popping with aftermarket exhaust

redcoupe86 · 01-03-2006, 05:24 AM

I have an 86 with a fabspeed exhaust with no cat (o2 sensor hooked up) and a Steve Wong chip by PO. Here is the thing, I'm getting a popping on downshifts which from what I've been reading could mean a momentary lean condition, but I smell gas and my tailpipes are black which would indicate rich to me.

Anybody have any direction to point me in?

Superman · 01-03-2006, 08:01 AM

Exhaust pipe popping is caused by exhaust leaks. Air gets in there, and mixes with the heat and unburned fuel and POP!

Lean conditions cause intake popping.

mb911 · mb911's Garage

I have seen most times with carreras that they need to be fattened up after a new aftermarket muffler is installed. It has to do with the tuning of the DME that makes this happen. Most go with a SW chip or some will just fatten it up a bit. Very rarely are the gaskets to blame but I have seen that as well

mb911 · mb911's Garage

oh you should also contact SW and see if he will reburn your chip for that muffler that would help a bunch

redcoupe86 · 01-03-2006, 08:40 AM

I'll try contacting SW. I wonder if I give him my serial number he can tell me how the chip was made b/c the owner did say it was made for the exhaust. As for "fattening" it up, I figured i was already too fat with the very sooty exhaust tips and the gaseous smell. How would you fatten it up? With the reprogramming of the chip?

Concider that the engine has 117,000 miles with NO rebuilds. It doesn't burn oil too badly and does not smoke on start up, accel, decel, etc. It does start kinda rough and then smooth out pretty good once warmed up. I have heard that on badly worn valve guides air can be sucked by them and you could get a popping sound.

redcoupe86 · 01-03-2006, 08:42 AM

Quote:

Originally posted by Superman
Exhaust pipe popping is caused by exhaust leaks. Air gets in there, and mixes with the heat and unburned fuel and POP!

Lean conditions cause intake popping.

what would intake popping sound like - i know, i know - POPPING, but where would the sound seem like its eminating from. Mine def. sounds like its coming from the tailpipes.

Danskman · 01-03-2006, 08:45 AM

Somewhat common on 911's after installing any exhaust, aftermarket or original - caused by exhaust leaks at the flanges. Retorque all attaching hardware, including the ones you didn't touch, and you should be fine.

Superman · 01-03-2006, 10:19 AM

I forgot to mention that this question seems controversial. For me, it's not. Danskman too, probably. Each time I mention exhaust popping to a professional mechanic I trust, he says "exhaust leak." This has been the case with a number of professional mechanics I have trusted. Chase down chip problems and mixture problems all you want. They can't get so far out of adjustment as to cause exhaust popping. A very lean mixture will cause subtle surging when you're trying to maintain even throttle position. A very very rich mixture can cause stumbling. But these are very lean and very rich conditions.

It's an exhaust leak. And as long as you don't have a CAT to destroy with the vibrations of the popping, it really doesn't hurt anything. If you don't like it, take it apart again, use good gaskets and coat the gaskets with some exhaust gasket stuff. Clean all surfaces well first.

To answer your question, intake popping is a bit subtle, but you can hear it standing next to the engine, at idle. If it is a carbureted car, you might be tempted to look down the throat of the carb, and perhaps goose the throttle. Don't do this if you like your eyebrows.

mb911 · mb911's Garage

yeah I misread the original post danskman and superman are right on. It will probably be gaskets in this case but what I have seen is to rich rather then to lean thats what would cause the poping out of the exhaust but it should be mentioned that this mostly only occurs with the carrera engines. I have put on hundreds of mufflers and only had them pop on carrera's 99% of the time. I was thinking out my a@# for a minute this morning

Emission · Emission's Garage

I have the Fabspeed and mine pops too. I think it is too rich, as it only seems to happen when the engine/exhaust is warmed up and the fuel can explode...

redcoupe86 · 01-03-2006, 10:54 AM

Quote:

Originally posted by Emission
I have the Fabspeed and mine pops too. I think it is too rich, as it only seems to happen when the engine/exhaust is warmed up and the fuel can explode...

this is good know stuff, thanks guys. Emission, do you have a cat or a cat bypass?

Emission · Emission's Garage

Quote:

Originally posted by redcoupe86
this is good know stuff, thanks guys. Emission, do you have a cat or a cat bypass?

No cat.

Mine is a dual outlet, but on the turbo it is really a single outlet on the muffler (from the cylinders), and another single outlet on the wastegate dump pipe.

Superman · 01-03-2006, 01:21 PM

I'd agree that a richer mixture provides a more ideal environment for this to happen. My mixture (in my antiquated CIS SC) is deliberately rich, for detonation insurance purposes.

Mine pops. I don't mind.

Emission · Emission's Garage

Quote:

Originally posted by Superman
I'd agree that a richer mixture provides a more ideal environment for this to happen. My mixture (in my antiquated CIS SC) is deliberately rich, for detonation insurance purposes.

Mine pops. I don't mind.

Likewise, with my turbo I like the rich mixture.