View Full Version : Does Size really matter?

09-02-2005, 11:19 AM
I went down to the shop close to my house to talk to the guy who's going to be making the exhaust system for my car. I told him that i wanted 2 1/2 in with a new cat with the borla muffler and tip for now. he told me that would be fine and that it would only cost 100-150 for everything (except the muffler). Then i came home and did some looking around and found an article from a fellow subaru owner who said that you'll actually loose power if you go with 2 1/2 or bigger and to either stay stock with a hi-flow muffler, or to go with 2 1/4. What do you guys think about this?

09-02-2005, 11:46 AM
Well I have a custom 2 1/2 in resonator-chamber back exhaust with a preformance muffler.

If anything I noticed an increase.

I dont know if it changes with the headers and cat....

I plan to buy some headers and a hi-flow cat and get my exhause done again.

Also I would think that if you have the engine power to match your exhaust, you won't have a problem...

09-02-2005, 03:24 PM

09-02-2005, 05:05 PM
Size does matter!

With our N/A vehicle you want a bit of back pressure in order to keep your low end torque. 2.5" would be just about the largest you want on a N/A engine. You do gain some top end, but loose low end torque

In not sure with turbo vehicle, but I think bigger is better. need to get that exaust out to spool the turbo faster

09-02-2005, 06:32 PM
i agree with lee....

with the n/a everyone seems to think that 2.5 is the biggest you should absolutely go...i'm running 2.25 and it works great for me. i really wanted to keep my low end...

as for turbo models, the bigger the better. the faster you can get that exhaust out, the faster you can spool up and the faster you can go...

09-02-2005, 07:48 PM
+1 for Jess and Lee

2.25 is really the way to go if you want to keep low end. considering you're going from 1 7/8 ----> 2 1/4, you are gaining high end, but still keeping backpressure. 2.25" is the way to go, IMO

09-03-2005, 12:31 AM
If you have your stock cat, I would imagine 2.5 would be fine. The cat is what gives you back pressure, but if you go to a high flow cat, then 2.25 would be the most I'd do.

09-03-2005, 03:25 AM
is it a mandrel bent or crush bent system?

09-03-2005, 09:46 AM
or press bent?

09-03-2005, 02:42 PM
i think its mandrel bent. The man has been in the business for about 20 years making header's and exhausts for his race car. He used to rip up and down my road with the race car, it was sweet :D . He said it would make sense to go with a hi-flow cat and had one for $50 so i might just have him throw that on. Since i dont have a turbo id think the 2.25 would be best. Its just suprising to me that if 2 1/2 actually makes u loose hp, then why is most performance exhausts that size? Maybe the expect you to run your engine higher then 3500 rpm all the time? :?

09-03-2005, 02:45 PM
more combustion = more exhaust

when you run at a higher RPM all the time (racing) to get the most power, there will be good exhaust flow out of a larger pipe, but when you daily drive your car, you generally won't go above 4k most of the time and the smaller diameter will allow exhaust to flow more smoothly. 2.25" mandrel bent would be a good idea for you.

With 2.5" you don't lose HP in the higher RPMs (you will gain HP) but you do in the lower ends (well, you lose torque) because the exhaust isn't going to flow as smoothly out the tailpipe.

gator gt
09-03-2005, 09:02 PM
but when you daily drive your car, you generally won't go above 4k most of the time

Well, until recently, I'd disagree with this. :lol: With gas prices skyrocketing, I'm trying to keep it away from the redline as much.

....but the higher RPMs are sweet sweet melodies.....I WANT MORE!!

Anyway, I'm going to agree with the N/A = 2.5 & lower and F/I = bigger the better.


09-04-2005, 01:12 AM
unless you have equal length headers, youre not doing yourself a favor with 2.5 piping. may sound cool, dont get me wrong... but sadly, subaru used 2.25 piping for a reason...

do some research on scavenging effects...

like gator said, 2.5+ exhaust piping should only be used in a forced induction setup where the object is to lose backpressure to free up a few more ponies.