What causes backfiring?

[speed220mph]

Here's my $0.02 worth:

The air pump, aka "smog pump", has been replaced by scavenging systems, averting the hp loss from driving the pump. After-burning is not needed on a Cobra, unless emissions stds. must be met.

With either system, air must be diverted from the exhaust to prevent backfires . . . if it didn't divert, then bang, whenever the throttle is lifted. Fuel would keep going for an instant out the exhaust after the throttle plate closed, but air stopped abruptly, allowing a slug of fuel to enter the exhaust port looking for oxygen to react with while still hot and go boom.

Leaks near the head will do the same thing as well as big vacuum intake-manifold leaks. At the exhaust, not only does exhaust leak out, air will leak in and cause a similar backfire.

As for leak prevention at the header/exhaust manifold-to-head interface, or where the suckers bolt up and seal, it depends on head and manifold materials. For headers, or tubes with header plates, and cast-iron heads, I prefer soft gaskets, 12-pt. bolts drilled and safety wired. Copper gaskets would work ok, too.

For aluminum heads, go with soft gaskets. Harder gaskets such as copper will seal, but will Brinell the softer aluminum head, making it tougher to get a good seal once the headers are R&R'd for any reason.

If you're using the more rigid cast-iron manifolds against cast-iron heads, rtv silicone sealer works fine high temp or not. A Winston Cup engine builder friend uses only rtv sealer at the header flange with success.