Ford Focus ST Forum banner
1 - 6 of 6 Posts

·
Premium Member
Joined
·
1,280 Posts
Discussion Starter · #1 ·
I thought I'd just come out here and ask, as there is no way I'm going to sift through 480-some pages in the exhaust thread! Question regarding weather it's okay to use a 2.5" catback if/when going big turbo. I've yet to throw on my downpipe, and got to wondering... I'm currently using the Mountune catback, and am considering getting the CNT. But then I thought, I plan to go BT next spring. If I do, can I use a 2.5" catback or do I need to go up to a 3"? Would there be an issue doing this? And/or is it a bad idea? Any help would be appreciated.
 

·
Registered
Joined
·
344 Posts
I thought I'd just come out here and ask, as there is no way I'm going to sift through 480-some pages in the exhaust thread! Question regarding weather it's okay to use a 2.5" catback if/when going big turbo. I've yet to throw on my downpipe, and got to wondering... I'm currently using the Mountune catback, and am considering getting the CNT. But then I thought, I plan to go BT next spring. If I do, can I use a 2.5" catback or do I need to go up to a 3"? Would there be an issue doing this? And/or is it a bad idea? Any help would be appreciated.
Instead of speculating an opinion on here, here is my measured experience going from my 2.5inch to 3inch turbo back:

Both exhausts on same setup - GTX2867r stratified tune 24psi 93 oct, 3 inch process west intake, turbo smart 38mm BPV

Original: 3 inch downpipe with high flow 100cell cat, 2.5inch cat back - made 332whp on vdyno

Next: same 3 inch downpipe with high flow 100cell cat, 3 inch cat back - made 349whp on vdyno

Note exact same temps, conditions, road ect. Did 3 pulls and for each and the exact same results. The power was the same up to 5300rpm then the 2.5inch became slightly restrictive.

So that is my experience with the difference, you are leaving some hp on the table between the two setups. Note also that the 3 inch is noticeably louder too.
 

·
Registered
Joined
·
38 Posts
There is a dyno chart on stratified's website that has a BT dyno that is running a 2.5 catback.



Sent from my iPhone using Tapatalk
 

·
Premium Member
Joined
·
1,280 Posts
Discussion Starter · #5 ·
So from the looks of it, a 3" may give a bit more power. I don't see me wanting to go much over the 350hp range, so it appears the 2.5" should be okay. Thanks for the input!
 

·
Registered
Joined
·
344 Posts
Yep the 2.5 inch restricted it by 17whp. For me I would not have gotten to 350whp on pump gas with the 2.5inch. YMMV.

If you run ethanol or WMI then that should be easy enough. It really depends on what 17whp is worth to you. For me a 3 inch cat back for a couple of hundred bucks is a bargain for 17whp extra.
 
1 - 6 of 6 Posts
Top