What's the optimal AFR?
 

Can somebody tell me what is the optimal AFR for a NA engine and for a supercharged engine running 9psi of boost?

Thanks!

Re: What's the optimal AFR?
 

"Perfect" air to fuel aka stoichometric mixture if N/A is 14.7:1, if boosted is 12.5:1. Won't necessarily give the most power though of course.

Re: Re: What's the optimal AFR?
 

So stoich produces the most power, but for safety reasons the AFR is kept rich. And that is why fuel maps for WOT operation of NA cars are characteristically rich?

The reason I ask is thay when my car was dynoed (NA) the dyno operator said I was running lean, even though the AFR was at about 12.4-13 (50 psi fuel pressure at WOT because of the Walbro fuel pump, no SARD yet). That doesn't seem lean for NA operation does it?

dyno with AFR

Re: Re: Re: What's the optimal AFR?
 

I am NA, and my AFR was around 14 for the most of my run:

http://forums.maxima.org/showthread.php?threadid=186878
http://www.glue.umd.edu/~nauman/images/dyno.jpg
http://www.glue.umd.edu/~nauman/images/afr.jpg

Why is it that 12.5 is considered ideal for boosted application? Saftey? Cylinder Temp?

mainly safety, I believe....

Perfect Stoich is 14.72 parts of air to 1 part fuel.

Okay, so the AFR at WOT is purposely set to be rich (12-12.5) solely for detonation prevention in both NA and boosted cars? That's what I'm trying to find out.

Too much inaccurate information!!!!! Stoichometric mixture will not yield the most power at WOT. 14.7 will yield optimal fuel economy. Most car engine will yield the most power between 13.1:1 or even less during acceleration and top speed.

Thank you. That is what I thought, but wasn't sure about.

Just to show a real world example. Take a look at Nigal's recent dyno. Notice he made extra 29hp because his A/F ratio was a tad richer. Take a look at the red compare to the blue in the A/F ratio graph. I would tune my car to 12.5 A/F ratio.

http://home.attbi.com/~nigelc33/429HPdyno.jpg