backfire

[tony9645]

Can someone give me the definative explanation of what causes a car to backfire, had a new twin exhaust fitted last week without a back box, sounds lovely but i get a backfire like a gun going off every few miles any ideas befor the neibhours kill me

Tony

97 GT3

[kmaier]

Well, backfiring occurs due to sufficient quantities of unburned fuel getting into the exhaust system and eventually igniting from the heat or a spark chaser from the cylinder.

In many higher-revving engines, the valve-timing may actually have both intake and exhaust valves slightly open (concurrently) where you finish the exhaust stroke (TDC) and begin the intake stroke. This allows a small vacuum at higher RPMs which gets the fuel mix flowing as the time the intake valve is open is much shorter at higher RPMs and would not allow as much of a mixture in. At lower RPMs you can actually get a bit of fuel thru and it may backfire once in a while.

Another possible source is valve-timing drift from the timing belt stretch or other mechanical wear which exacerbates the above situation. I notice that when I have a cambelt service on the V8 done, backfires are few. As I put mileage on the backfires do get more frequent.

In any case, having no silencer makes it more noticeable as you've already discovered. I'd suggest you check the valve-timing and clearance as a start.

Regards, KM
2000 V8