What's a safe afr to run with these compression on 93 pump gas:

11:1

12:1

13:1

I read that usaully for na it's 13.0-13.5 afr..........But what about a high compression as in 13:1 compression. Thanks

 

No matter what, the engine dictates what it wants. Read the plugs.

 

^X2..... Having a way to monitor knock while tuning is a sound investment.

 

Do this sound like a good method?

Lets say i started conservative with a 12.1 afr. Then I start leaning to 13.5afr and that's where i start seeing black specs or metal filing on the porcelain. I should richen it .5 afr back to 13.1 afr to be safe. Then start MBT advancing?

 

i would honestly grab yourself a knocksense unit. the whole thing will come in at less than 100 bucks. once calibrated i would use that for knock detection, and you can use the spark plugs as verification instead of the only source. i think the sensor will pick up knock before it becomes pretty obvious on the plugs.

im glad i got my knocksense. one thing it helped me through wasnt just wot settings but the propensity to knock at moderate load like 7-12" below 3500ish rpm.

 

AFR is only part of the equation, ignition timming also plays a big role on the afr mixture.

 

High-comp. with bad chamber (i.e high domes, improper squish)/port design (i.e. no swirl) = higher chance of knocking; run richer.

 

Cams too. If you're running sky high static comp with a stock/smaller can, you're dynamic comp is through the roof and you'll be really knock sensitive.

OP, consider looking at your mixture in terms of lambda instead of AFR. if you get comfortable with it it will make more sense in the long run. any "actual" AFR number is something that's been programmed into the controller based on what fuel you've told it you are running (which is almost definitely E10, not pure gasoline) and therefore the AFR number you see is wrong. All hydrocarbons make best power at .9, but some engines will do a little better a bit higher or lower... I would suggest tuning at .88 and then trying a bit of power leaning after you get the timing dialed. For NA, I've seen a couple of cases of best power in the neighborhood of .94, but that doesn't leave a a lot of error margin of the rest of the tune isn't spot-on and can make you more knock sensitive.