Stock STi AFR observations...

[d00f]

Ok, now that I have my WBO2 sensor installed (properly without wires hanging everywhere) I've made some interesting observations...

a) for some odd reason, disconnecting the stock O2 sensor does not cause a CEL. Can't figure this one out at all unless my wiring harness is incorrectly grafted with the STi ECU.

b) It idles at exactly 14.7 but occasionally deviates causing the idle to stumble a little. Odd... It did it before disconnecting the O2 as well.

c) If I floor it below where the turbo spools the AFR drops to a lean 11.9-12.4 range even though I'm making 5-10 PSI of boost.

d) Around 3200 RPM I hit 20 PSI of boost (where the EBC is set) and the mixture is still a fairly lean 11.5 or so.

e) The AFR quickly ramps down to a really rich 10.1-10.4 around 3800 RPM (max torque) and then gradually leans out to about 10.8 around 5000 RPM.

f) From there it leans out faster to about 7000 RPM where it's about 12.5-12.8 and still making about 17PSI.

g) The last 1000RPM it quickly falls back to 10.4ish.

Now, questions...
c - it seems to be fairly well tuned below the point where it spools. I assume this is to give some responsiveness and aid in spooling. Not sure how safely it can be leaned out in this area since the engine is probably in a fairly poor VE range.

d- Again, poor VE must necessitate leaning this out so the torque curve builds fast. This may also be for a less agressive boost control; not so sure here.

e- Obviously this is the area where the motor would be most likely to detonate and I'm guessing it's extremely rich here not only to reduce this tendancy but to keep the maximum torque output low. Possibly this area is detuned to keep the maximum output from going over 280hp.

f- initially I thought this may be the limitation of the fuel system but since it drops back to 10.4ish above 7000 RPM then I guess that is not the case. Perhaps they've leaned this out a tad to keep the torque curve flatter as the VE falls off. It actually seems quite lean but the motor has never detonated on me.

g - I think this portion is just to drop the torque so you shift before smacking the redline. Nothing like pouring in extra fuel to make less power.

These are my guesses. Anyone care to add to this? I'm interested in which areas to eventually lean out in order to make more power, and how far to lean them out...

-Michael

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