pistons and heads rather than needing new ones, or maybe the reason was that the extra torque of a turbo 2.6 compared to a turbo 2.0 would tip over into the gearbox longevity complications area.

to make onew last hypothetical about the sigma 2.6 if the rod to stroke ratio wa poor, and you could get off the shelf cheap rods and pistons that would fit and would improve it then go for it. but other than that forget it. the 2 litre might produce more power per litre, but there's no way it will have a snowballs chance in hell of compensating for a loss of 20% of engine capacity - there's no way the revability would account for it. in this case, you'd still have the same head to choose from and the same basic porting potential - there's just no way dropping down to litres would be of any value. that's not to be confused with engine condition though. a completely stuffed 2.6 is going to have a hard time against a fresh and sensibly built 2.0, but we are comparing apples to apples - or should be. I'm not suggesting you didn't know this, but when ever you hear statements tot eh effect that so and so had a 2 litre sigma and it thrashed the pants off a 2.6 - then immediately suspect and if need be investigate the condition of the 2.6. it's practically impossible for the 2 litre to ever top the 2.6 , unless the 2 litres came out with forged steel cranks, carrillo rods, forged pistons and radically improved head design. which they didn't

actually here's another example. you know the slant 6 valiant engine. it's got heaps of torque but isn't usually considerer a performance piece. there are actually 3 sizes - the 170, the 198 and the 225, the difference between them is purely the stroke. the 170 is a short stroke high revving motor (longer rods too), the 198 somewhere in between - and the 225 is a massively undersquare engine - with a stroke longer than most of the factory big blocks - though not all - similar to the b-o-p 445 engines - all of which were small blocks with big block capacity via very long stroke cranks - needless to say they are good for a smallblock weighted and sized engine, but they don't compete for outright power witht the decent bigblocks from the big makers - which as a rule have bigger bores and shorter strokes - better than the buick oldsmobile and pontiac 455 engines (which are 3 distinctly different engines btw). thankfully the crank materuial and brearing sizes were actually based on a bigblock, so the 2265 crank survives ok. but back to performance - the 170 was actually brought out with a huge long runner (hyperpak) manifold option - it's short stroke, long rods and advanced for it's time long runner sonic tuned inlet manifold made it very powerful for it's time. it was actually so good in modified form that chrysler completely dominated the 3 litre class in nascar which lead to the class being closed for good. that was probably the beginning of the now full circle move away from it[s grass roots that nascar has taken. you know nascar - the sc actually stands for stock car - they are nothing like it now - despite retaining carbed and pushroded powerplants... on that note - chrysler actually would hold an unbeatable crown - they ahve had more racing categories shut down or legislated to the point of impossibility as a result of their breakthroughs - not just engines either -- aerodynamics for another.

but back to the slant. the 170 will by far produce the best power per cubic inch - heaps more. but the long stroke 225 - even though the head was never majorly redesigned for the bigger engines - same port and valve size potential - and it's compromised bore to stroke (rod to stroke isn't woeful but can be improved - actually cheaply - by using 198 rods and 6 chrysler 2.2 four cyl pistons). it won't make peak power at anywhere near the same rpm - even with the wildest cams etc, but it will still make a lot more outright power and torque than the 170. so in effect - on the street, the 225 is the only option worth considering. the cranks are actually very strong - and I don't know of anyone breaking a crank or throwing a rod in a 225 - not bad considering the 4.125" stroke - (from memory) - that's nearly 105mm! probably diesel truck or tractor engine territory.

I'll shut up now

John McKenzie

04-04-03, 02:28 PM
Correct me if I'm wrong, but it seems that bore:stroke is basically irrelevant, and that rod:stroke ratio actually has a much larger influence in an engine's power delivery.

Of all of the motors that "don't rev", most of them are seriously over-square - think about most old school V8s, the Volvo B20/21/23 motors (OHC and OHV), Holden red motors etc etc - they are all big bore, short stroke motors!

I can see Jmac's logic, and I don't disagree... Except that revvy motors can be a bundle of fun... Anyone driven a 1.3 Laser back to back with a 1.5 Laser? Even if the 1.3 is slower, its still heaps more fun (I honestly don't know wich is faster).

04-04-03, 02:45 PM
A lot of the old school motors also have horrible cylinder heads.

So the top end breathing is going to be more limited by airflow than bore to stroke or rod ratio. That is what makes all of this so interesting, there are so many factors to think about.

I am curious to see how the new Ford four litre six goes. It must have about a 100mm stroke, and DOHC four valve head. Will it rev or not ? Anyone know ?

