What causes backfiring?

[Excaliber]

Air pump:
Driven by a belt on the engine it is used to inject fresh air into the exhaust port of the engine head. Usually right near the exhaust valve. The flow of the fresh air into exhaust chamber is controlled (via the computer on new cars, vacuum on old ones) by a valve on the pump that lets the air flow during decelleration.

This fresh air mixes with the spent exhaust gases and helps to insure complete burn of any residual fuel etc. that would normally pass through un-burned. Pollution control device.

Combined with an EGR valve (exhaust gas recirculation) these two items were the most significant pieces of the early smog control methods. The EGR introduces spent exhaust gas BACK INTO the combustion chamber BEFORE igniton takes place. Thus diluting the fresh air/fuel being introduced via the intake valve.

Obvioulsy this diluting of the fresh air/fuel charge negatively impacts performance!! So why do they do it? To lower the "peak combustion temperature" of the "big bang" to control the formation of Nitrous Oxides, which form a BIG part of the "smog" picture.

The timing may also be automatically "retarded" to accomplish the same thing, lower "peak combustion" temperatures. Now the engine (suffering from all this "tinkering") has to push the spent exhaust gases through a restrictive catalytic converter!

Man, now wonder the cars in the 70's lost a LOT of horse power in favor of "clean air". The SAME PRINCIPLES apply today. The engineering to accomplish it is FAR better and does not impact performance "that much".

I prefer paper exhaust gaskets and several "tightening" routines while the "paper" settles in.

Ernie