Perrin cat back 2.5" or 3"

[My_First_Subaru]

Hello everyone. For the past few weeks ie been trying to decide on whether or not I should get the 2.5 or 3. I'm not very good with cars so bear with me lol. I am possibly open to get sc sometime in the future but If I don't and stay NA will the 3 be useless or will it lower HP and tq. I mainly am looking at the 3 just for the fact that I may possibly sc. Thanks. Also I hear there are slight gains in the 3 but have not seen a dyno chart. Also the 2.5 is on amazon and this quarter I get 5 percent cash back on my chase card.

[Kiske]

3" no if your going to stay NA for a quite a while longer.

Also note that 3" catback won't do much good if the other parts of the exhaust aren't 3" as well.

[My_First_Subaru]

So most likely it would be a waste to get the 3 until the future if I decide to sc

[coolaron]

2.5" for NA
3" for FI

[My_First_Subaru]

But then I prolly won't decide till wayy later lol. So I figure If I get the 3 I won't have to worry about getting a new catback and I could just use that then change the overpipe etc.

[boredom.is.me]

Even if you get the 2.5", you won't "need" the 3" when/if you go FI.

[My_First_Subaru]

Oh ok thanks. So I can run sc on a 2.5" if I decide to. I'm assuming better airflow would be better

[boredom.is.me]

It probably would be better with a larger exhaust if you are really boosting, but many FI makers test their products on the stock exhaust. I know Innovative and AVO to name a couple.

[merlin2111]

I am having the same issue.

[Turdinator]

I haven't seen a dyno yet that shows a 3" losing power or torque with NA setup. Once you go FI a 3" will make more power even though its not "needed". If it were me, I would go the 3" to save buying two exhausts. Provided it wasn't too loud on an NA setup of course.

[reardrv]

I have not seen any evidence in either direction for our car saying what size is the best. A few company's do say there is a slight benefit to going with a 3", but I'll believe it when I see some dyno info. But why not prepare for the future? I've had multiple turbocharged cars and the do certainly benefit from 3" piping.

Before anyone says "well if the piping doesn't all match, you wont see any benefit". This is false. Exhaust gasses cool and expand as they travel through your exhaust system, so an increase in piping diameter as the exhaust moves to the rear can be beneficial.

And if anyone is about to say you need a certain amount of back pressure, stop yourselves right now and do some research. You do not want any back pressure. The reason using an over sized exhaust can hurt horsepower is due to excessive cooling of exhaust gasses, causing the gasses to slow. This will lead to more back pressure. Exhaust velocity and exhaust scavenging is very important to producing power.

Going on engine displacement alone, or NA vs FI is ignorant. A 2.0l ABA VW is certainly going to see negative results from a 3" exhaust, however that's not a 200hp, 12.5:1 compression, high revving performance engine.

Until I see some real numbers, I wont buy into the speculation. However since I would like to go with forced induction in the future, I will personally be going with a 3" exhaust when the time comes. No need to buy an exhaust twice.

[industrial]

Only drawbacks to 3 inch exhaust is noise, weight, cost and clearances. Those are pretty significant if you ask me. Only you can decide if that's worth a few extra hp.

I'm either building a NA motor or going FI and I'm pretty sure I'll be just fine with a 2.5 inch exhaust.

Also, gases don't expand on their own when they cool. If they do, you lose velocity. Not that exhaust velocities really matter beyond the header in our cars anyway.

[Kiske]

If your just going cat back it will do no good to go 3" as the front and mid pipe will still be smaller. I don't think perrin has a 3" mid pipe due to clearance issues.

Just going catless or with a high-flow cat will give you more improvements than a catback.

[DAN_BRZ]

Originally Posted by Kiske

If your just going cat back it will do no good to go 3" as the front and mid pipe will still be smaller. I don't think perrin has a 3" mid pipe due to clearance issues.

Just going catless or with a high-flow cat will give you more improvements than a catback.

your wrong perrin does make a 3" front pipe, but does not make a 3" overpipe

http://perrinperformance.com/i-13324...-fr-s-brz.html