04-04-03, 03:29 PM
As far as I know the new motor is pretty much dimensionally the same as the old one. So you can take the performance of the single cam 4.0 (wrt to the shape of the power curve) as a good guide to the behaviour of the new one.

I know the bore spacing remained the same, which is understandable from their production line point of view. If I were a Ford Engineer I would have jumped on the opportunity to mod the stroke, bore and rod length a little bit to make it feel like a nicer engine (for a twin cam that is)...but I'm pretty sure that they did not.

Hang on...just looked it up. Barra motor is 92.26 bore by 99.31 stroke. This is identical to the ED - AU motor.

And I also found this table which has the dimensions of quite a few engines laid out, including some of the ratios. http://users.erols.com/srweiss/tablersn.htm

04-04-03, 03:51 PM
Originally posted by jmac

I'll shut up now

no! don't!

those posts are some of the most informative information on engine efficiency in terms of bore and stroke i've ever read!

I think those posts should be put into the pool room

:worship: :worship: :worship:

04-04-03, 04:58 PM
i think his brain is empty now jzz30 :)

04-04-03, 05:03 PM
it is www.theoldone.com for anyone wanting to go there (reading it now, cannot talk anylonger, must go...)

04-04-03, 08:24 PM
I've been aware of larry's site for a few years and even had a forum conversation with him on a few topics. He has some interesting ideas on some things but sometimes has little hardcore proof to back his views up...not that this is uncommon in the tuning buisness!
There is an interesting article on rod/stoke on vizard's site. There is also an interesting rebuttal on the eng-tips.com site from the chief powertrain engineer for jaguar who's handle is marquis. He dismisses the rod/stroke arguments aiding burn unless there is a reason the burn is ineffecient. The rod/stoke being 'optimised' for a particular situation may only make more power due to an increase in mechanical effeciency...but read his post, its very informative (so is the forum, but beware they do not take foolish posts lightly and will not let non engineers post).
I think what larry was saying about the dyno tuning from your discription is 'transient response' which I have seen on a couple of occasions. This is the abilty of an engine to increase the amount of work/sec...there is more to it than this I agree, but I don't carry the formula's around with me, if anyone is really interested I will try and find it. Basicall it is measuring the engines ability to make power quickly and respond to load, rather than being allowed to slowly build up to a peak reading.

04-04-03, 11:35 PM
Correct me if I'm wrong, but it seems that bore:stroke is basically irrelevant, and that rod:stroke ratio actually has a much larger influence in an engine's power delivery.

It's not really the ratio of the bore/stroke, but more the length of the stroke itself when it comes to max revs. That's not a hard rule though as there's odd exception, such as the Honda S2000 engine.
Most engines red-line give them an average piston speed of 4,000 ft/min, but the Honda with its long stroke is 5,300 ft/min. They run extremely strong lower block girdles though (pics on my site) to stop the main caps from moving around.

My Dad's Honda CBR250RR motorbike is a 250cc four-cylinder thing with a red-line of 19,000rpm. That's sounds very high, but it's still only a piston speed of 4,000 ft/min.

05-04-03, 06:47 PM
your dad is very active in his old age :)

from a turbo falcon to a WRX and a CBR hehe

05-04-03, 10:51 PM
What is the bore and stroke of an F1 engine and what is the piston speed? I assume the bore is much bigger than the stroke to safely rev as high as they do.

I noticed that my ford pinto 2L in my cortina was much revvier than my brothers old L20 powered Bluebird. They both weigh about the same with similar mods but the corty went harder :)

The pinto is oversquare and I think the L20 is slighty undersqaure (or at least had a fair longer stroke than the pinto) The L20 had a fair bit more power down low and used less fuel but the pinto just loved to rev and out powered it in the higher revs.

06-04-03, 08:57 PM
OMG! jmac you are a gun!

so are the rest of everyone who has posted informative, fantastic info.

but my final ques is does everyone think it would be a good idea to put the 2l crank in the 2.6 to bring it up to 1.0 bore/stroke ratio so it is a nice revvy engine or is the 200cc loss in capacity not worth it, cos no matter how many times i keep reading this thread i can't work out what is the best thing to do

cheers everyone especially jmac.

06-04-03, 09:27 PM

it will rev better but you will gain nothing...

06-04-03, 10:05 PM
No.... do not reduce the stroke.

If you want a bit more top end zip, what it really needs is more airflow. Do all the usual things to clean up the induction and exhaust paths, and if you are still not happy, give it a bit more cam.

06-04-03, 10:16 PM
there are alot of factors involved her not just bore and stroke.
eg: the ecotech holden (buick) V6 has a longer conrod connected to the piston closer to the crown this was done to increase torque.
But the main thing is the piston speed destroking it will reduce the piston speed at a given RPM point hence raising the safe piston speed RPM higher. Lower piston speed in conjunction with the lesser capacity will give less torque at a given RPM.

