[Neutrino]

if i may bring my 2 cents:

From my research I have come to the conclusion that the backpresure advice is a myth. What does matter is the energy and of course the directly related speed of the exhaust gases.

Best scenario: the gases will exit very fast and hot creating a low pressure area behind. This will create a scavenging effect for the remaining gases in the combustion chamber allowing for a cleaner burn next cycle.

Now the problem is a small exhaust will keep the gases hot and fast but will be restrictive and create unwanted backpressure. On the other hand a dumping the gases in a large exhaust will not encounter as much backpressure however once gases meet the larger enclosure will expand and lose energy and ofcourse speed.

Therefore a balance needs to be struck to reach the optimal compromise. This of course its further complicated by the fact that this compromise will be change based on RPM and load. So you end up with effects such a loss of low end tq when using large exhausts systems .

One thing one can do when switching to a larger piping is: have it ceramic coated and/or thermally warped. This will limit thermal dissipation compensating a bit for the energy loss during expansion that I've mentioned earlier.