Im looking at getting a new down pipe but it is 3" bore all the way through, the stock pipe on my 2005 WRX is 2.5" I think. Avoiding buying a new centre section to, I have looked at these 3" to 2.5" flange adaptor kits, heres my silly question...

Do you need an adaptor kit? The flanges will still marry up but there will be a 'step' in the system where the flanges meet and ity steps down to 2.5". Would this be a massive flow disruption and cause any other damage? eg, heat or leaks etc?

Only reason I ask is 0.5" is not really a huge difference, considering it would only be 0.25" each side in the pipe

Any advice?

 

Why not just get a 2.5" downpipe?

Anyway, the adapter won't work as you've not left any space in the total length to allow for it.
Mating 3" to 2.5" won't work. 0.25" really does make all the difference when lining things up.

 

The one Im looking at getting - for a good price - is 3" only. Yes, there are others that have a 2.5" bore out there but they are quite a bit more expensive.

 

Found the answer myself, I will port the 2.5" centre pipe end that meets the down pipe. Similar to porting the headers, this way there will be a smooth reducing angle and not a flat 90 degree step for the gases to overcome.

 

The bore diam AND flanges are different sizes

nick