Safe to run 14.9 to 15.1 AFR in closed loop cruise?

[gesundlich]

In this day and age of $4.00/gallon 93 octane, I was thinking about changing my closed loop AFR to 14.9 to 15.1. My question is, is this a safe way to do things provided I'm not knocking and my EGTs are acceptable?

My second question is: How do I change? Would this be done in the primary open loop fueling map? Or, do I need to change the primary open loop map AND something else? I can DECREASE the values, but not increase them above 14.7 in the primary open loop fueling map.

Car is a 2006 WRX MT using ECUflash

[Multiades]

Replying a bit late...

Nothing wrong with leaning it out if possible to save fuel, but you won't accomplish much with a narrow-band O2 sensor.

Combustion efficiency potentially improves to 16.3:1 at the expense of increased combustion temperatures (desirable at low load) and NOx emissions. The catalyst may not be able to effectively reduce HC, but HC emissions will be naturally less.

If you could closed-loop with a wideband O2 sensor, you could do that. I don't know which modern cars come with widebands, but I'm under the impression it's more prevalent. One could always use an LC-1 and the analog 0-1v output re-centered around a leaner AFR...

-Chris

[Freon]

Very good advice. Subarus use a front wideband before the turbo and a single rear narrowband before the last catalyst.

The 2006 WRX (or any drive-by-wire Subaru) can be tricked to running lean by modifying the front wideband O2 sensor scaling table (transfer function table). I do think the rear O2 might still be a concern, but likely you can get away with turning off the rear O2 related check engine lights. I do know that removing the sensor or leaving it exposed to open air will make the car run richer, so running lean might do the same to some extent. The LC-1 trick would negate the problem.

The LC-1/LM-1 analog out definitely works for running standard 14.7:1 for rear O2 signal simulation on the DBW Srubarus. I've set it up on a several cars.

[repforenzo]

I'll just reiterate what the two previous posters said. I am close to 16.0:1 for idle and cruise with no ill effect. However, my car is set for Open Loop all the time and I have a Wide Band O2 that I installed. I also have an EGT gauge and found that idle increased from 690 *F to 820 *F, which is fine and keeps the combustion chamber nice and clean (the plugs are really clean).

I haven't had any timing issues, but I did shave a degree of timing off of idle because of a Knock Sum of 4 when it was really hot outside. No knock sum after the adjustment.

Note: I could not run the desired AFRs I wanted without a Wide Band O2 and having my car set in Open Loop. Unless you car is equipped with a Wide Band from the factory and you can alter the signal like Freon suggested, you have to do what I did.

[turbochips]

If you have your cath in place, bad idea... more than 14,7 AFR in cruise and idle will "eat" your cat. conv. very fast.

[repforenzo]

I'm glad you brought that up. I do have cats and recently I had them off because they are part of the center section of my exhaust. That section comes off pretty easily. Anyhow, the cats are in great shape. If I were too rich, that would create more damage than being too lean. I just have to watch the EGTs and make sure they don't get too hot.

[Sasha_A80]

Slightly lean mixture will not damage your cat. The main problem is much less NOx reduction. HC will be decreased by cat much better for lean mixture.

What is better with idle and cruise at AFR 15-16: cleaner chamber and plugs.

Bad things are: rust of your exhaust pipe (if not stainless) due to increased NOx level and, possibly, rough idle and hesitation.

Unfortunately you could not get measureable fuel improvements (1-2% max).

You also need to slightly advance ignition timing at light loads in order to improve idling and compensate for slower burning rate. And to switch off catalist testing by the post cat lambda.

Precat wideband lambda readings maybe fooled by appropriate resistor adding current offset to wideband controller curcuit.

Formally your need to correct closed loop TargetAirFuelRation table (desired AFR vs RPM and Load) if your ECU has this..

[C6ect.ect]

The only reason modern cars run cruise near 14.7 is because the cat can only work in that range. Running leaner (like 15.3 or more) results in lower EGTs and CHTs and is more efficient but can start to missfire at a point. 14.7 is the hottest burn, leaner or richer is cooler but for different reasons.

[unnefer]

Please correct me if I am wrong (pretty new to tuning Subarus), but can't you just change the "Closed loop base fueling target" instead of changing all the points in the "primary open loop fueling" maps?

[Sasha_A80]

Please correct me if I am wrong (pretty new to tuning Subarus), but can't you just change the "Closed loop base fueling target" instead of changing all the points in the "primary open loop fueling" maps?

You are correct. Just closed loop target AFR table should be changed.

From the other hand, I see the only reason for doing so at cruise. Namely - cleaner spark plug insulators. Fuel economy around 1-2% could not be noticed.

More promising (and risky) is engine leaning at high revs relative conservative factory calibrations. Engine power may increase but the engine is prone to break pistons if mixture is not rich enough.