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Thread: Quick and dirty MAF signal correction?

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    Quick and dirty MAF signal correction?

    So there's one more track day for us this year, on Monday, and it looks like both the 14point7 fuel piggybacks I have in my possession might be dead. I only did a quick check last night, but they are getting 13v and earth to the plug.

    Anyway, I just want to trim about 0.2 - 0.3 volts out of the MAF signal, to get the lambda back up to around 0.85 from the factory .076, at >5000rpm and 100% TP. Don't really care about mixture outside of that range. What's the quickest, simplest and cheapest way of doing this? A potentiometer? Or I found this article about clamping a maximum voltage using a Zener diode:

    https://www.aulro.com/afvb/electroni...pulator-5.html

    It's a one off, after this weekend, new engine and ECU going in over summer.

    Thanks guys.

  2. #2
    Purist, whats that? Jason Broadhurst's Avatar
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    Find yourself a fuel cut defender They rail MAF/MAP voltage. Adjust it down to nice AFRs.

    I've done this on NA cars to sneak more timing and less fuel in to OEM tunes and it's very easy.
    Jason Broadhurst

    Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

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    Quote Originally Posted by Jason Broadhurst View Post
    Find yourself a fuel cut defender They rail MAF/MAP voltage. Adjust it down to nice AFRs.

    I've done this on NA cars to sneak more timing and less fuel in to OEM tunes and it's very easy.
    Thanks Jason. Trick is finding one in one day in Sydney, just did a quick google, no luck. I've got a Jaycar just down the road, could probably put one together out of parts, I knew what was in it. Any ideas?

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    Found what I was looking for under "DIY Fuel Cut Defender":

    http://soarercentral.com/sc-forum/me..._cut-46559.pdf

    For anyone else wanting to do this, I found it via this thread, note the guy reckons he used different resistors to get it operating in range:

    http://www.toymods.org.au/forums/thr...l-Cut-Defender

    Thanks again Jason, very helpful as usual.

  5. #5
    Purist, whats that? Jason Broadhurst's Avatar
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    FCD is an active device which doesn't alter the signal until it reaches the limit required.

    The ghetto voltage divider will alter all signals scaled 0-5 linearly. So a 0.3 VDC drop at 5 VDC would alter the 2.5 VDC by - 0.15 VDC. Often that is pretty cool because your whole open loop OEM tune will get altered a little and run leaner.

    Couple that with my sweet little o2 sensor offset thing i made 10 years ago and you have quite an effective way to lower open and closed loop fuel consumption and pick up some torque/power for like $4.
    Jason Broadhurst

    Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

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    Quote Originally Posted by Jason Broadhurst View Post
    The ghetto voltage divider will alter all signals scaled 0-5 linearly. So a 0.3 VDC drop at 5 VDC would alter the 2.5 VDC by - 0.15 VDC. Often that is pretty cool because your whole open loop OEM tune will get altered a little and run leaner.
    That will be good enough for Monday, it's only got to handle about 1.5 hours of run time. Just to be clear, is the "ghetto voltage divider" you refer to the resistor + Zener diode?

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    Arrogant wankeler Slides's Avatar
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    Just voltage divider made of resistors will do what he is suggesting if you are happy to trim the entire airflow range not just peak flow.

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    Quote Originally Posted by Slides View Post
    Just voltage divider made of resistors will do what he is suggesting if you are happy to trim the entire airflow range not just peak flow.
    Thanks Slides. One more question, does the earth need to go to the ECU earth for the MAF sensor, or to the chassis, or doesn't matter?

  9. #9
    Arrogant wankeler Slides's Avatar
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    Tie it all in at the same point you intercept the loom, earth, high and signal in that wiring.

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    Purist, whats that? Jason Broadhurst's Avatar
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    I would take the unit earth to the sensor earth. The earth of the MAF/TPS.
    Jason Broadhurst

    Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

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    Thanks gents, I'm off to Jaycar now.

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    OK, back from Jaycar.

    Decided to make the voltage divider adjustable, so got a 3 position rotary switch and a range of resistors on the earth side. Also found a male plug same as the 14point7, so it will plug right into the existing wiring, just need to add the sensor/ECU earth. $12 spent so far:



    51k "main" resistor, and 1k, 2k and 3k "earth" resistors. By my calculations, that should be about 2%, 3.7% and 5.5% voltage reduction, and should be sufficient range to get the lambda back in the right zone.

    Junior likes electrical work, so I'll get him to put it together tomorrow. Will post the results here.

  13. #13
    Purist, whats that? Jason Broadhurst's Avatar
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    very cool man! let us know how you wire it up and it performs
    Jason Broadhurst

    Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

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    MAF adjustment contraption built and installed:




    testing the voltage output showed it matched the calculations.



    Having spent some time today fiddling around with the wiring for the 14point7, I figured out there's nothing wrong with the either of the units, the wiring is still picking up some interference from somewhere. So the 14piont7 is installed ready to use, but at some point tomorrow we are going to cut that entire loom loose, and connect the adjustable voltage divider direct into the MAF signal wire as close to the sensor as we could practically get. The earth is wired up, and we have packed the soldering iron:


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    This device worked a treat. We ended up having to solder another 51k resistor into the "R2" position, and with the switch set to the 1k "R1" resistor, for a 1% voltage reduction, the Lambda was spot on at 0.85 - 0.86.

    The first sesssion we tried it, it worked perfectly for about 4 laps, then the mixture went crazy, very lean. All the solder joints appeared ok, so I assumed that heat soak must have been affecting it. Gaffa taped a rag around it and it went fine for all of the following session.
    Last edited by Sprinkles; 03-10-17 at 09:41 AM. Reason: R1 and R2 arse about in original post

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    Couldn't you just run a potentiometer between the output and ground, with the wiper wire going to the ECU? Instant % adjustment between original and zero

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    Lambda confusion

    So I built a better version of the voltage divider, using wire wound resistors, and intended to test it last weekend. I cut out the old dodgy one and just reconnected the two extension wires, maybe 25 - 30cm long, with a plug. The idea was to take the car for a run to confirm the mixture was still rich, then plug in the new divider and confirm it corrected the lambda. However, I didn't get that far.

    In the first run, the lambda was perfect, at 0.85 - 0.86 at full throttle, with just two wires extending the MAF signal wire, and no voltage divider fitted. I thought maybe the lambda wasn't reading right, but the car's speed was good, even carrying an additional 35kg more than last time it went out. Unfortunately the OBD 2 port wasn't working, so I didn't get any data from the MAF. So I just left it as is, and didn't install the new voltage divider, and the car ran great all day.

    The only other thing that changed was the addition of a cold air intake (pic below), but I would have thought that if it was flowing more air than the stock intake, the MAF would compensate and the lambda would be the same.

    Any ideas as to what might be going on here?



  18. #18
    Arrogant wankeler Slides's Avatar
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    If flow distribution is changed in the MAF it could read a few % different, likewise if the pressure differential across the motor is a bit better with less restrictive filter/intake tbere will be a bit less charge carry over so you end up at a slightly different air mass flow/fuel command at the same nominal engine load/rpm.

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    Thanks Slides, that makes sense. So even if I did have the data from the MAF, it might be misleading as regards to the amount of air the engine is consuming.

    I guess it should go on the dyno for a back to back test against the stock intake.

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