Lean burn on a stock Subaru Impreza engine

[ever_green]

So I have always wanted to experiment with lean burn ever since seeing how people here were mostly fond of honda civic ex or whichever model that does do lean burn. Unfortunately I do not drive a honda as I drive a Impreza instead. There was no lean burn available for such cars (and most other cars on the market). However I do have a bit of experience self tuning my car and recently by accessing the maf scaling tables I managed to 'trick' the ecu in running lean AFRs (16.7:1).

how i did it this was by changing the air intake estimates (in g/s) in 2.0 to 2.35 voltage range of MAF sensor. I just underestimated the intake at these ranges and so the ecu injected less fuel. However in order to avoid the closed loop feedback system to correct this outrageous fuel mixture I had to create an Open Loop window in 1800-2500 RPM area in cruising load ranges. This way the O2 sensor couldn't correct the lean mixture!

here is a video of lean cruise

http://www.youtube.com/watch?v=mi0YvIvy0a8

the AFR is reported by the front wideband sensor. I have only experiment with 16.7 AFR at roughly ~44* of ignition advance. not sure if going any leaner would be beneficial or not. The injector pulsewidth is now lower by 9% at highway cruising compared to stock.

[Baltothewolf]

Hmm, I wonder what long term effects this will have on your engine. I know lean burn on the Hondas is ok because they specifically built the engine for it.

[ever_green]

not exactly sure but the extra timing and the already high compression ratio and the high swirl heads allow for a good burn it seems. no surges, abnormal exhaust gas tempratures or CEL so far.

[Baltothewolf]

Originally Posted by ever_green

not exactly sure but the extra timing and the already high compression ratio and the high swirl heads allow for a good burn it seems. no surges, abnormal exhaust gas tempratures or CEL so far.

Oh nice, well i hope you get a decent mpg bump from it! I know in high load scenarios my insight pings and once in a while knocks while in L/B, so I would definitely listen for that when over ~75% engine load and in L/B.

[ever_green]

Originally Posted by Baltothewolf

Oh nice, well i hope you get a decent mpg bump from it! I know in high load scenarios my insight pings and once in a while knocks while in L/B, so I would definitely listen for that when over ~75% engine load and in L/B.

yes pinging is from overly high ignition timing. most manufacturers like to inflate timing and run at the hottest 14.7:1 AFR. this is mostly for emission purposes. My AFR enriches above 55% to something like 14.4 AFR and timing gets retarded. no knock from data logs so far.

[Baltothewolf]

Originally Posted by ever_green

yes pinging is from overly high ignition timing. most manufacturers like to inflate timing and run at the hottest 14.7:1 AFR. this is mostly for emission purposes. My AFR enriches above 55% to something like 14.4 AFR and timing gets retarded. no knock from data logs so far.

Awesome. The last and foremost concern would be the long term effect on the catalytic converters. They weren't designed with the raised NoX from lean burn, I recommend a hard drive to burn off all the NoX once in a while.

[ever_green]

Originally Posted by Baltothewolf

Awesome. The last and foremost concern would be the long term effect on the catalytic converters. They weren't designed with the raised NoX from lean burn, I recommend a hard drive to burn off all the NoX once in a while.

Yeah that's what I had been concerned about. Not sure if my cat will like leaner mixtures.

[serialk11r]

Since you already have ECU access, why don't you get a narrowband O2 simulator, a wideband sensor, and have the ECU run in closed loop at 18:1, and make it go way "rich" as the load goes up? If it stumbles set it to 17:1 instead.

The drawback with this is that it might start having problems idling.

[ever_green]

Originally Posted by serialk11r

Since you already have ECU access, why don't you get a narrowband O2 simulator, a wideband sensor, and have the ECU run in closed loop at 18:1, and make it go way "rich" as the load goes up? If it stumbles set it to 17:1 instead.

The drawback with this is that it might start having problems idling.

i already have all those. The ECU it self is programmed in a way that it does not recognize values above 14.7:1 AFR. so if you define a value above 14.7:1 it will just ignore and do what it 'thinks' is best. usually 14.7:1.

in any case, I tihnk 16.7-17:1 is too lean? From books and online resource I see 15.5-16:1 being quote as best figures for economy:




even still i feel no difference in power at 17:1 vs 15:1 stock. throttle feels the same.

[Ecky]

Although going higher may not be the best for fuel economy with all else being equal, by leaning the burn out you can open the throttle plate more and run at a higher load, which helps too. The Insight goes as high as 25:1 or 26:1 sometimes, presumably to keep the throttle plate open.

You would probably be better off running only slightly lean and regearing to get load up, but lean burn is a more versatile and easier way of keeping load high.