Afr

[Lioninstreet]

Originally Posted by 1795

... He ended up tuning the carbs to run in the 12.7-13.1 range at WOT for maximum HP. A lot of the work that they do is for drag racers and high performance street cars.

When I discussed the AFR numbers with my engine builder later, he said that those numbers are fine for a drag race car, but not for a road race car. He wanted to see a high of 12.4 for an AFR to avoid being lean. I tended to agree with him, but wanted more advice..

One of the several factors influencing the answer to this question is air density. ie. Are you racing at the same altitude and weather conditions your dyno was tuning you at? If not, and for example you drop in altitude when you go racing, you just leaned yourself out and vice versa.

Also what kind of fuel are you typically running and is it always constant? ex. Pump gas these days (unless specifically a constant brand of 100% gasoline) will also effect AFR as the ethanol blends we get stuck with vary in consistency. I remember when running in a limited modification, pump gas carbureted class that we dynoed muled something like 5 different gasoline brands and trust me, they don't all burn the same, produce the same kinds of power, and (fun fun) they vary from batch to batch.

As for the higher octane race fuels, we surprisingly found that the lowest octane we could run without deto, the higher the BMEP was. This allowed for less ignition advance because of a more violent, faster flame front.

Just don't run down to your local Citgo with your high revving small block and tank up with their 87 octane pump gas because I told you so...

I know this doesn't directly answer your question, but it should cause some pause when tuning down to the last 2-3% to make a HP graph look better look. IMHO, for road racing (or even the aggressive weekend joy rides), you'd prolly be better served by honing driving skills, swapping stuff out looking for better tires & brakes, suspension tweaks, etc. This instead of working the AFR/HP game. For the 1320, it's a bit different of course.