2.5 downpipe with 3" catback... why?

[idt512]

I see a few of you that run this setup, question is.. what advantage does it give? why run the 3" catback when its choked at 2.5" at the downpipe? which size cat do you run?

 

[MNGSX]

It's not necessarilly choked.

A factory o2 housing is only so big an immediate jump to 3" is pointless.

Also hotter exhaust maintains velocity (read movement) and probably much volume thru a 2.5 inch downpipe as it does thru the tail end of a 3" catback. As the exhaust cools it starts dragging ass and you need a 3".

What I have is 2.75 at the flange tapering to 3" well before the first bend.

 

[Darktalon92]

The hotter a gas is the less dense it is thus it will flow better, in that case right after the 02 housing you can get away with a 2.5" DP since the exhaust gases are still very hot and good flowing...like MNGSX said. Also when a gas cools it expands, so thats why the catback needs to be bigger than the DP so that the exhaust will continue to flow well.
Ever notice how the Apexi N1 exhaust starts at 2.8" and goes to 3.3" at the canister...smart thinking
I actually run the same setup, 2.5" DP, cat and N1 exhaust and plan on wrapping the manifold, 02 housing and most of the DP before the cat with Thermo Wrap to hold in more heat and help it flow better.

 

[leet]

I did it because I was retarded and didn't think to get a 3" downpipe. However, it flows more than enough

 

[igs]

Originally posted by leet
I did it because I was retarded and didn't think to get a 3" downpipe.

That's the only correct answer.

 

[DSMSpyder99]

about 50% of the people on DSMtuners.com complain that they get excessive boost creep on there T-28, 16G, and other similar turbos because 3" Turbo-back exhaust is OVERKILL for those turbos. A full 2.5" T-Back is enough for a moderate size turbo. When i called DSMotorsports when i bought my exhaust they said a 2.5 leading to a full 3 is good up to a 20G/50 Trim(notice i said GOOD not BEST). If you go bigger than a 50 Trim then they said a 2.5" DP will be holding you back.

 

[DSMTurboEclipse]

Isn't 2.5'' good for almost 300hp?

 

[DSMJim]

Originally posted by DSMTurboEclipse
Isn't 2.5'' good for almost 300hp?

We made 425hp on a built B18 with a 60-1 using a 2.5" downpipe. So yeah it's good for whatever HP you want it to be. However going to a larger size reduces lag and increases overall HP. A larger exhaust system is alwasy better even on a small turbo so long as you don't have boost creep or any of those associated problems. I have proven this on the dyno with an almost stock Mazdaspeed Protege back-to-back running a 2.5" then 3" turbo back exahust. The 3" made 4hp extra on a 160hp car.

There was another thread on this topic earlier...

http://www.dsmtuners.com/forums/showthread.php?s=&threadid=100278

 

[idt512]

Originally posted by Darktalon92
Also when a gas cools it expands, so thats why the catback needs to be bigger than the DP so that the exhaust will continue to flow well.

Actually cool air contracts, hot air expands... and that is the reason I am not understanding the whole theory of why a bigger catback than the downpipe