Lean Burn in my Subaru - need help
 

EDITED

I solved the previous issue of higher fuel consumption with lean burn at highway speeds. The problem was too little valve lift (similar to insight vtec crossover). I just had to initiate the high lift cam lobe sooner (at highway speeds).

Now i'm commanding about ~16.5 AFR between 25-55% engine load (highway loads). As soon as one climbs above 55% load the ecu will inject extra fuel to purge the excess NOx from the 1 CAT I have (~13.9:1 AFR). It will also do the same once you let off the throttle. I know Hondas can go even leaner and I'm contemplating doing this as well...but i'm not sure how lean and also what sort of ignition timing they run and the consequences? I'm doing about 39-47* IGN depending on load at said speeds (10:1 compression head and a Honda-like pentroof chamber).

bump.

Whats the question?

If you try going leaner you're going to have to advance the timing as I'm sure you know since a leaner mixture burns slower.

yes my concern is how much more advance? I'm already up about 5* in timing ( mid 40s now)...is it safe to hit 50* IGN? will that actually improve torque? I would be curious to see how the insight reacts when it switches to lean burn operation in terms of timing.

also my main question I guess is which will be the best air fuel ratio approximately for best fuel economy...is 17:1 good enough?

I think variable lean-ness is ideal. You want your throttle open for lower pumping losses, and if you can adjust power output via AFR, you got what you want (aka a gasoline diesel basically).

Sorry, I have no idea on the timing. A dyno sounds like its really your best bet. Unless you get some instrumentation, thats going to be hard to figure out on the road.

I do have instrumentation. I can log cylinder filling in grams of air per revolution. Leaning afr past ~15.5 drops it quite a bit. It does not seem however that the extra timing does make up for it. Perhaps I have already exceeded MBT.