Quote:
Originally Posted by Dean
I understand the theory of why it would be rich between shifts, but only if it falls all the way to idle. All the way down to idle, fuel and likely spark should not even be present if the ECU is doing it's job. And time passes, so why would the ECU even take into consideration air that flowed over the MAF a second or so before?  |
IIRC, the car doesn't run rich when the throttle is closed w/ a VTA BOV. It's when the throttle is opened back up after a quick shift that there is less air in the intake track than predicted by the MAF.
Remember that the MAF is pretty far upstream, so the ECU is basically using the current MAF data under the assumption that what it's reading now is similar to the air the motor is actually combusting, when in actuality the MAF data represents the air the motor will combust several combustion events later. So if there's a sudden change in the air pressure in the manifold, the MAF data will not correctly represent the intake charge you're calculating fuel for.
Because of this, it's plain-old impossible to tune around the rich condition of a VTA BOV on a MAF based load table. Sure you could attempt to use the TPS to detect the closed throttle, and then attempt to adjust fueling properly for the VTA when the throttle opens back up after the shift by guessing how much pressure was blown off, but a) you risk running lean if you back out too much fuel with your "guess" about how much pressure blew off and b) I don't believe the stock ECU has a table for this sort of correction, which means you can't just tune the ECU, you have to reprogram it with new logic.
All this of course is a non-issue w/ a MAP based ECU because that does in fact read the current manifold pressure and the intake temp, so the ECU can properly react to a VTA BOV. But the MAF based OEM ECU isn't ever really going to be able to totally compensate for a loss of metered air. It's just a ton easier to route that air back into the intake making the MAF data more accurate. Plus, why waste compressed air you already spent exhaust energy on compressing?