I personally dont think it is worth the effort.

06-04-03, 10:50 PM
It is an interesting mental exercise comparing changing the engine stroke, and changing the diff ratio.

Don't laugh. There are quite a few similarities.

If you say increase the stroke by 10%, you get more torque lower in the rev range, and similar power, also at a lower peak RPM. So in basic terms, piston speed is up, torque and acceleration are up. New top speed ? probably less.

Now suppose instead, you fit a diff with a 10% lower ratio. (higher numerically). You get more torque at similar roadspeeds, and similar peak power, also at a lower road speed. So just as before, piston speed is up, torque and acceleration are up. And top speed ? probably less.

I am not sure the effect would be identical, but there are a few similarities in there. Changing the diff is a lot easier, and you can also change it back again. Changing crankshafts is a little bit more work.

What do you guys think of that................he he he

06-04-03, 11:00 PM
Sounds fair, I have thought along similar lines myself, but I doubt the diff gears would do as good of a job as lengthening the stroke.

07-04-03, 10:47 AM
It does have a similar effect.

Hence higher revving cars have relatively high diff ratios numerically, eg 4.4:1
where as lower revving engines such as V8s have a relatively low ratio eg 2.75:1
With all things being equal these cars should perform similarly

07-04-03, 11:20 AM
Actually it's not so much the amount of revs that the motor is capable of as much as it is the amount of torque the motor makes.

A big V* has wads of off idle torque and so will pull a tall diff ratio....and the tall diff ratio will give it a reasonable top speed because most of them don't rev too high. But it is easy to build a V8 that has good torque and will rev as high as a stock RB20. This engine will pull a tall diff ratio and run to a very very high top speed. Lamborghini motors easily fall into that type of category.

A small capacity 6 like an RB20 has little off idle boost and needs to run a short diff ratio to get it to drive away nicely. Because such engines usually rev well, they then get a reasonable top end speed out of the combo of revs and short gearing. But an example of a torquelss motor that doesn't rev would be easy to find (most motors made by BMC for example) and so they would have a short diff ratio and low revving and so would not end up going very fast at the top end.


07-04-03, 11:37 AM
cheers. ;)

07-04-03, 07:34 PM
GTS boy...V8...like a lambo???...except they are a V12 with a 77mm stroke. Not a big stroke at all, but with 5167cc you can get away with a bit.
In reality there are so many variables within any engine design it is pointless to take such arguments as rod/stroke issues seriously. Generally if there are serious flaws in such thing we just look elsewhere for an engine to suit our purposes.

08-04-03, 09:59 AM

If you want to get fussy, they are flat 12s for the most part.

But even so....if you look at my second paragraph, I actually typed "V*" meaning any type of big V engine......it is a little trick called a wildcard:) The following statements about building a particular V8 to rev and the one follwing that about the Lambo motors are only related to each other by the first sentence in that para.


08-04-03, 07:50 PM
AH there you go I thought you had just kept holding down the shift key given that the * is above the 8.
I have no record of a lambo with a flat 12, ferrari yes, but would be happy to be corrected...what model was it?

08-04-03, 09:47 PM
Originally posted by zac
Nice thread.
Can you guys agree that a greater stroke will be more fuel efficient?

I've got an old racetech mag with a story on the Corvette C5R. It has a really good story on the development of the LS1 for racing applications. Initially they destroked the engine by ~0.5mm and bored it by ~10mm to give ~5.8 litres. Eventually they stroked it to 101.6 (4 inches) to give 7 litres, but the engine is still oversquare.
Quite a good article actually, wish I could scan it!

I have an exec LS1 project being built hopefully finish the car in a couple of months.

4.125" stroke x 4.1 bore 436ci (7.2)

08-04-03, 11:15 PM
woops sorry I meant in my original post that the stroke had been decreased by 5mm, not 0.5mm!!

strife, the c5r 7l is 104.65 bore to make 6987cc. must be the maximum the block can be bored to. What length rods are you using?

09-04-03, 09:29 AM
Actually Nero.....I think you are right......now that I think about it all the cars since 1963 had V12s. Even the marine engines are usually V12.

I thought that at least one open wheel race car had a flat 12 but can't find any mention of it. Must be confused with the Ferraris after all.


09-04-03, 11:22 AM

rod length is 6.200.

Im not using a C5R block I am using a resleeved LS1 block, The price of the c5r block was out of my league when I first brought this engine.

