Decel exhaust popping?...answers

[cygnusx1]

 I have had the decel exhaust popping for a long time.  With the stock motor with 2.5" exhaust and then with my turbo motor with 2.5" exhaust, and even on my Triple Weber motor with 2.5" exhaust.  Well it seemed to be common knowledge that popping in the exhaust during decel was because of fuel or exhaust gases igniting in the exhaust system.  Common explanations were that the engine was too rich during decel and fuel cut should help.  Contradicting this, I read somewhere, while dialing in my webers, that there is such a thing as "lean condition pop".  I always thought that the popping was cause by too much fuel being dumped so I always tried to lean the mix during decel.  Well it turns out that by richening the mix during decel, the exhaust popping is much less and even goes away totally.  It worked great with the webers.  With a best lean idle established, I richened the idle volume screws about 1/2-3/4 of a turn each and the decel pop disappeared.

 

I took the same idea to my megasquirt maps and added fuel in the cells used for decel and bingo, the exhaust popping during decel just about went away.  I find that maintaining 13.5-14.0 during decel minimizes the exhaust pops and back fires.   

 

 

 

 

I hope this helps someone and sheds light on the decel pops.

Edited July 27, 2010 by cygnusx1

[Challenger]

You dont like them?? haha

 

I assume its making the exhaust rich enough that it cant easily ignite?

[jacob80]

This is something I will keep in mind. Its as if I would tune for a good cruise AFR of ~14.7-15.0 but then when you're coming down from some high RPMs in decel it'll pop like a mother and you're forced to richen it up to get it to go away. Still battling this...

[proxlamus©]

man i love decel pops.. sounds awesome

[Nismo280zEd]

I was always told this was a sign of a healthy motor when it pops on decel with an open exhaust. However the Z has some LOUD pops on decel, great info!

[jacob80]

I was always told this was a sign of a healthy motor when it pops on decel with an open exhaust. However the Z has some LOUD pops on decel, great info!

 

This is true.

 

Its as if the pops become louder as you come down from high RPMs, sometimes to the point where I just disengage the clutch because I am afraid to harm my motor (in some way that is beyond me).

[FlatBlack]

This is true.

 

Its as if the pops become louder as you come down from high RPMs, sometimes to the point where I just disengage the clutch because I am afraid to harm my motor (in some way that is beyond me).

 

 

The only way you're going to 'hurt' an L28 is with det or running it without oil. Beat on that thing! It was made to be beat on!

Edited July 29, 2010 by FlatBlack

[letitsnow]

Why not setup decel fuel cut on the MS, it's what all the modern efi systems do. There won't be any popping because the injectors won't spray ANY fuel.

[Lazeum]

I actually love decel lean pop too! Sounds like the race cars I have seen recently at Le Mans Classic (GT40's, Porsche 904 gts, etc...)

 

I've been also wondering if it could hurt the exhaust valve with resonance and back pressure in the exhaust system...

 

 

[cygnusx1]

If you ever heard a set of triples on a heavy decel without exhaust pop you know how sweet that baffle sound is.  MSII, in particular my revision, does not have fuel cut on decel.

 

 

 

 

My revelation was that making it richer eliminated the backfiring in the exhaust, not leaner.

Edited July 29, 2010 by cygnusx1

[Zmanco]

After enough big "pops", muffler baffles can work loose. That happened with my MSA Dynomax muffler. At idle it sounded like I had bad muffler bearings...

[jacob80]

Bad muffler bearings will get you good. Where can i get more?

 

At any rate, the main concern with this decel pops is my turbine wheel.

[mtnickel]

Bad muffler bearings will get you good. Where can i get more?

 

At any rate, the main concern with this decel pops is my turbine wheel.

 

You can make your own decel fuel cut using the fuel map. This is what I did for my Dad's Porsche 914.

 

Basically,

 

Datalog: figure out what vacuum you pull on decel. take a reading at a low RPM (perhaps 1500-2000rpm) because you'll get more vacuum at high rpm and if you use that reading, it could be work spotty.

 

So say we get a reading of 25mBar, Setup the VE table as follows

 

RPM 500 1000 1800 3000 ...

Map

100 80 80 83 84

..

..

40- 30 30 30 30 ...

28- 30 30 30 30 ...

27- 30 30 0 0 0

 

So what happens is when vacuum drops below 27 (signaling closed throttle decel), it cuts the fuel.

Don't worry about running lean anywhere because since the map values (28,27) are so close, it's either on or off. No interpolation between them causing a lean condition. Moreover, you won't experience any cruising loads with this low of a map.

I don't make the entire row 0 as it can cause a stumble coming back to idle. Instead have it bring fuel back in somewhere between 1000-2000 rpm. This is a guide, I set it up years ago, so the numbers could be much different.

 

Anyways, it worked like a charm. I later got MS2 with the decel feature, but it didn't even work as good for whatever reason.

 

Hope this helps someone.

 

Mark

 

 

 

Perhaps even the lowest amount you can pull (so

Edited August 18, 2010 by mtnickel

[niner11]

Dave, I was just going through this older thread and saw a post of yours that talked about MSII not having fuel cut on decel. I think in the accell enrichment wizard in tuner studio you can create any percentage of decel that you're looking for. I've got mine set at 85% without any bucking but for the longest time I left it at 100 % with pretty good results.

 

I was also wondering if you are still using the same accel enrichments that are posted from a few years ago. Thanks

[cygnusx1]

Dave, I was just going through this older thread and saw a post of yours that talked about MSII not having fuel cut on decel.  I think in the accell enrichment wizard in tuner studio you can create any percentage of decel that you're looking for.  I've got mine set at 85% without any bucking but for the longest time I left it at 100 % with pretty good results.

 

I was also wondering if you are still using the same accel enrichments that are posted from a few years ago.  Thanks

 

 

 

 

That decel value is ONLY when the gas pedal (TPS) is closing.  Once the pedal stops moving, you are back on straight MAP, even while the car is decelerating. This applies to the code version of MSII I am running.  You can cheat by pulling ve values under high vacuum on the tables as described above, and I have tried that.  I was just surprised that when I brought more fuel in during deceleration to get in the 13.5-14.0 afr range, the popping just about stopped.  When I had decels in the 16afr range, it popped like crazy.

 

I will go to the MS thread and post the map I have been running for the past year or so with much success, on the street and at a track day full bore.

 

 

Edited November 17, 2010 by cygnusx1

[mxgsfmdpx]

The straight 6 design is also very known for its loud pops

Edited November 17, 2010 by mxgsfmdpx