Originally Posted by Boostin'Xr4

The internal combustion engine doesn't need backpressure. Backpressure is detrimental to performance in every way. What it needs is exhaust pulse pairing for reversion and scavenging, as well as high gas velocity.
Lowering the exhaust temperature by installing a pipe larger in diameter than the engine can efficiently fill will cause the exhaust velocity and temperature to drop, making the gas more dense, thus becoming harder to push.