Joined: Mar 2004
Actually it has nothing to do with backpressure but rather the combination of ignition timing and air:fuel ratio. When the coolant is at less than normal operating temp the AFR is richer to prevent stalling, ie. the same effect as a choke in a carburetted engine. When you release the throttle fuel flow is shut off until the engine enters the RPM range controlled by the idle system at which point fuel flow starts and this richer mixture ignites in the exhaust system and you hear the popping. The engine could be tuned to do this all the time by raising the RPM at which fuel flow begins on coastdown, enriching the mixture, and retarding the timing. However this would increase emissions substantially with no benefit other than the amusement factor so I doubt any tuner would do it.