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Thread: Compound turbo setup

  1. #61
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    Well he's sort of right. Strictly speaking, efficiencies compound just as boost does, however when operating in a compound setup, the two chargers will increase each others efficiency if that's what you're referring to?

  2. #62
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    Quote Originally Posted by Slides View Post
    nope
    Am I wrong? If you have one compressor at .75 efficiency blowing into another at .75 efficiency, what's the overall efficiency?
    Last edited by Skepticism; 14-11-16 at 12:33 PM.

  3. #63
    I <3 Mang. klampykixx's Avatar
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    I agree with the logic but I'm thinking it's still 75% efficient.

    However spooling one with another would make one more efficient and the other less so, so would possibly even out

  4. #64
    Arrogant wankeler Slides's Avatar
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    Quote Originally Posted by Skepticism View Post
    Am I wrong? If you have one compressor at .75 efficiency blowing into another at .75 efficiency, what's the overall efficiency?
    The efficiency is proportionally weighted based on the PR/normalised flow across each based on the operating condition of each. If each one is operating at a normalised 65% efficiency point, the efficiency will be 65%. If one is at 50% and doing 1/3 the work the other 70% doing 2/3 the work you have to take the weighted average aprox 63%. They don't just multiply, they would be fucking uselessin practice if that was the case, tractor pull guys would all just be using internal compression screw compressors not compound turbos if that was the case.


    Think of it this way, within a reasonable gas density range, they are just a pumping device that loses a certain portion of energy beyond ideal, yes absolute inlet and outlet conditions will effect the absolute flow figures and energy numbers but mapped efficieny for air at any normalised operating point will change SFA in practical use range. If you staged a bunch of centrifugal water pumps in a cross country pipeline would you expect efficiency to approach zero? That is what multiplication would suggest.


    An assessment of total forced induction system efficiency taking into account both turbine and compressor operating points and intercooling effects is also far more important than trying to get one component operating at "peak efficency". Might be where you were heading? Yes removing heat from the charge between/after stages will allow more mass flow for the same input energy but staging them doesn't majically change the physics of one or other or boths compressor's behavoir. Absolute heat rejection obviously has to be higher to get the same intake temp at twice the charge density/mass flow if you go from 2 to 4 bar absolute but that doesn't mean the efficiency of compression is any different.


    Edit: that might come across a bit agressive, wasn't my intention.
    Last edited by Slides; 17-11-16 at 07:28 PM.

  5. #65
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    This is going to sound dismissive or reductive, but just work out a turbo or supercharger reasonably sized to your engine and fucking go with it. It's 3.8 fucking litres, not some tiny engine that needs all the help it can get, you'll be able to find an option that will give you enough boost (within a realistic budget) to shit on 95% of the road drive vehicles out there. Don't fucking overcomplicate it, you'll reach the realistic structural limitations of the engine/ comp ratio well before the intricacies of which method of forced induction becomes a particular issue.

    In the 'olden days' of the internet, people used to argue about which was better - drawthrough or blowthrough carbed. I have my own peculiarities, for sure, but the advice I ALWAYS gave was 'use whichever method you have the technical skills to have the best chance of actually completing' because in 9 out of 10 cases that would produce the results and produce a result they'd actually get IMMENSE enjoyment out of. For those 1 out of 10 outliers, the truth is they had the technical expertise that they didn't even need to ask the question in all honesty, as they could weigh up the pros and cons and proceed as necessary.

    As an example of this, about 15-20 years ago now, I took on a bit of a dare with a mate - make a car run 14 seconds for $1400 bucks including the purchase price of the car (and we achieved it, and with the price of an old moo now, it is a joke compared to then). Ended up grabbing a smokey ke30 4 door corolla, did 'everything' wrong on the manifolding etc and the fucker still outgunned an EA that ran consistent flat 15s, just ran enough boost to make it work. The intercooler we got for free had a massive leak and repair attempts failed. Having that time again (and now knowing what toyo 3k motors had better breathing heads etc) we could have run a 5k and likely broken into a high 13 for the same sort of money.

    The point being, people get too hung up on the intricacies, just get out there and do it, even at a bodgy level, and you'll quickly discover that boost is the cure all, the specifics are the icing on the cake not the bulk of the deal.
    John McKenzie

    Science flies people to the moon.
    Religion flies people into buildings.

  6. #66
    I <3 Mang. klampykixx's Avatar
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    I like it!

    As I previously said I'm not actually going to do any super complicated multi boosted thing, I genuinely wanted to talk about it.

    My van will likely get a single turbo off a falcon xr6t and I think the commodore will get a blower and call them both done.

    Also after having the previous motor tweaking experience of my red vr, I fully appreciate the boost factor having way more influence compared to getting every last watt of power out of an NA setup.

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