911addict

Here's what wikiAnswers says:
What ignites the exhaust fumes on race cars to make flames? How do flames come out of exhaust when shifting? How do you spit flames out of your exhaust? What makes flames come out your exhaust? Why do flames appear out of exhaust when shifting? Answer: race cars run a very rich mixture. when the engine dies, the unburnt fuel in the exhaust system catches fire from the heat in the exhaust pipes. when the engine is refired, the fire blows out.

This contravenes often cited opinions on this board that the cause is LEAN running.

This is a subject close to my heart as I now regularly flame (between gear changes?) since my headers have completed my straight through exhaust system...

PS: Many of us who flame have good, consistent mixture readings, which also disputes the 'lean' argument.

38D

It comes from the CIS not being able to meter the fuel accurately enough on a rapid decel + the hot turbo. It is a rich condition. It typically comes from the turbo side when you suddendly lift off the gas and have a hot turbo. My old turbo would shoot a 6" flame on the track.

Chris M.

What about from a NA car? I saw flames coming from Charlie's car in your Mosport video. Maybe he's got a secret turbo in there!

JBL930

I was told it was a lean condition too, the theory being that the mixture was too lean to be ignited by the plugs and then igniting in the hot exhaust, i know when i shoot flames and when it pops and bangs the LM1 is showing it going lean. I'm not going to argue with Colin though I'm not an engineer

Flachbau

It is an overly rich condition. When you lift off the throttle the throttle body closes and yet the CIS has already committed the anticipated amount of fuel. As as result not enough air is allowed into the cylinder chamber to allow the mixture to burn. The unburned fuel is then dumped out the exhaust port into the (flash temp hot) turbo where it ignites. If there is nothing blocking its exit the resulting explosion finds its way out the end of the exhaust pipe. The richer your AF ratio the more the effect. If your turbo is not hot enough then there will be no flame. It easy to make the car shoot flames simply by letting off the gas when on boost as you approach redline.

On a NA car the same thing can happen if the exhaust headers are red hot.

nathanUK '81 930 G50

The reason people are saying it's lean is because when running lean your exhaust and turbo will be much hotter than when running rich.

So, for perfect flaming you need your car to run lean (not advisable with an air cooled engine) and then for the flames you need lots of fuel to ignite in the turbo or headers

So yes, the mixture is too rich to burn in the engine when you lift off the gas, but running lean while on boost makes the turbo, headers & muffler hotter

Gandara

Can a regular turbo do this if it doesn't have any fuel mods or modded AFRs??

nathanUK '81 930 G50

For some reason the stock headers seem to be too long and you won't see any flames.

Aftermarket headers are shorter, free flowing mufflers and no cats help too.

911addict

My car flames from the w/g side and the exhaust side, although not at the same time, and of course, not all the time. Never had flames before the headers were fitted.

911 2

i get flames 6ft long out both exhausts

38D

You can get flames from the stock heat exchangers on the track. You can actually see the turbo & headers glow red hot when on full boost (easier to see when its a little dark out)

early85944

I just love watching that fireball in the rearview mirror. Once I thought it was the cops oops!

nalband

has anyone actually scorched the paint above the exhaust? I bubbled mine up on my last track run. I know that this is common when you have a header setup, but I'm a bit concerned when I can see the flames in the rearview. Should I get it checked out or just keep smiling and repainting?

lwilkins

Well, I learned something recently that might be useful to this post. A friend of mine (Myron and I won't give his last name without his consent) has a very nice 78 or 79 turbo that was set up for racing, and was raced by Peter Gregg.

I had previously assumed that my 92 turbo was shooting flames, but Myron told me that if I didn't have a muffler (I don't, and have pretty good headers), shooting flames was unlikely. I later confirmed with fellow racers that my car does not shoot flames on decel.

By contrast, I recently had a student who had a stock 91 turbo, who I drove around the track to show him how to optimize the unique characteristics of the car. He shot flames, a lot, according to folks behind me in braking areas. (I shouldn't have been going slow enough for them to observe this, but I confess (a) it was his car, (b) on hard tires, and (c) he was a green student, all of which made me drive at about 75%. Good thing, too, as the brakes got mushy in the last lap.)

I use race gas (104-105 octane) in my car and I am pretty sure that he used 92-93 octane street gas, as he was at his first or second DE.

So I surmise that the flame shooting phenomenon is related to exhaust gases collecting unburned in the mufflers, then igniting on decel. No mufflers, no flames. At least no noticeable flames. This is just like Myron told me.

I would be most appreciative if others could give info that would confirm or deny this theory.

early85944

But the 935's shot flames with zork tubes