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Thread: VVT oscillation during cold start

  1. #41
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    Quote Originally Posted by spoolboy View Post
    Any tips on what to log and how to make adjustments for throttle body airflow? I would imagine a larger throttle body would be one scenario and positive pressure from boost would be another. Combine them and it's happy fun time!
    I have some ideas, sure. I've never done it, but it should be possible by setting up as controlled an environment as possible and bringing the throttle adaptives into the throttle flow model. And realistically, installing a $20 baro sensor would do wonders to simplify things.

  2. #42
    Quote Originally Posted by smokeshow View Post
    Lol, so it goes like this. Say you have a hole in a thin metal plate and you want to move air through the hole. You have to either build up pressure on one side or reduce pressure on the other side. You can calculate the mass flow rate through the hole if you know the effective area of the hole (absolute area * coefficient of discharge), velocity of the flow, and the density of the air on the high pressure side. Now, concerning the throttle body...due to physics, we know the velocity is equal to the speed of sound when the pressure ratio is below 0.528. We know the density of the air upstream of the throttle because of the modeled baro pressure and intake air temperature. And finally, we know the effective area of the throttle (as a function of flow) due to the bench calibration that the throttle goes through. Now the rules of compressible flow tend to fall apart a bit when the pressure ratio goes above 0.528. That's what the phi correction is for. Below that pressure ratio, the airflow becomes subsonic. So the phi ratio is a multiplier on that bench throttle flow calibration for pressure ratios above 0.528 to continue giving us a usable model of compressible flow through the throttle to continue estimating throttle mass flow.

    Now, phi is a variable itself and is updated based on models and measurements. Since we are measuring MAP and also have a running estimate of manifold pressure in the form of CALC_MAP, we can calculate an inferred flow discrepancy in the form of a set of throttle flow adaptives. Those adaptives are comprised of an adder, a multiplier, a proportional and an integral component. The phi variable is calculated with the throttle flow adaptive integral - phi is then used to calculate a new compressible flow factor to generate a corrected throttle mass flow when the air moving through it is subsonic, or P_Ratio > 0.528. Finally, the throttle adaptives all together are used to correct the values that come from the throttle body model.

    It's a nasty algorithm, isn't it? I won't go into any more detail now, but tl;dr...air goes through the throttle, we model both that flow and the manifold pressure; model the baro at low TPS, estimate/calculate baro at high TPS; compare the modeled MAP to measured MAP, calculate phi and throttle adaptives to correct for subsonic throttle flow and any offsets the throttle might have (manufacturing tolerances, throttle coking, etc). There's a lot more to this crazy algorithm but I'll stop there
    So we have to know the bench flow rate of the TB, The amount of air (CFM) the engine requires at any given RPM and load (MAP/KPa), which is calculated using Phi to really understand which numbers should be populated into the tables for any given RPM?

  3. #43
    So if I interpret your numbers correctly you are stating it is Pratio?

  4. #44
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    Quote Originally Posted by McCloud View Post
    So if I interpret your numbers correctly you are stating it is Pratio?
    Yes, VE tables have Pratio as a function of RPM where the cells are % volumetric efficiency. Pratio is unitless because it is pressure over pressure...more specifically MAP/Baro. On NA engines operating at sea level MAP varies from ~13in Hg (45kPa) at idle to ~0in Hg (101kPa) at WOT. In terms of Pratio that is ~0.4 at idle and ~1.0 at WOT.

  5. #45
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    Quote Originally Posted by McCloud View Post
    So we have to know the bench flow rate of the TB, The amount of air (CFM) the engine requires at any given RPM and load (MAP/KPa), which is calculated using Phi to really understand which numbers should be populated into the tables for any given RPM?
    Yes, the whole challenge here is not knowing the new throttle body flow model when you change the throttle body or anything immediately upstream or downstream of it.

  6. #46
    Quote Originally Posted by Homer View Post
    That it is. Best way to see if VVT is disabled is logging exhaust cam position.
    Logged the exhaust cam, sits right at 125 degrees, so those settings must lock it in.

  7. #47
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    Quote Originally Posted by McCloud View Post
    Logged the exhaust cam, sits right at 125 degrees, so those settings must lock it in.
    Yeah if it is not budging from 125 it is most likely locked.

  8. #48
    Quote Originally Posted by Homer View Post
    Yeah if it is not budging from 125 it is most likely locked.
    I downloaded the info from HP Tuners on adjusting VVT. I'm going to play around with it once I get a little more compfortable with the PCM. I'm really tempted to get a couple of hours on the dyno to see what will happen. Retard the cam around ~4500 RPMs see what happens to my top end. I have a steady climb in power right up to 6500 now so not sure how much it will do with the Cam I have, but still curious. That is with the assumption that 125 is currently full advance so nothing to gain down low.