What causes backfiring?

racer135 · Sep 25, 2009, 05:30 PM

I have seen many tuned cars backfire. I understand that at the 2 step some people remove timing and add fuel to increase heat at the exhaust and build up boost while static. That usually makes an engine backfire. But what about full throttle WOT or shift backfire. Is it caused by fuel cut at high shift points? Is it cause by a bad tune (not enough timing?). My car does not backfire but I have seen many tuned cars backfire.

mr-derer2 · Sep 25, 2009, 05:41 PM

im no expert but maybe when the car dumps the boost between shifts there is still fuel spraying for a split sec or left over or whatever and then u get the boost back and then its a bit rich and bang! or the fuel sprays when u start to build boost after the shift and then its rich. I know i get 3ft flames out the back that stay there til i lift. usually only in 5th at WOT. But i run multi-nozzle meth injection so im spraying alot. plus im tuned a bit rich anyway. I just know that when ur rich u backfire.

Jack_of_Trades · Sep 25, 2009, 06:09 PM

It has to do with the cam timing opening during the end of the combustion stroke I believe, allowing the flame front to ignite fuel either in the intake manifold or exhaust depending on where the timing allows. More fuel would make this more pronounced I'd imagine.

AMSevo8man · Sep 25, 2009, 06:11 PM

Its gas or gas fumes in the exhaust igniting. Your air to fuel ratio has a lot to do with whether or not your car backfires.

Jack_of_Trades · Sep 25, 2009, 06:15 PM

Quote:

Originally Posted by AMSevo8man

Its gas or gas fumes in the exhaust igniting. Your air to fuel ratio has a lot to do with whether or not your car backfires.

I've run as rich as 9:1 AFR with no backfiring. It needs to be ignited from the combustion chamber somehow, usually the ignition timing allowing that to happen.

racer135 · Sep 25, 2009, 06:43 PM

The evo stock is pig rich. Mine was in the 9.X AFR. I know since my AEM Wideband only goes doen to 10 AFR. When the car was stock I think it never backfire so it is not only the AFR. I believe it has to be started bad timing. Most likely very low. So that means that a car that backfires will be slower due to bad timing???

Jack_of_Trades · Sep 25, 2009, 07:04 PM

Quote:

Originally Posted by racer135

The evo stock is pig rich. Mine was in the 9.X AFR. I know since my AEM Wideband only goes doen to 10 AFR. When the car was stock I think it never backfire so it is not only the AFR. I believe it has to be started bad timing. Most likely very low. So that means that a car that backfires will be slower due to bad timing???

People get into really low timing during shifts sometimes and they also do it during their stat. rev limit to try and build more boost, which can also cause a few flames to peek out sometimes. You see it a lot more with race cars with the exhaust coming out of the front bumper since the exhaust path is so damn short.

jrohner · Sep 25, 2009, 09:34 PM

Exhaust leaks will also cause backfires. I was getting a big blue flame out the back everytime I let off when my steel turbo-to-manifold gasket blew out.

GaBe511 · Sep 25, 2009, 09:54 PM

^^^ Usually non-restrictive exhausts cause backfiring, like straight through styles

lan_evo_mr9 · Sep 25, 2009, 10:08 PM

Nothing to do with exhausts....I've had stock VW's backfire after doing a fuel injector service or decarbonating the intake/throttle body. Seems like too much "fuel" to the air ratio. A "real" backfire is pretty nasty to hear. They sound very dangerous. You can hear a backfire more in the engine than out the exhaust.

clockworktoy · Sep 25, 2009, 10:11 PM

Fuel cut will result in a big puff of smoke if its harsh enough, bad plugs/wires will also cause you to missfire and backfire at times. Like jack of trades said, it has to do with cam timing and not so much to do with fuel. If you run extremely rich you'll just have some smoke out the back (with a straight through exhaust) But hardly ever flames.

lan_evo_mr9 · Sep 25, 2009, 10:45 PM

Quote:

Originally Posted by clockworktoy

Fuel cut will result in a big puff of smoke if its harsh enough, bad plugs/wires will also cause you to missfire and backfire at times. Like jack of trades said, it has to do with cam timing and not so much to do with fuel. If you run extremely rich you'll just have some smoke out the back (with a straight through exhaust) But hardly ever flames.

So, if you have bad plug wires/ignition, you are dumping fuel, which will cause the backfire. Cam timing does not have a major effect on backfiring, at least on a stock motor.

clockworktoy · Sep 25, 2009, 11:19 PM

Quote:

Originally Posted by lan_evo_mr9

So, if you have bad plug wires/ignition, you are dumping fuel, which will cause the backfire. Cam timing does not have a major effect on backfiring, at least on a stock motor.

The OP asked about "tuned" cars and by that i assumed cars with aftermarket cams.

lan_evo_mr9 · Sep 25, 2009, 11:43 PM

^ True that. Aftermarket cams nor a tune should cause backfiring. That's that. If you have backfiring on a normal basis, something is really wrong.

SoCalRedLine · Sep 26, 2009, 10:40 AM

my car only backfires in these situations:
when i hit fuel cut at WOT
when i hit boost cut at WOT
when I try to launch with the AC on

Ive noticed that I only backfire when the exhaust is already over 1000°F and it runs uberrich 9.5 or more.