Max bore I have seen on both LS1 blocks and the C5R is 4.155 (105.537), there is work on 4.2 bores as well.

I have about 12000 kms on a 4.1 bore ls1 block with stock 3.622 stroke with no integrity issues so far.


09-04-03, 07:31 PM
sorry haven't been online much in the last few days.

bottom line on the sigma - for all the trouble of destroking the sigma - you'll get absolutely ZERO gain - unless you modify the head, alter the cam and probably the whole induction package for a higher revvign engine. the thing to remember is it's only _theoretically_ a higher revving engine.

4 cylinder engines actually have a very bad harmonic problem - inherently. I honestly don't understand it, but it is the reason that V8 engines use a 90 degree crank - that is each crank throw is rotated 90 degrees from the next, instead of having a large version of the 4 cylinder crank. there is actually such a thing as a flat crank V8 crank - sometimes called a 180 degree crank - and I asked people who know more than me why they are so uncommon - as from a manufacturing and machining point of view they would be a hell of a lot less trouble - and the response was that then they would have the same inherent harmonic problem as 4 cylinder engines have. obviously it's not life and death - but it's enough of an issue that production V8 cranks aren't made that way.

but back to the sigma - all you would honestly do - and I'll put money on it - is end up with less power and less torque, and probably zero gain in the rpms it likes to visit. the thing to remember is, the breathing and cam etc, in their current form aren't configured to support massive rpms - it would be pure madness of a manufacturer to design something like that at the time they were built because it would have sacrificed a lot of usable torque and power in the rpm range the engine would have actually seen day in day out.

if you have that money to spend, and really really want to get a bit mroe out of it, I'd suggest finding someone who knows about the heads on these motors(probably easier said than done but I'm tipping these forums would be a good place to look), and then getting any obvious inherent impediment to good airflow cleaned up and perhaps look at new cam. If you aren't worried about the epa - to covr my ass - lets say you are driving this off road only.....then although they aren't glamourous, a small holley with a home made adapter plate would work reasonably well and are easy to sort for practically any application. if you either have, or have access to the later efi manifolds - then one of these with either straight lpg, or with a modest priced aftermarket programable computer would be preferable - the efi manifolds apparently flow quite well. I'm told they are able to be fitted to earlier engines - but have zero experience here.

the big issue with a smaller bore is the maximum valve sizes you can fit in there - it's obviously crucial on a pushrod engine, but is still a big factor with higher output overhead cam engines which have valve inclinations tha allow big valves unshrouded within a smaller bore.....

there's one other factor - esp with the economy comment. all else being equal, a larger bore is more prone to detonation. so with identical chamber designs, a smaller bore will tolerate a slightly higher compression ratio. the higher the compression ratio, the better the potential for part throttle economy, even at the same a/f ratio, timing, and amount of air/fuel ingested. so yep - a smaller bore longer stroke can pay off. it's also to do wih the cranbk throw offering more leverage, but it's not the main reason.

John McKenzie

10-04-03, 10:54 AM
jmac you are a gun!:worship:

cheers for that ill prolly put my cash into a late model head (TR), pistons and efi and a slightly larger cam.

i heard talk of the de-stroking issue with my motor from a few people, cheers for clearing it up and not allowing me to waste my time and cash! ;)

10-04-03, 11:53 AM

Flat plane V8s sound lovely though. A much sexier noise than a normal V8. I am pretty sure that the 3 literish V8s in the Ferrari 308 328 era cars (and that would include the F40) are flat plane.

The vibrations are half the fun!


11-04-03, 07:25 PM
No kidding, the comment about the 180 crank V8s sounding great waqs actually almost word for word the comment that was made in aus.cars about them that made me ask what the reason behind them not being used more actually was!

John McKenzie

11-04-03, 11:42 PM
Most manufacturers don't use flat plane v8 cranks for NVH reasons. Ferrari can get away with it.

AFAIK the only advantage of a flat plane crank (apart from the sound) is it is stiffer (allowing more RPMs) than a 90 degree crank.

there's one other factor - esp with the economy comment. all else being equal, a larger bore is more prone to detonation. so with identical chamber designs, a smaller bore will tolerate a slightly higher compression ratio. the higher the compression ratio, the better the potential for part throttle economy, even at the same a/f ratio, timing, and amount of air/fuel ingested. so yep - a smaller bore longer stroke can pay off. it's also to do wih the cranbk throw offering more leverage, but it's not the main reason.

All else being equal hey? If only it were that easy! With regards to performance engines, I find nothing worse than having decent sized valves in a small bore engine, while trying to maintain high compression ratio and good crown/chamber shape! It gets ugly! But yes, all manufacturers are going for smallish bores for their econoboxes.

