Is this normal?

Since I got my SSI's and Dansk 2 in 1 out on the car I have sometimes noticed a slight "popping" from the exhaust pipe when I lift off my foot from the gas pedal at perhaps 3000 rpm in second. I often also get a loud "bang" from the exhaust when changing gear under maximum acceleration.

I thought "popping" only occured on modified turbos?


Is this normal?

Anyone?

[BGCarrera32]

From everything I've read, sounds like a fuel/ignition issue (yeah duh right?) Sounds like it could be time for a custom DME chip...might want to look up Steve Wong on this forum (Steve W)

Take a peek at this thread for a start...
Steve Wong custom chip installed...

-BG

[don911]

After removing the cat and installing a pre-muffler, I get the same popping on deceleration. Did you have a cat converter installed before the SSI's?
I think the popping is normal for non cat exhaust. Don't know about the loud bang. Might not hurt to have your air/fuel mixture checked.

[AirCooledExcellence]

Mikkel,

My SC with SSI's & Dansk does that as well.

Saw your write up in 911 & Porsche World Today!

Cheers,

[Superman]

When exhaust popping is mentioned in front of the better mechanics I have known, their first remark is "Exhaust leak?" My exhaust does have a leak, and it does burble and pop pretty good when I release the throttle.

Thanks a lot superman I thought it was a mixture thing.

[1fastredsc]

I think it's normal, especially if you don't have a cat-converter. They're common in CIS cars cause the sensors bounces under immediate decel. I think it has to do with going lean but don't quote me, but going lean under no load while coasting is no problem.

[Superman]

Okay, I'll elaborate. When those trusted mechanics get told "no, it doesn't have an exhaust leak, but it does pop on deceleration," the mechanics scratch their heads and ask "well then how is air getting in there."

You see, when combustion is finished, there is not enough oxygen in those exhaust gasses to support further combustion, even if there is fuel (not unusual for there to be enough fuel left). That is why an exhaust leak always results in popping, and also why a secondary explanation is hard to find.

It also happens to be the reason flames can be seen coming from the pipes of race cars. The fire is not burning in the exhaust pipe, but the mixture leaving the cylinder is pretty rich and hot. It ignites at the end of the tailpipe when it hits the atmosphere.

[Garyo4]

I replaced the cat on my '79SC with a test pipe, no popping. Sounds like you have an exhaust leak. Did you use new gaskets? The old ones might have been too crushed to seal again after being disturbed.
Regards,
Gary

[350HP930]

My 930 with a straight pipe disguised as a muffler where the factory cat and muffler used to be liked to gurgle a lot on deceleration.

After my Andial fuel enrichment device died and I tweaked the mix up using the CIS adjustor screw to fatten the mix up across the board.

The richer idle and decel mix completely got rid of the decel exhaust gurgle.

I take this as a case in point that a 911/930 with reduced back pressure and a correct ~14:1 A/F ratio likes to gurgle on deceleration.

For this reason I assume your options are to live with it or fatten up your idle mixture.

[Adam]

My 2.7 (with Monty), has a fantastic growl/gurgle on decelleration. It does run a little rich, but I think it sounds great!

The loud bang should be looked into, though...