Is this downpipe doing any good?

[Cool Mr Steve]

Ok, the RRE Downpipe I got starts out about 2.5", then immediately turns into 3" piping, but right at the catalytic converter it necks down to about 2.25" - 2.5", then back to my 2.5" catback. Is the downpipe doing any good, or do I need a bigger cat so it doesn't have to neck down?

 

[Thomas91169]

its gonna have to neck down anyways if youre using a 2.5" catback.

having the neckdown closer to the turbine is better, since the gases will be hotter and less dense and able to make the neckdown transition more easily.

and i always thought using a DP with a Cat still in place is kinda negating itself. youre gonna flow alot better through the DP but its still gonna get caught up in the Cat. but hey, what do i know?

 

[Cool Mr Steve]

I understand that it will have to neck down, and that it would be better to be smaller near the turbo instead... It would be better to have a 3" turbo back and a 50 trim... But with the setup I am stuck with, I just want to know if the downpipe will make a difference, and if that difference will be noticable from a stock downpipe.