Slight backfire on shifting

[DaddySS]

Gang - take a look at this log please, note from frame 3230 how the spark climbs when I shift - can this be part of the problem or am I on the wrong track?

Thanks!

[Black02SS]

Disable DFCO and see what it does.

[DaddySS]

Interesting - what's the best strategy - raise enable temp, RPM, ?

[Black02SS]

Yup raise the enable temp to something that won't be hit.

[SSpdDmon]

All you have to do is disable the M6 DFCO, not DFCO completely.

[DaddySS]

OK thanks, I'll give that a shot and let you know.

[DaddySS]

Whoa - excellent! Made a big difference in how it responds on shift - nice and smooth and no backfire ! Thanks!!

I changed the m6 temp enable to 255 and that's all, timing acts very differently surprisingly enough.

[mistermike]

It's not reallly a backfire, but ignition of unburnt fuel in the exhaust. Disabling DFCO usually fixes it, but IMHO it's a heavy handed solution, because you're still dumping fuel into the exhaust, but it's too rich to self ignite easily. The popping occurs as DFCO kicks in and the the additional air causes the mixture in the exhaust to approach a level that easily ignites. More aggressive DFCO settings can also minimize exhaust explosions and save fuel and your cats at the same time. Commanding a very lean condition, either through B3647 or the VE table, and pushing timing in the overrun area will also quell exhaust explosions and there's no delay as there is with the DFCO.

[DaddySS]

It's not reallly a backfire, but ignition of unburnt fuel in the exhaust. Disabling DFCO usually fixes it, but IMHO it's a heavy handed solution, because you're still dumping fuel into the exhaust, but it's too rich to self ignite easily. The popping occurs as DFCO kicks in and the the additional air causes the mixture in the exhaust to approach a level that easily ignites. More aggressive DFCO settings can also minimize exhaust explosions and save fuel and your cats at the same time. Commanding a very lean condition, either through B3647 or the VE table, and pushing timing in the overrun area will also quell exhaust explosions and there's no delay as there is with the DFCO.

Yes understood, but what I couldn't understand was why the spark was dropping from 38 to 5 when I let off the throttle to shift and now it doesn't do that (see the log in the first post).

The '02 system doesn't support 3647 and I'm not sure what strategy you mean by more agressive DFCO settings.

[mistermike]

dfe1 posted some settings that enabled DFCO almost instantly, but it looks like he withdrew it. I'd be interested in whether there was a problem.

Even without 3647, you can make the VE table settings very small in the overrun zone (high RPM-high vacuum) to pull back fuel.