03-09-03, 10:26 AM
someone should use this thread as info on writing a tech article..

either that, or it should be put into the pool room or something.....

it's the best thread i have seen on here, ever!

03-09-03, 02:36 PM
A good read guys.

I have made 330hp at the wheels and over 1300ft/lbs of torque with a standard stroke / rod length 2.6. This was using a standard TR Magna head, and a large turbo of course ;)

This car was set up for drag racing, and the motor is well suited to this application. I use a 3.6 ratio diff and 2 speed powerglide box. I only rev the motor to 6000rpm. The car has run a best of 11.4@120mph. The torque certainly gets the car moving without having to rely on massive revs.

The balance shafts have been removed, it does have a little vibration, but nothing to be too concerned about. The balance shafts should be the first thing to go if you plan on revving the motor at all.

The TR magna head conversion is good, as you get a big port head with big intake valves as standard equipment. The exhaust valves are a little small though. The only downside is the Comp ratio will be a little low if you put the head onto a standard bottom end. (great for turbo)


03-09-03, 03:43 PM
Originally posted by RX22NV
A good read guys.

I have made 330hp at the wheels and over 1300ft/lbs of torque with a standard stroke / rod length 2.6. This was using a standard TR Magna head, and a large turbo of course ;)

Hi Dan,

Sorry, but no way are you making 1300ft/lbs (sic) of torque :p

If your peak torque is at 3500rpm that would equate to 866hp at the engine, if torque peaks at 5000rpm you would be making a whopping 1237hp at the engine!

You probably mean 1300 "Tractive Effort" as measured on a dyno dynamics dyno, which is different to engine torque.


03-09-03, 04:30 PM
I have said that before, and you picked me up for it then too.

But yes, that was tractive effort as measured on a dyno dynamics dyno.

Is there any way of finding out what torque my car is making? Although I wouldn't mind saying my car has 866hp at the engine! :D

03-09-03, 05:21 PM
rx22nv would you suggest that i upgrade from my astron 1 head to the later model tr head and manifold without swapping pistons (alot more work than just doing the head) is the compression ratio to low for n/a or will it be ok? note: running straight gas.

03-09-03, 05:52 PM
to get engine tourque, or an idea of it, divide the torque at the wheels by your diff ratio, assuming the run was done in 4th gear, and assuming that your 4th gear is 1:1.

04-09-03, 09:34 AM
I know that it does make it lower but the exact figure I'm not sure of. I think the chamber volume is a lot bigger in the TR head.
While it would be a bit lower that the astron, the gains from the better flow would probably put you ahead.
Mine have always been turbo, so I have not have had first hand experience.

Do you plan to go turbo in the future?

shinybluesteel: The run was done in top gear which is 1:1.
So 1300 / 3.6 is 361.1 Thats with a powerglide.

Is that ft/lbs?

04-09-03, 05:04 PM
Originally posted by shinybluesteel
to get engine tourque, or an idea of it, divide the torque at the wheels by your diff ratio, assuming the run was done in 4th gear, and assuming that your 4th gear is 1:1.

What about wheel size?

04-09-03, 05:18 PM
We need more info that that.
The 1300 figure, is that a force, wheel torque, or roller torque?
If the tyre diameter and roller diameter aren't the same (and they're prob not), it's like another set of gears meshing and you need to know which one you're talking about.
In the future leave out the 1300 lb.ft because it means almost nothing. I could pull more than that if I did a run in 1st gear with my stock Verada :)

The following is to do with stress in bottom end.
The conclusion is that for a given bore, destroking to raise revs will not give you more power.

"Lets limit this to a displacement increase through only a stroke increase on an engine RPM limited by the bottom end (doesn't work if you keep the revs the same for both cases).
This is a very crude calculation BTW.
P = power, w = RPM, D = displacement/cylinder, l = stroke, s = stress in bottom end (assuming same strength because it's too complicated otherwise)
Bore is constant so D proportional to l
P proportional to l x w
s proportional to l x w^2

After playing around, power is proportional to the square root of the displacement if we are stroking. So a 10% stroke increase will only give a 4.9% power increase because the revs are reduced.

As I said before, it's a damn crude calculation that assumes a lot of things and ignores even more . But you can probably still find examples of it being true.
Replacing a stock RB26 bottom end with a slightly longer stroke mega tough bottom end isn't one of those examples
But a mega tough 2.6 L vs a mega tough 2.7 L bottom end with the same bore size might follow the above relation.
This relation would apply best to something like an F1 engine."

