On alot of exhaust systems and downpipes they narrow down to 2.5" to fit standard parts.

Will this affect the performance of the system even though it is only 2.5" for about 2" then its back to 3".

 

yes it would affect the flow of gases.for example would be much the same as say,squeezing a garden hose midway.this slows down the flow rate and total volume of water at exit.

 

So as my current blitz nur spec RX has a 2.5" joint at the center to downpipe

when im changing to a new downpipe should i get a 3" one and fit an adaptor so it fits the 2.5" center so that only a tiny portion of the system is 2.5" (about and inch)

As if i get a downpipe that has a tapered end to fit it means more the of system would be 2.5" (about 3inches)

 

anyone