Lean Burn Success
 

I've been tuning my 2015 Subaru WRX to run in lean burn and I finally got around to doing some proper A-B-A testing. This car has a direct-injection turbo engine that is pretty knock-limited at moderate loads, so I wasn't sure how much of an effect a lean burn tune would have, but I'm pleased with the results.

In order to tune it, I used engine management software (Accesstuner Race), and re-calibrated the stock wideband oxygen sensor to trick the ECU into running lean. I'm currently running 17.5:1 AFRs at lower loads, which transitions to 16.3:1 at higher loads, and switches to normal AFRs at loads above 1.0g/rev of airflow (~40% of maximum load). The engine runs in full-time closed loop. Ignition timing is advanced up to 7.5 degrees over stock. Lean burn is active from 1200-2800rpm, and activates as soon as the oxygen sensor comes up to temperature, unless coolant temperatures are under -10C. I go into a lot more detail in This Post on NASIOC. I've also made a lot of changes that improve economy during engine warmup at the expense of emissions, which are detailed in that post (lower idle, disabled variable valve timing warmup settings, disabled stratified fueling idle mode, disabled post-start ignition timing tables).

I did some A-B-A testing to compare my tune with the stock map. Testing was done on a flat highway with the cruise set at 100km/hr in 6th gear with a warm car. There was a pretty steady cross wind that was more of a tail wind in the A-B direction, and more of a headwind in the B-A direction. Fuel consumption was determined by datalogging A/F ratio and mass airflow and then calculating the fuel flow in kg/hr. I also datalogged coolant, oil, and intake air temperatures to make sure they were consistent across all tests.

Average Fuel Consumption Results:
Stock Map:
A-B: 5.075 kg/hr
B-A: 5.587 kg/hr
Lean Burn: (17.0-17.1 AFRs)
A-B: 4.644 kg/hr
B-A: 5.274 kg/hr

Difference A-B: 8.5% less fuel with lean burn
Difference B-A: 5.6% less fuel with lean burn

The results were probably better on the A-B trip due to the lower engine load. (Edit: On the A-B trip, ignition timing was advanced an average of 6.1 degrees over stock. On the B-A trip, ignition timing was only advanced 3.2 degrees. At lower loads I can advance timing up to 7.5 degrees over stock). With a knock-limited engine like this, there is less to be gained by running lean at higher loads. I may do more testing at 40-80km/hr to determine if the results are even better at low engine loads.

Congrats!

I never got great mileage out of my Forester. In fact it seems to me that Subies in general are not mileage masters (in terms of fuel efficiency. In terms of longevity, all bets are off). If you can get this kind of result consistently, it could be worth a couple of MPG for you, not a bad result at all.

I haven't read your tech post (but it's nearly lunchtime and I will), did you include a periodic cookoff cycle to regenerate your catalytic converter?

No, I hadn't thought of doing that. Would this be purely to reduce emissions? The stock tune seems to cycle from lean to rich maybe once every few seconds or so. The logic for this is not accessible with my engine management software, so I don't think it would be possible to do that with my tune. The car still runs "clean", i.e. it passes the sniff test, but obviously NOx emissions will be higher with my lean burn tune. I live out in the boonies though, so reducing emissions is not a really high priority for me.

I think the regen cycle is partly about emissions but also partly about extending the life of the cat. Hopefully someone will weigh in on this to clear up any misconceptions, but my understanding is that the regen cycle briefly drops the car back into stoich to heat the cat back up and keep it operating normally, and to burn off anything that has accumulated within it while it was running cool under lean burn.

I don't live in the sticks (maybe I do - small town of about 1600) but the metro area doesn't have annual inspections here either. As long as your lights work and the wheels don't come off, you're good.

According to user Oil Pan 4 in This Thread, running lean doesn't seem to damage the cat. He seems to have quite a bit of experience running lean, so I took his word for it.

I have been trying to get this done in my 2015 Impreza premium for a while! Do you think this method would work on a n/a Impreza like mine?

I don't see why it wouldn't work well, but I don't think COBB engine management is an option for the NA Imprezas. You might want to see if RomRaider works for your engine.

I’m just a little envious of you guys which have aftermarket tuning support on factory PCM, lol... I want lean burn so bad, because the engine is at relatively low load at a decent cruising speed, so I imagine lean burn would help lots, but the only thing I can do is use a SAFC or go standalone...

A dual EFIE will give you up to 15.2:1 which is better than nothing

Dual EFIE? I don’t follow...