DFCO Timing?

[N0DIH]

Ok, can someone please explain something to me?

WHY do we care what the ignition timing is during DFCO?

If there is no fuel, why the heck would we care about what we are sparking? In reality, we should shut OFF the spark to keep the coil cooler.

Thanks!

[SSpdDmon]

That's not quite true. During DFCO, there is a little bit of fuel that is injected into the engine. Try logging your injector pulse widths and you'll see. On mine, I see pulse widths close to 0.5ms during DFCO. Idle is close to 3ms and normal decel (before DFCO activates) is in the 1.7~1.8ms range. That's with SVO 30's...

My question related to this was going to be, how much can we safely raise DFCO timing to eliminate any popping or backfiring between shifts (M6 car) or when re-applying throttle?

[N0DIH]

On my LT1 (OBD1) my O2 sensor readings drop to 0.004v (which is pretty much what they read engine off) and PW's are shown as 0.

Is it more of the timing so when you apply the throttle again, the timing isn't very high initially and then it picks up as soon as the engine is essentially running again? Takes only a split second, but ?

[dfe1]

Hard to say what to expect with software from the "dark ages", but on the LS1-based systems, I regularly see pulse widths during DFCO between .2 and .3 ms. As for timing, I've experimented with settings between -5 and +10 degrees and haven't seen any noticeable effect. I think one of the considerations is, as SpeedDemon of the frozen north has pointed out, there is some fuel being injected during DFCO and too much spark lead could lead to detonation, even though there's no load on the engine. Engine braking is another consideration.

[N0DIH]

How quickly does the LS1 O2 voltages drop to 0ish when DFCO kicks in? And do they vary at all? Mine on my OBD1 LT1 don't at all, 0.004 is where they stay until fueling starts again then they start to come up and fluxuate again. I don't get any issues with popping or backfiring when DFCO is on, or when it returns. I have heard some people with loud exhausts complain, but my stock one I can hear it in the decel mode some, but no irregular noise.

I wouldn't be surprised is GM did kill 100% fuel on mine and later reduced it to almost none, but kept some. Somewhere along the way Clear Flood mode went from 0 fuel to 80:1 A/F. So maybe that was the timeframe change?

Maybe completely no fuel is hard on the engine where a really lean mix isn't so bad? I can't see it being hot when no fuel is present, and emissions should be zero. Maybe it promotes too much cooling of the cat/O2's?

I don't know if my 99 L29 454 ever gets into DFCO, You can be cruising in traffic @ 35 mph and not touch the gas for 30-40 sec and hardly drop 2 mph. It just cruises along. If I disconnect the IAC at idle and then drive it drives fine. So maybe the throttle follower is too slow or too far out to drop the air and while it is so far out it keeps adding fuel and keeps me cruising along. Might explain some of the crap fuel economy 454's get....

[dfe1]

What are you using to program and data log. Part of the problem could be the software, although some of the controls in the LT1 software may not be implemented. I wouldn't take it for granted that the software you're using gives you access to all the tables in the PCM. I don't remember if you have EFILive-- if you do, I can send you a log file so you can see transitions into and out of DFCO.

[mistermike]

There's no danger of too much spark causing detonation during DFCO. To say that there's no engine load would be an understatement. Pushing spark in "overrun" areas ensures that unburned fuel isn't pushed into the exhaust causing popping noises or making the catalytic converter unhappy.

[N0DIH]

I am running Freescan. I have a few runs left of TTS Datamaster that I primarily save for trans work. I wouldn't be surprised if the HC11's that the LT1 PCM has doesn't do that detailed resolution on the bus. But you do know for sure, when DFCO kicks in, the engine responds noticeably. My 99 K2500 Suburban you can't even tell if DFCO ever does kick in. It is absolutely unnoticeable, I hate it....

So in the DFCO operation range, wouldn't you want really high timing (40-46 degrees or so), being it is running very very lean and light load? I guess up to a point where NOx emissions would take off...

[ZL1Killa]

i'm not tring to hijack this thread or anything but also, my car tends to sit rich on decel and such and I was wanting to know if it would be ok to just kill fuel (for the most part..cept for the little it does put in)...so how do i go about cuttin DFCO off.. or making it so that on decel it doesn't go rich. or would it not be adviseable to cut DFCO off??

[mistermike]

Cutting off DFCO will just make it worse. More aggressive settings can minimize sucking excess fuel on decel. These (http://forum.efilive.com/showpost.php?p=48176&postcount=25) settings seem to work for me. Refer to the parent thread for more details.

[N0DIH]

unburned fuel in the cat is bad news, melts things....

[TAQuickness]

Adjusting the DFCO spark ramp rates helps smooth out the DFCO transistions.

[SSpdDmon]

Adjusting the DFCO spark ramp rates helps smooth out the DFCO transistions.
Faster? Slower?

[dfe1]

Faster? Slower?

Yes.

I think it depends on the individual vehicle. I ramp mine in and out fairly quickly and the transitions are seamless. I don't get any popping noise on decel.

[2002_z28_six_speed]

changed all these values. will repost later

[TAQuickness]

Faster? Slower?

Slower ramp-in for spark. The RR makes this very easy to dail in to your liking.

[SSpdDmon]

Slower ramp-in for spark. The RR makes this very easy to dail in to your liking.
Yeah...I think I'm going to have to pick me up one of those soon. Maybe Santa will have one in his bag for me this year. :lol:

[DaddySS]

My question related to this was going to be, how much can we safely raise DFCO timing to eliminate any popping or backfiring between shifts (M6 car) or when re-applying throttle?

Has anyone determined a good timing or ramp in rate that will help or solve the pop on shift issue?

[DaddySS]

BTW, the ramp rate only affects the rate when the throttle is released, when the clutch is depressed in ramps in immediately.