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What causes a backfire?

Right now, I get a "puff" and a pop when I shift (like a muffled backfire). I'm wondering if, when I upgrade my exhaust, will I backfire constantly on shifts?

I always thought a backfire was gas hitting the hot cats.

Matt's car doesn't backfire much with the test pipe, but Jason's car backfired like a son-of-a-bitch as I recall.

So what causes it?

I do believe thats what causes it, or just fuel being burned outside the combustion chamber.

Any hot condition in the exhaust combined with excess fuel. Severely retarded ignition could do it, but you'd know that. A bad gasket could possibly do it if you were running rich and had a sudden introduction of O2.

Steve

In a fuel injected, computerized, modern system, backfires should be non-existent. Among other things, I would think that any CAT matrix would take unkindly to that sort of abuse. Even if there is extra fuel getting into the exhaust, somehow (valves, stuck injector, etc.), you'd also need some source of oxygen getting into the exhaust, somewhat tricky on a pressurized system. Otherwise, the likely source is not just fuel, but rather fuel/air mix, getting dumped, in large enough spurts to ignite.

Sometimes, sounds can be misleading. Are you sure it's a backfire?

In the day, everything ran rich, and excess fuel was used to cool exhaust valves. At $2.50 a gallon, you don't see a lot of that anymore.

Backfire=bad , afterfire=good , right?

Backfires come back through the TB, don't they?

Anyways ya, I rarely get afterfire but I do sometimes when I'm at wot and either shift or engine brake.

Mine pops a little at the start of loaded engine braking when cold.

Steve

Mine backfires all the time since i pulled the cats. It's probably unburned fuel hitting hot turbine.

Mine pops once in a while between shifts. Mainly 1-2 and 2-3. I've never heard it from 3-4.

I didn't use a gasket for the downpipe, though. I used the BEP as the gasket. So far my car doesn't seem to pop any more often than it did when the old exhaust was on there.

So the pops etc arnt bad?

Does your bov vent atmospherically?

Yep. HKS Super Sequential. I'm going to buy the recirc kit though, I think.

nzKAOSnz wrote:So the pops etc arnt bad?

Nope, they're normal.

probably just runs rich between shifts. Shouldn't be a problem at all.

when I had a DTM on my 91, I had a kind of popping on engine brake.. that was when I'd downshift to a lower gear, and just let the clutch out. I thought it was kinda cool.