04-09-03, 05:20 PM
I noticed the above mention on the flat plane crank being used by ferrari, what is the technology reasons behind the "dual plane" crankshaft being used in maserati's new V8 engine?

Sorry for being slightly OT

04-09-03, 05:26 PM
Regarding the 2ltr-2.6 power/revability i think i can comment.
Ive had BOTH 2.6 and 2ltr in my galant with same gearing/exhaust/intake....

2.6 Blows all over 2ltr...sure the 2ltr will rev to near on 7k, and the 2.6 runs outa puff around 5 but it is undeniably loads faster.

In my opinoin the greater capacity is pulling more air(through what is basically the same head) so the 2.6 runs outa airflow before the 2ltr cos of greater volume of airflow.

If ure using a late head on a early 2.6(dished pistons, astron2 an magna have flat tops) you will get near on 6.5-7:1 comp...

04-09-03, 06:04 PM
There is a guy in perth thats been playing around with the 2.6 astron motor in his galant drag car.This car has been developed over a few years and has had a few different combinations used in this motor and from what i can gather its quickest with the shorter stroke crank giving it something like 2.4L.It makes something close to 200rwhp and runs 12's.

04-09-03, 06:26 PM
Same guy has developed a twin turbo kit for Magnas.

05-09-03, 09:14 AM
Why leave it out? other people that use Dyno Dynamics dyno's might be able to draw a comparison from it.

05-09-03, 09:58 AM
No, it's totally dependent on your gearing. As I said, I could get my stock Verada to put out more (about 1700 lb.ft) in 1st gear if I desperately wanted to prove how useless it is to quote torque at the wheels.

05-09-03, 10:17 AM
Tractive effort is measured in pounds, not foot pounds.

If you tie a string to the towbar, it would pull with a force of 1700 pounds on the string. That is the force accelerating the car.

When you put the car on dno rollers, it is 1700 pounds at the tyre contact patches pushing against the surface of the rollers.

The diameter of the dyno rollers does not matter.

If your tyres have a one foot radius, there WOULD be 1700 foot pounds developed at the drive axle.

If your tyre radius is larger, there would be more axle torque required to get that 1700 pounds of pull (or push) at the road.

You then divide axle torque by the diff ratio to get tailshaft torque, and then divide it again by gearbox ratio to get flywheel torque.

So you start off with say 1700 pounds of tractive effort.

If your tyre radius is thirteen inches you multiply 1700 by 13/12 and get 1841.7 foot pounds of axle torque.

If your diff is 4.11, you divide 1841.7 by 4.11 and get 448 foot pounds of tailshaft torque.

If you are in third gear, and your gearbox ratio is 1.22, you divide 448 by 1.22 and end up with 367 foot pounds at the flywheel.

Your tyres, diff, and gearbox will be different to this, but that is how it works.

05-09-03, 10:32 AM
You could say I'm pretty familiar with the concepts of force, torque etc.
AFAIK dynos don't always show tractive effort. I'm pretty sure I've seen some giving torques. It's all related anyway so I don't care what it's quoted in as long as sufficient info is provided.
If the torque given is torque about the roller then you certainly do need the roller and wheel diameter to convert to wheel torque.

I actually calculated that 1700 lb.ft figure using my engine torque and gearing, in the same manner you just said except in the other direction.

05-09-03, 12:58 PM
You are quite right.

Torque of the roller is measured through some sort of load arm against which the dyno offers resistance.

But direct torque only really means something if you have a hub dyno or an engine dyno.

And measuring torque at the dyno roller does not really help either unless you know the roller diameter. Tractive effort is much more useful, because it applies equally to anything you drive onto that same dyno.

For the same reason roadspeed is usually given instead of engine RPM. Knowing that you have xxx pounds of tractive effort at yyy roadspeed, you can work out yourself the torque and RPM values that apply for your vehicle.

I would rather see Engine RPM, engine torque, and brake horsepower (or Kw) myself on a dyno sheet, but most dyno places cannot be bothered working out the correction factors.

It is also just one more thing they can stuff up (or cheat on) depending on how you see things.

05-09-03, 02:37 PM
To add a bit to both discussions here is a dyno pic. It shows an RB25DET before and after the bottom end was changed to an RB30 to make an RB30DET (the only change is in stroke) As usual the dyno sheet only shows speed and not RPM which is not the best way to demonstrate the engines new power curve. I tried to work out rpm by using road speed and RPM of a standard R33GTSt. The yellow lines are my approximate RPM values.

