DFCO exit timing - Table b5150

After recently having the cat back fitted to release the noise that had been thus far strangled by the stock system I have steadily become tired with the DFCO exit pops and bangs. With the new system it sounds like I am doing the old kill switch trick we used to do on bikes. The subtle pops/bangs are nice but at certain loads the thing sounds embarrasing.

So looking to smooth the step back into normal fueling from DFCO, the table that looks best placed to do this is b5150 but the question is to lessen the bangs, should timing be increased closer to normal spark timing or decreased further to lengthen the time taken to return to base spark?

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Originally Posted by stigmundfreud

After recently having the cat back fitted to release the noise that had been thus far strangled by the stock system I have steadily become tired with the DFCO exit pops and bangs. With the new system it sounds like I am doing the old kill switch trick we used to do on bikes. The subtle pops/bangs are nice but at certain loads the thing sounds embarrasing.

So looking to smooth the step back into normal fueling from DFCO, the table that looks best placed to do this is b5150 but the question is to lessen the bangs, should timing be increased closer to normal spark timing or decreased further to lengthen the time taken to return to base spark?

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Running a different OS, but can't you just disable DFCO by temperature (a.k.a. LS1's)? If you can't then you want to slow down the spark ramp back to normal timing to lessen the sensation of engine braking. Though that will make it last longer, it is a very gradual braking at that point. I don't know if that will eliminate the 'popping' sound..short of disabling it..:grin:

more than happy to disable and yes even in this OS we have similar enablers/disablers. I was leaning towards lowering the exit spark start point so less spark = less bang though at the same time was thinking the closer to end spark rate you start the ramp in from would lessen the bang cycle so could reduce the actual big bang. You know the nice rumble when dfco exits? Thats what I want to keep but at town speeds the exit from DFCO ends up with a massive pop, its all on stock so the noise is purely related to the lack of silencing but I want to reduce it. Will try both lower and higher settings to see which is closer to what I want. The trade off if lower settings work is that I will spend longer waiting for true throttle response which can be a bad thing if it ends up like a bmw330 which can take up to 2 seconds for throttle response!!

anyway its time you and Joe both upgrade to e38's to share your enthusiasm on a newer platform!!

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Originally Posted by stigmundfreud

anyway its time you and Joe both upgrade to e38's to share your enthusiasm on a newer platform!!

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You may be right. I was reading an article in GM High-Performance today that was insinuating the same thing. I know.. I have gotten too specialized and narrow in my understanding of tuning..:).

I would be interested in your DFCO Spark results. I have played around with spark ramp rates and amount of DFCO spark. My goal is a little different. Running up and down the Mountains around here, I try to find that DFCO 'sweet-spot' where I can go down a 2-3 mile decline..hold gear and never use the brake.

If I recall that initial pop is right when DFCO releases? I hear it, but it kinda rumbles with the exhaust. But, I agree under certain circumstances you don't want it to do it.

Let me know what you end up doing.

Good luck..

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Originally Posted by stigmundfreud

anyway its time you and Joe both upgrade to e38's to share your enthusiasm on a newer platform!!

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I would like to, but GM USA doesn't make a 2 door performance coupe in my price range...:nixweiss:...anything good is promptly EOL'd.

You could weld up the rears in the g8!

Current settings are nice for duration of dfco meaning good brake rate whilst staying off the power but the ramp in is causing the issues. I'll report back on the findings though I might just go and disable dfco (assuming it retains cfco)

Pontiac is being dissolved.

blimey!

Well I took 20 degrees off the timing through the range, improved things a lot at sub 2500rpm = much more subtle but feels almost like a slight surge when tipping back in, presumably as it now takes that little longer for the spark to catch up. Still getting big pops about 2500 but much less than before, a bit more timing pulled here could improve things but there is only so much you can do when you have the vacuum issues to contend with.

So in the end i just disabled it, I cannot see it making that much of a saving but certainly the DFCO cannot be too great for cat life. So bye bye big pops and bangs but still get a nice run down sound and cfco still in effect.

Thanks for the info..

I manage a lot of my DFCP by MAP Enable/Disable. So it really only kicks in going down the big hills. Good information though.

Thanks for sharing!

ah you are COS3? Using map to control triggers is very nice but I got bored messing with things so preferred to use traditional triggers. I know ringram used to run his entire fueling off his VE by using MAP trigger points to adjust fueling for PE mode. Much more accurate and less interference from other elements but required a very accurate table

No 'regular' LS1 OS.

With my cam and altitude I Idle ~45-50 kPa MAP. I adjust my MAP enable to 20 kPa and let it ride to 30 kPa.

For me, that only activates on moderate declines and with a 10 kPa MAP spread, it holds DFCO 'forever'. Very handy for my day trips.

Later..

To stop the "popping" on the entry / exit of DFCO, simple set the timing values in {B5149} and {B5150} to something like 10'. The exhaust backfires occur when the timing drops to very low values and unburnt fuel is ejected into the hot exhaust, where is ignites as an uncontrolled burn.

Simon.

might give that a try when the car is back from the garage

Well its taken a while ;) Having run with no dfco for the last 9 months I figure it time to try your 5150 settings out!

I wanted to add a bit of input here as I have been trying to fix a tune on a 2010 Camaro LS3 with long tubes and fairly open exhaust. The thing crackles and pops very badly upon decel until the DFCO kicks in.

My fix in the end that is almost perfect was to copy the values in the first column of the low octane timing table {B5102} over 2 {B5149} and {B5150} as both those values are set to VERY retarded timing.

The car is 100% better and people on the street aren't staring at me any longer from the obnoxious noise.

Hope this helps someone struggling like me!

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