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Originally Posted by shadrach74

I never said that his number accurate. What I said is that it's unlikely that his engine would not run at an AFR of 18:1.
With an 11:1 compression ratio and pump gas, an m96 motor will ping like a big dog at 18:1 A/F ratio under full load on a dyno. Narrow tuning window. It may run.... for a little while.

And yes, I do much leaner than stoichiometric (14.7:1) all the time when running a 360CI, magneto timed (fixed @25BTDC) air-cooled flat 4 with the throttle wide open. Can you guess the application?

Sounds like a Lycoming 0-360 variant. A fine aircraft engine with a very wide tuning window. With an 8.5:1 compression ratio and avgas it will probably purr like a kitten at 18:1 A/F ratio. It won't be making max power though. My father was an aerospace engineer and certified aircraft mechanic. We rebuilt a few of these in our garage when I was a kid. I am guessing you still prefer to do your sea level takeoffs at full rich.