Fuel, DFCO, Timing and EGR tuning
 

So I read increased timing and less fuel can help you save a lot of gas. My Subaru recently was tuned mostly for power due to additional powertrain modifications such as 11:1 pistons, bigger camshafts, ported valves and headers. This tune included much richer fuel ratios, higher timing under load and less under cruising due to disabled EGR and disabled fuelcutoff/overrun. My 4000 km average for trip from work to home before my economy map was 13 L/100km. Therefore I have now decided to tune for economy for this summer since I only drive to work in downtown area and back home. I think it's possible to run full lean 14.7:1 on this car without issues below 3000 RPM.

My first problem is whether I should enable EGR? I know EGR should allow me to run a lot more timing during cruising, but i'm not sure if that extra timing will make up for loss of oxygen content introduced to the combustion chamber?

Second problem is whether running 14.7:1 really makes much of a difference compared to high 13s considering that I will probably lose torque and possibly have to reduce timing as well?

Finally overrun fueling or DFCO, will this help? I notice with DFCO car generates strong engine braking where as in without it the car rolls down the hill with no resistance at 650 RPM.

I'm wondering if this is worth the effort. thanks.

Sounds like its time for some A-B-A testing.

If you can flip a switch, and change programming maps, I'd say that you should indeed have an economy tune that uses EGR. Maybe DFCO would also be good - I rely on it for braking sometimes.

that's the wrong way of thinking about spark advance.

more advance means more time the piston fights a combusting air/fuel charge to get to TDC(and start going down in the correct direction for the power stroke). anything you can do to allow output to peak with less advance will return you more power per unit of fuel injected.

introducing EGR with no other changes(or the PCM accounting for it) will cause two really noticable things to happen: the engine will run rich and you'll lose power. it runs rich because the EGR gasses displace oxygen that would otherwise be in the cylinder, it drops power for the same reason.

with correction/accounting:

the PCM has a good idea of how much of the air/fuel charge is EGR gasses and reduces fuel flow accordingly to maintain whatever AFR it is targeting. a lot of factors contribute to how accurately the AFR is maintained, but the O2 sensor will dial it in pretty soon after.

the engineers who setup the calibration for the PCM know that once EGR is introduced, the amount of advance to get back to peak output per fuel unit is greater(because of a slower air/fuel mixture burn speed), so advance is increased.

for a given manifold pressure/vacuum, you CANNOT create the same amount of power with EGR compared to without it, since the EGR gasses will bring the manifold pressure up, requiring the throttle to be closed to bring manifold pressure back down. you can get back MOST of the power lost through EGR operation through EGR-added spark advance.

the economical gains are from the reduction of throttle losses and reduction of exhaust backpressure, since the throttle has to be opened further to maintain a given power level and when dealing with gasses, an area of higher pressure will flow towards and area of lower pressure.

When people say timing gives big gains when tuning, I think it has more to do with the fact that from the factory, at WOT, cars will run leaner than optimal power at low rpm and richer than optimal at high rpm. When you adjust the fuel maps, you have to then adjust the timing and together those will give you noticable gains in torque.

What RobertISaar said is exactly correct, EGR "allows" more timing, precisely because it reduces the burn speed, and is less efficient not considering throttle losses.

To me it sounds like you don't really care too much about keeping your car NOx emission compliant (on that note, diesel trucks emit several orders of magnitude more NOx/PAHs/PM than cars, so it makes so much sense to regulate passenger cars tightly right?). With an e-throttle car, the way I would set up the tune if the map is not switchable and I didn't care about emissions compliance is the following:

1. Keep the highest load cells tuned for maximum power, because there's no reason not to make maximum torque while your foot is all the way down.
2. Lean out the next few cells down and increase throttle position to compensate. Control knock with valve timing if possible, then ignition timing. I would guess that the factory tune in a lot of cases is pretty close to this.
3. Figure out how lean you can run the engine at lower loads, and then set an rpm range where the engine runs lean (a 2009 car probably will need to be put into open loop for this to work), and then adjust the throttle map accordingly to get a smooth transition from lean to stoichiometric. Essentially, running as lean as possible at lower loads, controlling torque with fuel instead of air.
4. Lower the idle speed.
5. DFCO on, throttle opened fully while decelerating if possible.
6. No EGR, if that's an option.

so yesterday I l just enabled open loop delay for about a second, switched to stock closed loop fueling except for under load, added couple degrees of timing for light loads under 2400-3000 RPM (highway speeds) and switched to a smooth linear throttle map. I also managed to calibrate the on board trip computer (added +4% correction just like my scangauge), which was neat trick. Car runs at ~ 14.5:1 AFR now compared to ~13.7:1 before. I also disabled rear o2 sensor feedback so it does not enrich the mixture (i rather run lean if anything). EGR i kept on to allow me 48-49* of timing at cruising load. The throttle response just feels much better with more timing. However i'm not liking this linear throttle map from a performance perspective, the car feels like it has last ~50 HP at partial operation but it really allows me to precicesly place the manifold at the pressure i want.

so far today drive to work I scored 9.1 L/100km which is MUCH better than the 12.3LHK i scored the day before and this was with a heavy traffic caused by an accident today. Now to see how my drive home mileage will be, it is usually about 12.7LHK depending on traffic.