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What causes the Exhaust-Bang?


Forum Addict
May 29, 2004
I saw a Fiat Croma today and it was tuned (I think).
I was biking next to it and it revved when it was at the trafficlight when the revs went down there was this machinegun inside the exhaust destroying my eardrums. Anybody know what this is?

It's so nice :love:
As I understand it....
Mixture gets lean enough where it can't ignite and unburned gases go to exhaust, where it fires. Typical with old carbs.
So the unburned gasses go to the exhaust, why do they burn there?
I assume the exhaust is not hot enough (no compression either) to burn it...
MXM said:
As I understand it....
Mixture gets lean enough where it can't ignite and unburned gases go to exhaust, where it fires. Typical with old carbs.

Heard with the f40, if u let go of the gas, it'll dump all (in the engine :p) the unburned fuel in the exaust system, and that how u get that bang there. Not sure if it's always the same everywhere. But I do notice it on loads of race cars.
Hmmm, At leat on the rally cars it's caused by the turbos wastegate dumbing the unburned fuel into the exhaust system... (could be wrong)

Theory being that when you keep the turbo spinning (injecting fuel) and therefore having the boost on, you have full power immediately when you leave the corner (where you slowed down and dropped the revs for). This is just something I recall from the back of my head so not sure about the correctness (is that a word?)
Hmm...If the turbo is spooling, it is sucking air out of the cylinder right?
Cause the exhaustgasses coming directly from the cylinder will flow into the turbo.
You say that if the turbo spools the engine is injecting fuel..?
As far as I know modern cars have this system that they don't inject fuel when you release the throttle. I hear this rattling with new WRX's too..

Thanks so far for the answers :)
In the rally cars this is a special setup as fuel consumptioin is not an issue and therefore they are injecting even when the throttle is released.

You can also set it up yourself in your own car if you want the "cool" sound. I have no idea if this is what causes the WRX to do so, maybe some subaru owner could clarify...
I searched on google, but since I don't know the correct word for it I searched for "backfire".
A car backfires because unburned fuel is getting to the muffler. The heat of the exhause system causes the fuel to compust, thus a backfire.

Ever hear a popping or backfiring noise occur when stepping on the gas? Maybe it?s during acceleration from a stoplight, or perhaps climbing a hill. This is called a backfire, specifically, when combustion of the air/fuel mixture takes place in the upper half of the engine rather than the bottom half (or combustion chamber) where its supposed to. Backfire occurs when either the intake or exhaust valves are open when they?re not supposed to be or when a mistimed ignition spark takes place either from crossfire of wires or maladjusted ignition timing.

This is the popping or banging sound sometimes heard in the exhaust when decelerating. It can indicate a problem such as over-rich carburetion, a bad exhaust valve or an ignition problem (retarded timing or a cracked distributor cap). If the backfiring occurs through the carburetor, it may mean over-advanced timing, a bad intake valve or a cracked distributor cap.

a loud noise made by the explosion of fuel in the manifold or exhaust of an internal combustion engine

But I'm not really sure if that is the same as the popping sound in rally cars.

I'm sure somebody on this forum knows..
Basically on rally cars they used to have an extra fuel injector in the exhaust manifold when you let off the throttle, fuel is then dumped into the manifold and explodes, this creates the flames out the back, it is designed to keep the turbo "spooled". This is known as an anti-lag system, basically it cancels all turbo lag at the cost of fuel economy and stupidly high manifold temp's, it's also not road legal and used to be turned off between stages. Modern WRC engines aparently don't need this extra injector because the spark is completely computer controled so basically can dump fuel air mixture un-ignited into the manifold.

The pops and bang you hear from normal cars is usually down to timing or a stupid cam shaft making the timing a bit funny, modern engines are so well computer controlled it pretty much gets rid of this which is better for economy but not as cool

He's a good tip if you want have fun with back-fire, check if your car has a mechanical fuel pump or an electric one you can control manually. Find a hill, stick it in a low gear and let the engine brake, now shut off the ignition for a few sec's then turn it on. BANG!! :D becareful not to go to crazy as you can blow your exhaust apart :shock:


Ok just read up some stuff and the above deffinition isn't quite right, I just decribed an extra injector system


here a good decription of an Anti lag system

Thanks Ruuman, I read the bangbang doc partly yesterday but I thought it was different.

So they flame out of the exhaust uses all air (oxygen) in the manifold and creates suction and therefore it spools the turbo up?

btw: I don't think my mom will like here Punto to be transferred in a machinegun:D
^^ huh?? no.

It retards ignition so much that the cylinder fires while the exhaust valves are open. This creates excessive amounts of exhaust to keep the turbo spooled up.

if you don't have this, the turbo has much much less exhaust to power it and also when the intake butterfly closes it creates a vacuum. When this vacuum is created, the higher pressure air in the inlet manifold tries to force its way back through the turbo to equalise the pressure. This also slows the turbo down. Wastegates prevent this from happening.
all turbo cars have this problem even if it is stock... I had it intentionally done with my old Subaru when I flicked a switch... whatever exhaust gases will be mixed with some un-burned fuel and you have flames about 3-4 feet long coming out. On my RX-7... it would happen sometimes due to the fuel being too rich.
btw... on Rally Cars... it is because of an anti-lag system, I don't know how it works off the top of my head though... have to do some research
but most steetcars never has it as it would HUGELY decrease the life of your turbo as your turbo would always be spooling at 0BAR when you are off the throttle
ok street cars don't have AL systems as Andyhui01 pointed out your turbo wouldn't last much more than 100 miles before it exploded nevermind damage to your exhaust maifold, cat and muffler.... however alot of older turbo cars will backfire if you switch from a re-circulating blow off valve to a atmosphereic BOV because the computer thinks there is more air and runs extra rich which causes you to get a burst or flame out of your tail pipe or in some cases like as mentioned with the RX-7 you get a sort of blow torch effect... it's actully a sign that your tuning is slightly off... either way it's not super good for your car

my car makes fireballs when i shift
Carb'ed cars don't have very good fuel management, and hence take a bit of unburned fuel through the cylinder and into the exhaust.

If you wind a turbo car up hard, you get traces of fuel into the exhaust when you shift gears (as the large turbo spools down it momentarily pulls some fuel through during the valve overlap). An FQ-400 is a good example.

A lot of turbo cars have blown their exhaust off from what is called detonation. It's similar to pinging, and can blow a cylinder block/head/piston, or the exhaust, depending on how far the unburned fuel gets. Overboosting a turbo can cause this easily (along with pinging).

Oh, and turning your ignition off and back on while driving is really, really bad!!!

When i spent time in Dubai, my mechanic told me of an arab who used to do that for fun, at 100mph, in his new ferrari (348 i believe). He turned the ignition off for a fraction too long, and blew the engine cover and exhaust clean off. He then gave the "wreck" to the mechanic (and probably bought a lambo), who after a couple of thousand pounds had a brand new ferrari!! :)
A misfire isn't exactly the same. It is caused by a plug not igniting the fuel. You can get ignition too early, too late, or not at all in a cylinder. That is a misfire.

A backfire is fuel burned after exiting the cylinder, so isn't exactly a misfire, but can be caused by one.