Flame when downshifting
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,437
Likes: 1
From: Gainesville Fl
Flame when downshifting
i know this is like my umpteenth thread in like ten minutes but lets just say i got really bored at work today and started thinking about nothing but corvettes! ( im sure that never happens to any of you!)but when C5-Rs downshift to take a corner flame shoots out the side out the exhaust tip. what causes this? will ours do this if there is little enough exhaust restriction? do you have to be running a certain amount of power? or run a certain type of fuel? i am clueless about what causes thisbut it looks cool as hell. please help inform me on this subject!
Le Mans Master
Joined: Feb 2002
Posts: 5,970
Likes: 19
From: San Marcos Texas
This is caused by unburnt fuel, race car+huge cam with lots of valve overlap+ straight exhaust...Not likely to happen on a car with a stock cam and catalitic converters or even a semi crazy engine like mine... 
Drifting
Joined: Dec 2002
Posts: 1,474
Likes: 4
From: Tracy CA
A friend of mine had that with a VW Baja bug. The exhaust was a header leading to a single purple hornie. When you'd let off the gas quick - a 2 foot flame would fire out.
Of course this was a 1800 pound car that got 13 MPG.
Of course this was a 1800 pound car that got 13 MPG.
Melting Slicks
Joined: Apr 2003
Posts: 2,150
Likes: 6
From: NS
You mean like this?
This is one of the short track cars I work on. With the exhaust setup we had this particular year the car would flame up like this everytime the driver dropped the throttle. It's a combination of things....
1. Throttle goes from WOT to closed throttle at high RPM. It takes the fuel system a little time to react to the sudden air cutoff so the engine is basically "choked" for a short period of time. This dumps a bunch of fuel in the exhaust system. Valve timing makes a big difference as well.
2. We need air to burn the fuel. I don't know about the C5-R but in our case we had a crankcase evac system installed on the exhaust. Basically, each valve cover is vented to the exahust via 1-way valves. The momentum of the exahust moving out of the pipe applies a small vacuum to the crankcase. Through this system we get some oxygen in the exhaust, along with small leaks in the pipes.
3. The exhaust must be VERY hot. It's not the engine that lights it off, it's the hot pipes. Our car doesn't flame up until we run a few laps to get everything nice and toasty. The flame lasts for about 2 - 3 seconds.
The following year we changed the headers to a new style along with the camshaft. No more flame. It's all in the tuning. As for a street car, it would only do it if you removed the cats and mufflers and ran the guts out of it.
This is one of the short track cars I work on. With the exhaust setup we had this particular year the car would flame up like this everytime the driver dropped the throttle. It's a combination of things....
1. Throttle goes from WOT to closed throttle at high RPM. It takes the fuel system a little time to react to the sudden air cutoff so the engine is basically "choked" for a short period of time. This dumps a bunch of fuel in the exhaust system. Valve timing makes a big difference as well.
2. We need air to burn the fuel. I don't know about the C5-R but in our case we had a crankcase evac system installed on the exhaust. Basically, each valve cover is vented to the exahust via 1-way valves. The momentum of the exahust moving out of the pipe applies a small vacuum to the crankcase. Through this system we get some oxygen in the exhaust, along with small leaks in the pipes.
3. The exhaust must be VERY hot. It's not the engine that lights it off, it's the hot pipes. Our car doesn't flame up until we run a few laps to get everything nice and toasty. The flame lasts for about 2 - 3 seconds.
The following year we changed the headers to a new style along with the camshaft. No more flame. It's all in the tuning. As for a street car, it would only do it if you removed the cats and mufflers and ran the guts out of it.
Race Director
Joined: Sep 2004
Posts: 13,793
Likes: 0
From: St. Peters MO Sometimes you have to prove yourself by doing alot of killing or alot of dying...
1996man... you had a lot of time on your hands today and you must be a pyromaniac... 
...
just playin... it happens to me all the time. At least you were thinking of vettes and not some stupid way to make a ricer go fast.
...just playin... it happens to me all the time. At least you were thinking of vettes and not some stupid way to make a ricer go fast.
Team Owner
Joined: Aug 2000
Posts: 40,156
Likes: 45
From: San Pedro CA
Certain turbo charged race cars will do it, too. But for a different reason.
WRC engines use an extra injector on the exhaust to keep the turbo spinning at optimum RPMs, even if the engine is near idle. So raw flame is filling the exhaust manifold. It will pop out the pipes.
Turbo engines do it a lot, too, because of the extra amounts of fuel used to combat detonation at high boost levels (thats in addition to cam and exhaust issues).
I am pretty sure Champ (CART) cars do it, but they burn alcohol, so you won't see it.
WRC engines use an extra injector on the exhaust to keep the turbo spinning at optimum RPMs, even if the engine is near idle. So raw flame is filling the exhaust manifold. It will pop out the pipes.
Turbo engines do it a lot, too, because of the extra amounts of fuel used to combat detonation at high boost levels (thats in addition to cam and exhaust issues).
I am pretty sure Champ (CART) cars do it, but they burn alcohol, so you won't see it.
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Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
I once had a really good explanation given to me for this, but I can only remember portions of it.
Essentially it has to do with the timing events that happen when you let off the throttle. For an instant your timing is retarded, which allows it to possibly still be burning when the ehuast valve is open. This allows it to ignite the unburned fuel in the exhaust. Often times though if the car is too rich there won't be enough oxygen for your exhaust to light off. Toss in some exhaust leaks though and you have a fresh supply of air, a bunch of fuel and a spark.
I've done it myself on the dyno. The car was running too rich, it had some exhaust leaks, and it threw a 3 foot flame out of the tailpipe. The really annoying part about it is that I didn't see it because I was driving. There was a guy with a video camera but he turned away just before it happened. That would have been a cool video to have.
Essentially it has to do with the timing events that happen when you let off the throttle. For an instant your timing is retarded, which allows it to possibly still be burning when the ehuast valve is open. This allows it to ignite the unburned fuel in the exhaust. Often times though if the car is too rich there won't be enough oxygen for your exhaust to light off. Toss in some exhaust leaks though and you have a fresh supply of air, a bunch of fuel and a spark.
I've done it myself on the dyno. The car was running too rich, it had some exhaust leaks, and it threw a 3 foot flame out of the tailpipe. The really annoying part about it is that I didn't see it because I was driving. There was a guy with a video camera but he turned away just before it happened. That would have been a cool video to have.
