What causes backfiring?

[THUB]

To add a question to the discussion, Why do Evo's just coast down in gear quietly while some cars (SRT4's come to mind) burble and pop the entire time?

[3gturbo]

Originally Posted by SoCalRedLine

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.

I was hitting fuel cut at 2500 rpm's in second gear lol. You should definately fix that. BTW 2.4's own.

[clockworktoy]

Originally Posted by THUB

To add a question to the discussion, Why do Evo's just coast down in gear quietly while some cars (SRT4's come to mind) burble and pop the entire time?

Exhaust systems and tune come to mind. Im not too familiar as to why SRT4's do that (but i know what your talking about, i hear it all the time) But if you retard your timing in the load cels your in while your coasting you can make your car grunt POP (although i don't recommend it for a DD its kinda fun to try out).

[Fast_Freddie]

If you are referring to cars that run the 1/4 and backfire or "pop" between shifts, it is retarded cam timing and a rich fuel mixture that is tuned to keep the turbo spooling during shifts so you get better boost response after shifts, instead of using the NLTS feature.

[GaBe511]

Backfires typically are caused by unburned gas in the exhaust system, mainly the headers, being ignited by the amount of heat in the exhaust.

Thats Why SoCalRedLine said he noticed it when his EGT is 1000F

Since most tuned cars run higher amounts of fuel, its more common in those cars.

Thats why i said exhaust. Plus the less restrictive the exhaust, the more pronounced it will be.

[Jack_of_Trades]

Originally Posted by GaBe511

Thats Why SoCalRedLine said he noticed it when his EGT is 1000F

1000*F is nothing on a turbo engine. Most of use exceed 1500*F on a pull. If that was the case, we'd all have backfire's constantly. I agree about the backfire between shifts with the 1/4 mile cars though. Insanely retarded cam timing combined with very rich mixtures is definitely a cause for that.

[StreetEvo]

GaBe511 is on the right track. When you start to mess with exhaust systems, i.e. less restrictive, better flowing, the scavenging effect becomes more pronounced. In most cars there is a period of valve overlap, both valve sets remaining open at the same time. Because of this, a portion of the unburnt air/fuel charge is being sucked into the exhaust system. The popping you hear is that portion of the air/fuel charge igniting, either in the exhaust manifold or the turbo/downpipe area. SRT4's have a factory straight pipe exhaust system, so they have an unusually high scavenge effect.

[Spoolin' By U]

Backfires can be amplified by running an unrestricted exhaust system, which does not
utilize a catalytic converter and has a high flowing silencer and or muffler. Since there
is almost always some unburnt fuel present in the combustion chambers after a WOT
condition. This exhaust stroke byproduct then makes its way out via the piping and
will spontaneously combust from the heat and pressure resulting in a mild explosion.

[thermaldynamics]

I experienced a lean pop between shifts during acceleration ~1/2 throttle and at WOT. I have cams and open exhaust. My problem was completely eliminated when I increased the values of the ECT vs acceleration enrichment table. The lean pop didnt happen until the car was close to operating temp. The backfire did not help with spool at all infact the car fell flat on its face during and a split second after the pop then would pick up on acceleration again.

Thanks to all who helped find the real accel enrichment values! The car is 100% better in daily driving.

[skochur]

what if your car backfires when you let off gas for a couple of seconds and then get on gas again? only happens once in a while though...