I think the sheet is amazing, the 3 liter bottom end only gained a very small amount of extra PEAK power, however it shows power basically has DOUBLED from idle to 4000 RPM with considerably more power from 4000-6000rpm, from about 6500 to redline the engine is down slightly on power compared to the 2.5 liter. IMO the slight top end loss is negligible and the 3 liter would shit all over 2.5 in a race not to mention streetability and response. I think the peak power stayed similar because the turbo could only supply a certain amount of flow, and done so at lower RPM's because of the increased capacity/stroke.

The bottom end was the only power adding change done to engine. Also remembering that the CR was lowered from 9:1 to 8.3:1 which would have lowered the RB30DET's power a bit, if the 9:1 CR was kept there would have been even more difference in power. The original sheet was taken from http://www.skylinesaustralia.com/forums/index.php?s= a guy there done the conversion and posted the original sheet

05-09-03, 03:51 PM

Yeah, that torque curve tells the story. Look at the shape of both torque curves as the engine comes on boost. Damn near identical, except for the 3.0L happening at much lower revs and with a much bigger number.

Then the 3.0L torque curve drops from there on instead of staying hozizontal like the 2.5L...indicating that the turbo (or something else, but probably the turbo) is limiting possible air flow.

That is a monster conversion. Shame it isn't legal in any Skyline.


05-09-03, 03:59 PM
Yeah, impressive hey! I thought it was legal in a R31 because the block stays standard?

05-09-03, 04:12 PM
I don't count R31s as Skylines (because they were built here and don't actually have as much in common with Japanese Skylines as most R31 owners would like to think).

It would naturally be legal in an R31, but you'd not be able to swing it in a 1990 onward R32, or R33 (which is the car that really really needs it!!)


05-09-03, 04:25 PM
You said ANY skyline, what about HR31's? Anyway point taken.

05-09-03, 04:27 PM
Yeah that looks pretty good, but as usual GTSBoy raises some interesting points.

That particular turbo is slow to wind up on the RB25, but when it does the power between the two engines is the same.

But what if you put a smaller exhaust turbine in the turbo on the RB25, it would reduce top end power slightly, but it would bring up the middle, so it looked more like the RB30 shaped curve, but lower everywhere.

Suppose you then fitted a diff with a numerically higher ratio, say about 20% more gear (RB30 is RB25 + 20%)

You could then add 20% torque in every gear for free throughout the range, but you would drop your peak roadspeed by 20% as well.

So you might just end up with almost identical results by changing the diff and getting a slightly better matched turbo ? ? ?

05-09-03, 04:29 PM
Well in the case of HR31s...they don't have RB30s in them. But they would be wicked if they did. And now that I think about it...they would be legal too.

That would make for a little rocketship of a car wouldn't it? Cheap too.


29-09-03, 07:45 PM
I agree with that theory of the undersquare engine,I don't get why ford chose that design of bottom end then put a twin cam head on it as they would make better power and torque if they destroked the engine,I had always wondered why the Holden V6 was seemingly more powerful (despite figures) and torquey than the Ford.
My brother was told this by a performance engine builder,it was to do with Fords cam grind being suited for peak power output at the top end but the bottom end being undersquare to try and get better low down torque but the end result is stuffed up as there is less power in the bottom to mid range than the Holden V6.
He actually said Holden make their engines so they are quicker in the mid range as they deliver more power & torque from low RPM's to the mid range,the Holden V6 is an oversquare engine where its stroke is smaller than the bore diameter.
That's probably why I have never liked any of the Falcons as they don't seem to have any power,they would be a far more powerful engine destroked and with the bore taken out,imagine a 240 KW twin cam 4 litre with no turbocharger.

26-10-03, 02:59 PM
i think a lot of people get caught up in a lot of stuff that doesnt apply to a street engine....most of you guys are boost teddies.....and u have no sanctioning class restrictions that would make u chase every last ft-lb of torque that u can squeeze out of a restricted engine ........ with resetricted engine rpm limits etc.....................just wind up boost .....

leave stroke./ rod length / bore size combinations alone .. go for off the shelf combinations... they are cheaper readily available and arae proven combinations

04-05-04, 07:09 AM
Interesting read, now I have a headache and am going to bed.

Jmac: could you tell me if the new ACL range, are any good and do the oil rings passing the gudgeon cause any issues. They are locked so they can't rotate?
If this is inappropriate place to ask, sorry.

While this does not directly assist the sigma,
ACL has on their site (www.acl.com.au) a piston range that allows stroker motors to use longer than std rods by using shorter pistons to bring r/s ratios closer to original spec. By positioning the oil rings over the gudgeons, they save space and maintain crown thickness. It explains issues of bore wear due to excessive r/s ratios. Its not so hard to follow and uses simple real life examples. Stresses are a significant issue for longevity of any stroker. Big $$, so you want it to last.

