pikey7

Still this answer eludes me......

I guess for those who don't mind taking their car to the dyno everytime a small mod is done there's no problem, but even with the software, chips, MAF controllers and Widebands available, no-one (that i've seen anyway!) has truly said what all these numbers, curves and diagrams should look like. What I have seen is a few "that looks good" when charts have been posted, but...

I guess each individual vehicle takes different tweaks to get to the same settings........

There, I've asked it! (feeling better now!)

rage2

Finding the "perfect" A/F ratio for a motor and it's intended use takes a lot of playing around with. Depends on a lot of factors, as different A/F ratios affect your EGT's, fuel economy, cylinder/ring wear, emmissions... list goes on. NA motors and FI motors like have different sweet spots for AFR (generally in the 1-1.5 point leaner for NA applications). What dictates the perfect AFR totally depends on the hardware in your motor, what kind of EGT's it can take, type of pistons, combustion chamber design, etc. There's no one AFR that's perfect for every motor, but there is one AFR that's perfect for one motor with one purpose.

I spent a LOT of time dialing in A/F ratios both on the track and on the dyno with my 951 when it was 2.5L. The sweet spot that I found was under boost, I'd aim for 12.5 AFR with timing no less than 17 or 18 degrees at torque peak. Off torque peak, I can get away with a leaner mixture with a bit more timing, and off boost, I use a fairly lean mixture (high 13's low 14's). Part throttle is completely taken care of by the closed loop system, which holds AFR at around 15-16's. Very good power, don't need absurdly high boost, and very reliable... my car saw more track time than street time and I never had a failure. On the street, it gave excellent fuel economy numbers. Now, if I didn't track my car, I could get away with a different AFR under boost because I don't have to worry about sustaining high EGT's... short bursts aren't terribly bad (ie drag racing).

TurboTommy

There's two things we're trying to achieve with air/fuel ratio: max power and heat management/anti detonation. It's worth mentioning that the sweet spot for max power is much wider than you think. There's really only a 1% variance in power between 13.5/1 and 11.5/1 from a chemical/burning standpoint.
However, you can get away with a higher state of tune without detonation when you go more to the richer end of this spectrum.
So, with our turboed engines, which are basically always at a high state of tune a good air/fuel ratio is probably around 12 to 12.5. But, like Rage said, it's slightly different between engines. These slight differences amount to very, very small changes to power once your in the fat part of the air/fuel ratio spread.

rage2

Good point. Even though there's only a 1% variance in power at a wide AFR, you trade off different things... economy, combustion/exhaust temperatures, detonation.

A good example is the honda b20 motor from the CRV. I've tuned tons of these prior to turbocharging, and they make the same torque regardless of 12:1 AFR all the way to 13.5:1 AFR, so you basically choose the leanest mixture for economy benefits, since the motor is nowhere near EGT or detonation limits.

It's the balancing act of all the factors when you tune to the edge of detonation on a certain fuel that's tough.