Backfire is an explosion produced by a running internal combustion engine that occurs in the intake or exhaust system rather than inside the combustion chamber.
An explosion in the inlet manifold, carburetor/throttle body or air cleaner of an internal combustion engine will typically occur when the intake valves are not shut prior to fuel combustion.
The same term is used when unburned fuel or hydrocarbons are ignited somewhere in the exhaust system.
A visible flame may momentarily shoot out of the exhaust pipe.
Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio.
Backfiring can occur in carbureted engines that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced.
As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion.
The condition that causes this is a misfire.
The result of a misfire or incomplete combustion is that unburned fuel or flammable hydrocarbons are delivered to the exhaust manifold where it may ignite unpredictably.
Another backfire situation occurs when the engine is running rich (with excess fuel) and there is incomplete combustion during the Otto cycle, with similar results.
- http://en.wikipedia.org/wiki/Back-fire
Just did some Wiki reading for ya!
So it can be common for Turbo Charged cars, in your case, your R32 GTR having Twin Turbos.
My car sometimes gives a few "Pops" and "Pongs" with my high flow cat and standard catback exhaust.