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Old 09-25-2009, 05:01 PM   #1
illinipo
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"safe" afr for high compression NA...

After 6 months of PRP ownership with mechanical issue after mechanical issue, my tune is FINALLY in the correct zip code. Now begins the lengthy and fun process of fine tuning. Im so happy to finally say, "im gonna go tune " instead of, "I HAVE to go tune "

Part of that fine-tuning is AFR. I dont have a dyno in my garage (I dont even have a garage) so I can't play around with things to find out what makes best torque. Nor am I concerned with the absolute best torque... its my only car and i'd rather be safe. That said, I believe I have some torque to gain from where i'm at.

I know afr and spark are always doing a waltz when it comes to detonation. Less fuel heats things up, and more spark increases cylinder pressure, both of these things can cause detonation. I recently spent a few hours finishing up the .8-1.0 load cells of the BKT using my method of listening for pinging (it works!), and they are still way under MBT. This is at .867 lambda or about 12.74:1. Datalogs are within .01-.02 lambda of that so the MTF is good.

Here is the question:

Would it be better to lean it out a bit to say .875 (12.86:1) or .882 (12.96:1) and lower the spark a bit? This is a 10.5:1 zetec and I often see IAT's over 100, would it be okay to even go over 13 due to the heat? (I would think no...)

Or, should I keep it at .867 and the borderline spark I just tuned to? Is there any torque to gain from where I'm at?


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