[ByronRACE]

Mixture = Air/Fuel Ratio

For a combo like yours, peak power should occur at about 13.3:1 AFR, but you should check everywhere between 11.0:1 and 13.5:1 to be sure. I've seen NA engines make substantially more power/torque quite far from "the norm".

And, for durability reasons, and the fact the the power change isn't that dramatic...I wouldn't recommend tuning it leaner than 12.7:1; there is little to gain and the richer the mixture, the less likely it is to detonate. Too rich and you'll start fouling plugs, pistons, chambers with carbon...don't want that either. 12.3-12.7:1 at WOT is where you want to be.

Sorry about the length, I type fast and get carried away.
A couple more things... The cam plays a pretty significant role as well. Generally speaking, you want peak torque to occur at as high an engine rpm as practical and within reason. Cams that build a lot of cylinder pressure when the piston isn't moving very fast tend to detonate more easily...RV cams, short duration stuff. Think of it this way...you light the mixture, pressure is rising and chasing the piston down the bore. The faster piston is moving to start with, the less likely it is that you'll hit critical pressure/temp and detonate the mixture. Picking the right cam will help as well...and when you're ready, I'm happy to run simulations and make suggestions.

And yes, combustion chamber (both head side, as well as piston side) play a role. Quench, squish, all that stuff...yes. But, it's a lesser role. You will avoid bad combustion chamber design by buying popular/proven aftermarket heads/pistons...so I wouldn't worry about that too much. Make sure to take off all the sharp edges on valve reliefs, around the spark plug hole, flatten/smooth any casting flaws or ridges anywhere in the chamber. Edges and protrusions glow in a hot combustion chamber; and can cause preignition (cousin to detonation), and run-on to occur. If you want to get nutty about it, polish/smooth the J electrode on your spark plugs as well. I had a '93 Cobra (Mustang) that wouldn't quit pinging under load and couldn't take hardly any advance. Eventually I upgraded to better heads, and upon removal...the reason for the detonation was quite obvious. The bottom thread of the spark plug hole in #3 was hanging down about 1/4" into the chamber. That must have created a very effective glow plug.



Byron