disabling deceleration fuel cutoff

[ever_green]

With an automatic it is hurtful to transmission to switch from neutral to drive when car is moving. However neutral is really benifical for pulse and glides and general coasting. In drive the car just engine brakes. Recently I noticed this engine braking goes away once the injectors come on at low rpm around 1100rpm.

So I opened my ECU ROM in Romraider (a subaru open source flashing system) and found overrun fueling tables. I just raised overrun fuel injection rpm from 1100 to 4500 rpm and now my car doesn't enter fuel cutoff at all.



Its really helpful for gliding and coasting as the car literary goes the extra distance now. I don't need to drop the tranny into neutral and induce wear and tear. Of course the consumption is not as low as in neutral but It's close and more practical.

If you coast a lot I find fuel injector cutoff to be useless and unhelpful. My mileage improved now since I pulse and glide a lot but I'm guessing for an average Joe consumption might actually go up.

next up is lean burn xD

[awcook]

Nice job. If I didn't use DFCO so often on the highway then I would totally do this. I realize that shifting from N to D is bad for the tranny, but how bad is it for you to shift from D to N? I've tried to do research on this subject but results have only been from enthusiasts who say "Why do they include N anyway? DFCO uses no fuel while Neutral uses fuel, what makes more logical sense?" or something like that.

[ever_green]

Originally Posted by awcook

Nice job. If I didn't use DFCO so often on the highway then I would totally do this. I realize that shifting from N to D is bad for the tranny, but how bad is it for you to shift from D to N? I've tried to do research on this subject but results have only been from enthusiasts who say "Why do they include N anyway? DFCO uses no fuel while Neutral uses fuel, what makes more logical sense?" or something like that.

My mileage is better without dfco for my style of driving specially in hilly terrain. With dfco I bleed speed and momentum too much and my time with throttle open increases vs without it I coast a lot.

[serialk11r]

I haven't seen a Subaru automatic transmission before but if it works anything like the other cars I've seen, you can blip the throttle in neutral to "rev match" the auto when you put it back into D and it'll be super smooth.

[awcook]

Yeah, I've been trying to rev match lately, though I'm not sure if rev matching would be worth it if you are just going into DFCO anyway, since you will be using more gas to get up to the higher RPMs.

[user removed]

I shift to neutral in the Sentra and back to drive without issues. The torque converter does not lock up until after the engine is up to speed going back to drive from neutral and thats at speeds as high as 70 MPH.

No way it could even come close to a hard full power upshfit or a passing downshift at any speed beyond 50 MPH at least in my experience.

Removing the DFCO portion of programming would also mean you can not use engine braking for unwanted deceleration (caught by a light, etc) or downhill where coasting would mean you would go beyond a safe (or legal) speed.

regards
mech

[awcook]

That is true Mech. Removing DFCO can be dangerous, but if you live in an area where the only hill that is really steep has a stop sign at the bottom, you will have to use friction brakes anyways. That's how it is for me anyways, and if you know how the stop lights in your city work, then getting caught at an unexpected light is not common.

[oldtamiyaphile]

I don't think Old Mech is saying it's dangerous, just that sometimes using DFCO down hills is the best option.

My TJ's DFCO only works above 2000 RPM and with very tall gearing, it's never engaged at normal speeds (only for a second or two at highway speeds).

[user removed]

First, it's reversible (the mod) so a great opportunity to test both configurations and see which is better for mileage and it could even be vehicle specific and certainly location specific.

In driving scenarios where you would never use DFCO, obviously DFCO could be a disadvantage. In scenarios where you can get nailed by tripped lights, DFCO might win out and with a modern, fairly expensive car, if I could beat my DFCO scenario mileage without transmission stress then it would obviously be the better choice. I try to avoid DFCO as much as I can, but there is always the car that trips the light at the precisely WRONG time and that forces me to stop.

38.64 MPG is impressive especially when you consider the best 06 Imprezza is 27 highway. That fact alone points to paying attention to the OPs position, since the proof is there. Even if a technique that eliminated DFCO such as this experiment offered little or no improvement, the lower overall transmission stress justifies the means of accomplishing that alone.

Interesting thread

regards
mech

[mechman600]

Originally Posted by ever_green

With an automatic it is hurtful to transmission to switch from neutral to drive when car is moving.

Says who?
Unless the transmission shifting logic is poorly engineered and makes a sizeable clunk/jarring when doing so, there is no harm in going from neutral to drive. The Matrix is smooth as glass shifting to drive, even at freeway speeds. The feeling is similar to making a routine 3-4 shift, so I highly doubt the effect on the transmission is much different.