How to pick an AFR?

[Slidn]

Context I'm asking about AFR in the hemi forums, but some twat told me to come in here.
I figure you guys may be more helpful.

Q: How do we pick a target AFR in general?
It's great if someone tells me "Running an AFR of 12.2-12.4 is ideal for engine X."
What this doesn't tell me is how someone reached that conclusion.
How do you find an AFR range for any engine? Start from stock and work from there?

I have also seen multiple people claim "See what the car responds best to."
What data are we logging here? Butt dyno? 0-60 E.T?
I'm looking for a measurement here.

Q: What is an acceptable range of AFR values? 0.5? 1.0?
Going from 13.0-12.0 seems like a large swing.
Going from 12.4-12.2 seems like a small swing.

Maybe I am over emphasizing the importance of AFR, but i don't understand how we figure out what AFR is best for any given engine.

[JayRolla]

Tuners that dyno tune hundreds of setups are who I usually trust. You could be safe at .90 and .70 but the power difference especially on e85 is negligible so I just shoot for safe rich numbers. Been tuning my own boosted and N/A vehicles for over 25 years and always shoot for the same numbers on any setup. .77-.79 boosted and .82-.85 N/A. Yes, there may be a few HP left on the table but it's not worth running a setup leaner IMO. Even OEM tunes you can see manufacturers always shoot for way richer numbers and are pretty much the same on all the different motors and setups. The big swing is between N/A and boosted. The 90's boosted cars would dip into the 9.0 afr range sometimes. Most in the 10's.

So yes I feel you may be over emphasizing the importance of AFR. My rule of thumb is run a safe rich AFR and then make power with compression, timing and boost.