How lean is too lean???

Aftermarket ECU (w/o knock detector), 4AGE smallport(in rwd configuration), OEM cast pistons(less durable then aftermarket forged)

Current AFR targets in the highway cruising "cloud"* range from 12.8 to 14.0.

So my question is........ How lean should I go with the AFRs in this configuration?

"cloud" - grouping of cells large enough to allow for SOME movement in the vacuum level -MAP signal- based on road grade

At highway cruise? You should be able to be safely to stoich (14.7), and of you're monitoring egt, anywhere up to 15.5 or more..

The Samurai, at cruise, bounces between 14.1-14.8. That's with the OEM ECU and factory internals, and it isn't knocking. At anything over 1/2 throttle it goes into closed loop and drops to 11.9-12.5.

My Insight at cruise is about 22:1 but of course that's lean burn mode so it doesn't count I guess.. I just wanted to have the highest number here.

My turbo rotary sees numbers in the high single digits at max boost. I just wanted to have the lowest number here.

My S2000 goes into the 16s while cruising on the freeway.

I've run a stock 20V pretty darn lean at cruise. I mean, like, to the point where it starts to lean surge, then barely rich it up one. I put 90K miles on one like that.

Dan that's pretty rich for cruise. The miata runs around 15.5-16:1 under cruise. There's almost no load at cruise, almost impossible to melt a motor unless it's so lean it struggles to run.

For economy, lean it out till the pistons melt, then back it off a bit.

I always lean until it starts to misfire then add a little fuel.

This is generally not a problem when your engine is only making 20-25hp. If you melt pistons at 20-25hp then your engine, quite frankly, was too weak to live, deserved its death, and should not be lamented.

I've seen charts that showed peak economy happening at 16:1. Leaner than that, so the chart says, causes power production to fall off faster so that you end up needing more fuel to make the required power despite the leaner mixture. (BSFC starts going up again.) I've never been able to get that lean, although I had a stockport 13B happily cruising at 15.5:1 before.

Got worse fuel economy than my bridge port cruising at 12.5:1. Figure out what that means, 'cause I sure can't!

With hamburger, anything more than 80/20 is too lean.