Porsche Newbie

I get a back fire thru the exhaust on decel. My pre muffler/CAT is basically empty..see right down the pike in there.. could this be a cause of the backfire? Also, I am assuming there is little back pressure now that there is no material inside the CAT/Premuffler.. is this a bad thing? replace or no?

Chris57

mine does it ocassionally now and never did before. I put in cat bypass and sport muffler and thats when it started. I am going to purchase a european computer or a steve wong chip and think either one will cure it. My car is a 1985 turbo look carrera.

Droops83

Any 911 will backfire a bit on decel if you put in an aftermarket exhaust/cat bypass. The backfiring is from unburnt fuel (hydrocarbons) combusting in the exhaust from the heat. This is why many turbocharged/race cars spit flames out of the exhaust on decel--turbo cars generally run richer AND the exhaust gas temps are higher, so with an open exhaust this greater amount of heat will ignite the excess fuel, and it looks cool! The reason this generally does not occur with a cat/stock exhaust is that the cat will oxidize much of the unburnt HCs, and a stock muffler will muffle smaller popping sounds that an aftermarket unit wouldn't. I personallly think that it sounds cool and would not worry about it.

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Chris Andropoulos
Schneider Autohaus
Santa Barbara, CA

Dan Cobb

I couldn't possibly have described it better in more words.
+++

84_Carrera

Original to me...
Stock Cat, Stock Muffler, Weltmeister Chip = no pops

Fabspeed Bypass, Stock Muffler, Chip = no pops

Fabspeed Bypass, Bursch, Chip = minor noises

Fabspeed Bypass, M&K (de-packed), Chip = frequent pops

Fabspeed Bypass, Fabspeed Dump Pipe, Chip = Glorious Music

One of the other car guys at work insists I have a backfire through the intake / elsewhere, as those kinds of pops can't be coming from the exhaust...

Noah930

There was a similar question on Pelican awhile ago. The replies ran a couple pages long. No clear cut consensus on whether it was a lean or rich situation. (I voted for rich, using a thought process similar to the one presented above.) Eventually someone (Steve Wong?) chimed in that he believed it was due to a lean condition, not rich. Apparently, there's not enough fuel to detonate, so it runs through the motor/combustion chamber, accumulates on the exhaust side, and eventually there's enough to ignite for the backfire. In retrospect, there are so many variables (is it really a full-on backfire, or just a little popping/gurgling on decel? what's the state of tune or modification of the motor? etc) that I'm not even sure everyone was talking about the same thing. Personally, my stock '87 930 does do that light gurgling and popping on decel, even when cold, so I'm not so sure it's because of superhot exhaust pipes.

84_Carrera

yeah, sorry, let me rephrase... it's the minor popping, but when you have a cat bypass & muffler bypass, little sounds aren't so little anymore.

Yes, decel lift / back on throttle lightly for mine.

Porsche Newbie

I'd describe it as "a little popping and gurgling" not full on "bangs".. not gonna woory about it then..I do need to check the CO% for sure..

nkgosselin

I had a similar noise on my 83sc that I thought was completely normal. The car ran great. But when I took it in for a 60k service a month a go, the guys at Storz garage in Denver found something out. They said that one of the injectors, or maybe it was an injector washer was not seated correctly allowing/creating a bit of a miss most noticeable when decelerating. Anyway, it was something to do with one of the injectors and its position/tightness/washer. Anyway, the car feels like it gained 10 horsepower, does not pop on decel and idles like a 60k mile car should. I had no idea such a small thing could make such a huge difference. Really night and day.

Droops83

You are correct to point out there are a few different causes. CIS 930s like to run rich, especially when cold. Remember that fuel needs enough oxygen to ignite, and if the engine is running rich, there is not enough oxygen in the combustion chamber for it all to ignite. Once the unburnt fuel exits the combustion chamber out the exhaust port, however, there is plenty of oxygen. Even when the engine is "cold," it is still hot enough for unburnt fuel to ignite, but not violently, just little pops and gurgles like you say happens on your 930.

But on the other hand, a car will often backfire if it has an exhaust leak. This is because even more air is being introduced into the exhaust, making it even easier for anything in there to ignite. Remember introducing air into the exhaust makes it hotter, that's why you need to rip out the cursed smog pump/air injection on your 911, it makes it run too hot! Well in CA just remove the belt since you need the system for smog of course.

Steve Wong's comment on Pelican makes sense, if a car is running lean then the EGT is hotter, and the little bit of unburnt fuel that accumulates can make a big bang all at once. So like you said, it can vary!

But I will admit, I am a mechanic, not an engineer, and I would love for anyone that has some great scientific knowledge of this to chime in!

---Chris A.