07-05-04, 01:29 AM
I honestly don't have enough personal experience with them specifically (and it would vary from make to make of piston, and for the specific app, aws both would affect how high the rings are placed) so I can only make general comments.

I'd take a (bloody) long hard look at the specific part, and just how much of hte oil ring is unsupported, and exposed, and I'd further email acl specifically to see what if any modifications they made to the oil ring segment (anything from ring tension to the specific shape and design of the centre insert of the oil ring 'pack' )

For a racing motor, as a concept I'd be in favour of it for sure if it was the difference between a rod / stroke ratio or say 1.4is and the shorter piston would get it to nearer 1.6-1.7 . For a streeter too, possibly, but if the r/s ratio wasn't less than say 1.6:1 I'd probably lean toward retaining that (and the better supported oil rin package) than the advantages of going to a better r s ratio.

The other thing to consider is that unless oil control is good, even on a race motor, the oil contamination would ultimately negatively affect the amount of compression you could safely run, and it wouldn't be long before you started losing power. As a dumb (and obsolete) example of oil control effects, on mini engines (the original ones) you can bore and stroke them all the way up to 1600cc (from a 1275 original capacity) but the bore wall thickness, and r/s ratio (and even the stroke length itself, as the already have a longer stroke than bore in std conmfiguatiuon) means they make less power than a std or very mild stroke increase and a more moderate overbore. In a full race motor the smaller capacity is consistently quicker. The only time they ever had any advantage is in something like motorkhanas or tight dirt speedway sort of stuff,. where the longer stroke, despite less peak hp, has a slightly better mid range torque....

But back to the specifics. I'd also check the bloody r/s ratio on the engine in question. It's not an engine catered to by these new pistons, but as an example I referred to earlier, the valiant slant , despite a 4.125 stroke, has a rod stroke ratio of 1.62:1 which isn't nearly so bad as a lot of modern engines. It probably shows in the fact you can get over 1bhp per ci with the 225 normally aspirated (considering it's small bore and a head designed circa 1960 for a 3 litre engine that was never changed port wise throughout their production). As a general comment, the better the rod to stroke ratio (and bigger isn't necessarily better) the more power you can get from a given size port. If you have access to heads that flow better than you ever need it's of no reall concern, but if it's already stifled on that front, it can help to have more like a 1.6-1.8 ratio (and the specifics therin is a matter of bvoth debate from one engine builder to the next, but also heavily dependant on the rpm range. If you were sticking below 4500rpm a lot, then 1.6 is possibly the end of the scale to head toward. Any shorter however, and (imo) the extra side looading of the bore wall etc will not only wear it out way quicker, but also cost power.

Sorry it's not a definitive answer, but it's not really possible to be. I'd be very interested if you could share anything you heard back from ACL.

John McKenzie

28-11-04, 02:58 AM
re the 180 degree or flat plane crank, it was commonly used in V8 applications(nascar and f1 to name just two) as it allows for equal timing of the exhaust pulses allowing better scavenging, this alone makes them worth while even today, but even more so years ago when the art of building extractors was still being explored. In most cases the harmonics were managable and it was purely done as a means to make more power while keeping the exhaust tuned length and compactness of the exhaust within relatively confined spaces, especialy with the formula cars.

As for rod/stroke ratio, no one seems to have mentioned it yet but it is also possible to go too long in the rod. Consider the point that piston speed aids cylinder fill, and if its slowed down (but it all depends on rpm of course) it can lead to a slower moving charge of air/fuel to fill the cylinder with, effectively reducing torque. in fact its common for nascar engine builders to change the rod ratio for certain tracks as it has a decent impact on the particular rpm band that the engine will see (depending mainly on whether its a super speed way, short track or road course) . this may seem like hard work for little gain, but when your stuck with a 4speed box with fixed ratios it can be the difference between winning and losing.

Whilst it may seem like its worth debating the rod ratio effects , you need look no further than the Mopar/chev engines to see that its not all clear cut. If you agree that both makers have small blocks worthy of racing,(and FWIW i think they do) and that both seem to be able to make about the same power based on factory parts, then its not to hard to see that even though for years mopar guys laughed at chev guys because of there short rods , that the chev guys have done ok for themselves. And despite differences in heads and everything else (including bore/stroke or rod ratio or valve angles and so on) both small blocks seem to be as capable as each other, so as JMAC said "it aint etched in stone".

and as for destroking the astron motor, dont do it, if it needs its rod ratio fixed look at the rod or piston, if its the head, port it, if its something else restricting it hang a dirty big turbo of the side and give it about 20psi, it still wont rev, but it will make a hell of a lot more torque...