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Joined: Apr 2001
Posts: 2,898
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From: Fort Worth
You should very much worry about a 14 or 14.5:1 AFR when you are running the car hard. The ECU holds 14-15:1 or so but ONLY at part throttle.
I've seen track motors die at 13.7:1 when run that way, hard, for a few seasons.
The simple fact is that the VAFC utterly ignores the fuel trims in the ECU. If the ECU had stored a LTFT value that was adding fuel, and then later dials that out, the VAFC will still be operating at it's original fixed adjustment and you may very well end up running lean. Those LTFT values get applied even when open-loop (WOT/VTEC and cold start).
One of the BIG items to note here is that the VAFC should NEVER be used for part throttle tuning. Most people know that, but what most people DON'T know is how to actually achieve part throttle tuning. The throttle setting MUST be reset from the factory setting which will start modifying fuel at as little as 10% throttle input. I usually bump that number up to 33% or so, which is roughly where a stock S2k (pre-04, not sure on the newer ones) will start to go open loop. Note the stock ECU does this based on MAP, NOT TPS, but since the VAFC doesn't adjust that way... you're stuck with an approximation.
Suffice to say that the VAFC isn't adequate for much other than changing the VTEC point, and if you're using it to dial out some of the WOT richness, you should never attempt to go leaner than about 12:1. I suppose you're now showing me that even that number isn't conservative enough...
I've seen track motors die at 13.7:1 when run that way, hard, for a few seasons.
The simple fact is that the VAFC utterly ignores the fuel trims in the ECU. If the ECU had stored a LTFT value that was adding fuel, and then later dials that out, the VAFC will still be operating at it's original fixed adjustment and you may very well end up running lean. Those LTFT values get applied even when open-loop (WOT/VTEC and cold start).
One of the BIG items to note here is that the VAFC should NEVER be used for part throttle tuning. Most people know that, but what most people DON'T know is how to actually achieve part throttle tuning. The throttle setting MUST be reset from the factory setting which will start modifying fuel at as little as 10% throttle input. I usually bump that number up to 33% or so, which is roughly where a stock S2k (pre-04, not sure on the newer ones) will start to go open loop. Note the stock ECU does this based on MAP, NOT TPS, but since the VAFC doesn't adjust that way... you're stuck with an approximation.
Suffice to say that the VAFC isn't adequate for much other than changing the VTEC point, and if you're using it to dial out some of the WOT richness, you should never attempt to go leaner than about 12:1. I suppose you're now showing me that even that number isn't conservative enough...
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