So I was thinking about ways to get more performance out of my car and came to the question of downpipes. Which would provide the best setup, a downpipe where the pipe inner diameter closely matched the diameter of the turbine wheel or a downpipe with a inner diameter larger than that of the turbine wheel? Wastegate gasses are not a factor.

Can anyone shed some light on this?

 

My turbo has a 2.5" exhaust wheel and exit (4 bolt). It flares to 3" coming right off the flange, so it is a bitch to get the DP bolts tightened, but other than that it is great and it spools a lot faster than the 2.25" DP I had for a while.