Ah yep i had this page just sitting there for a while and hadn't refreshed it

your comparing a large capacity low revving high torque diesel engine that needs as much torque as possible at lower rpms to a smaller higher revving efi petrol engine..
look at high performance or race cars, there is next to no back pressure..
so as long as the engine in question has reasonable headers and matching size cat the rest of the system is either or neither imo... having a to small exhaust can hinder the max output of an engine, but within reason a larger than required exhaust isn't going to kill power out put...
I had 2 1/2 and a 3" system on my au i6 and other than the noise difference it made no difference in how the car performed....

Jmm said they gained 4rwkw on a ba going from their 2.5 to a 3 inch exhaust.that was on an unopened engine.

Back pressure is the most evil word ever. It is a by product, not a target.

A choke can be a benefit in a designed exhaust system, but it is far from the simply being "back pressure"
A choke is normally used in a merge collector to speed up the flow through it, and restrict the back flow.


An example I use is if you have your manifold exit in to a pipe 1m in diameter that has a cap on the end with a very small hole to simulate the 'back pressure' of say a 2" pipe. The results would be very different to actually using a 2" pipe.
What this demonstrates is that there's more to it than just size.
Some exhaust builders actually use something similar to this example, called a wave termination box. this lets the header act they way they planned while still being able to attach to an exhaust system.

With the right header, you can have any size exhaust system you want. (as long as it's not a restriction)