[VaSteve]

Seeing flames shoot out the back of a 911 is nothing new. But I always associated it with the older cars that liked to be run rich and not burn all that gas. I'm surprised to see this happening in a modern car... why would this occur assuming the DME was doing it's job?

Also, I don't recall seeing this much (ever?) from non-Porsches. Surely some other cars like being tuned rich, but do they just backfire within the tailpipe? Is the flame effect due to the position of the engine and the short length of the tailpipe?

[smdubovsky]

Its not a backfire (there is no popping associated). Turbos do it. 930s, RX7s, WRX, Evos can all shoot flames when they are off the gas. They deliberately overfuel the motor during a throttle 'lift' so gases burn in the tailpipe creating the flow necessary to keep the turbos spooled. An anti-lag trick. Its kinda neat to have one pass you at the end of the straight and burp flames right in front of you.

SMD

[Lucky]

An even more important question: why is there a soda can on the roof?

[Gator_86_951]

Rick deman explained that car is a turbo car. Somewhere on rennlist.

[mikeherman]

I used to have an evo8 with a 3" turbo-back exhaust, and it ran pig rich. I would be lucky to get 3 run sessions on a tank of gas. I was regularly seen shooting 3 foot flames out of the tail pipe or black plumes of unburnt exhaust/fuel. Typically this would happen when I was up-shifting down the straight aways or down-shifting at end of them. It was a nice trick to keep the slightly faster cars off of my a$$

[scott]

I have often ponder the use of the term backfire...I was taught that a backfire happened when the intake charge was ignited in the intake manifold causing the cylinder firing to come out of the intake or "back" through the system...bad timing or over tight valve clearence the cause? fire out the tail pipe was the result of a really rich intake charge ratio

Funny how terms get used to describe conditions...I have always heard people decribe the gunshot tail pipe explosions as a back fire....

[VaSteve]

There's a whole list of misapplied terms... "full throttle" is really "fully open throttle". Anyway, the fire is in the back so for me "backfire".

Are the flames in this case ignited by the spark plugs and travel through the manifolds and muffler or are they ignited by the hot exhaust pipes (as I learned it)?

[Jase007]

Are the flames in this case ignited by the spark plugs and travel through the manifolds and muffler or are they ignited by the hot exhaust pipes (as I learned it)?

Neither.

Look here for the true reason you get the flames ... though no one wil admit it.

[scott]

There are two types of backfire. One is an explosion in the engine's exhaust manifold or exhaust system, this is most correctly referred to as afterfire. The other is an explosion in the engine's intake manifold.

Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Usually, backfiring occurs in carbureted engines that are running lean where the air fuel mixture has insufficient fuel. ("Running lean" is typically a sign of mal-adjusted carburetors or fuel injection where there is not enough fuel for the amount of air). Afterfire, occurs in engines that have an emission system malfunction (air injection system diverter valve), exhaust leak or unburnt fuel in a decatted exhaust system. When a driver shifts up and lets off the gas, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system. The leak itself is the most dangerous aspect. Without it, the mixture would cool enough not to explode. A fuel injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective.

Common causes of backfires are:

Timing – If car is distributor-less
Timing – If a two stroke engine is backfiring every other stroke.
Fuel pressure, fuel filter and pump – leaks and corrosion could cause lack of fuel during the fuel injection event.
Bad wiring in ignition – ..if car won't run for more than a few moments.
In older engines, backfiring may be considered normal. Backfire is rare in modern cars with fuel-injection and computer-controlled fuel mixtures.

Common causes of back fires in the intake manifold are bad spark timing, or incorrect (usually lean) fuel ratio.

When starting an engine, timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner. A common air filter will allow the gases to escape, but will block the flame front. On many small marine engines, no air filter is used, but a screen is placed over the intake of the carburetor as a flame arrestor to prevent these flames from escaping the intake, and potentially igniting fuel, or fuel vapors in the enclosed sump or bilge of the boat and causing a fire or explosion. Improperly adjusted carburetors that create a lean condition during acceleration can cause the air fuel mixture to burn so slowly, that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire.

In drag racing, backfires in the intake usually result in the complete destruction of the intake manifold, the carburetors, blower, and sometimes engine.

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