Perrin cat back 2.5" or 3"

[DAN_BRZ]

IMO you would be good with either choice, i personally own the perrin 3" catback with 3" front pipe and 2.5" overpipe. the sound is by no means loud, but it is louder then stock. compared to a greddy evo, invidia it is quiet. the exhaust note sounds mean and aggressive, zero rasp, but not deafening. i have ppl telling me my car sounds amazing at local meets and just cruisin on the road at stop lights. u can hold a conversation at hwy speeds without screaming. ive heard the 2.5" resonated and 2.5" unresonated. unresonated sounds awful, raspy as hell, and recommend against buy it. the resonated sounds nice, not much louder then stock, but doesnt compare to the exhaust note of my 3".

whether u choose the 3" or 2.5" resonated ull be happy, if one makes a few more hp then the other ur butt dyno or ur foot isnt going to notice a difference.

[My_First_Subaru]

Thanks everyone for their input

[Efenys]

I got a 3" and have no regrets. FI may/may not be in my future in a few years after i see some longevity tests. I got it because the HP increases are negligible and i like the 3" deep sound a bit better.

After all "You buy a catback for the performance right???"

[reardrv]

Originally Posted by industrial

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.

Exhaust gasses do expand as they cool, which will cause a loss in velocity. Why do you think people wrap/ceramic coat headers and down pipes? In my example a better way to phrase it might be that they cool as they expand, also causing a reduction in velocity. And velocity does still matter past the headers, just not nearly as much.

[Mobius357]

Gases contract when they cool, not expand, given no change in pressure. PV=nRT is idealized but Volume and Temperature still vary directly and not inversely.

[reardrv]

Remember exhaust gasses aren't stagnant in a chamber, they are rapidly expanding as they travel through an exhaust. And what happens when gasses expand, a temperature drop. Just like when you crack the valve on a tank of compressed air, the air will come out cold, sometimes even producing small ice chunks. Inversely when you compress a gas, like in forced induction applications, it gets warmer.

If the gasses expand too rapidly, they cool too rapidly and lose velocity, increasing back pressure. This is why an exhaust that is too big, especially in the beginning can hurt performance.

However since you can not stop the gas from expanding and cooling, you do want to give it more room as it travels down the exhaust.

A good example would be a high end motorcycle exhaust. The best performing exhausts from company's like Akrapovic, Arrow, or Yoshimura have very carefully selected pipe diameters that taper out as they move towards the end. The old school Kerkers and Vance & Hines exhausts use the same principal, which is one of the reasons they made big power in those early bikes.

Now on our specific car, I would say that despite a 2.0L displacement due to the specific output of the motor there may be a benefit in going 3". Remember exhaust volume is a factor too. And the volume produced by our 200hp, high revving, high compression motor will be greater than that of most other cars of similar displacement.

Personally I think that the jury is still out on whether a 3" exhaust helps or not. If we are talking a forced induction application, 3" all day long. Maybe bigger if you want a giant turbo, huge horsepower setup. But in my opinion if you plan on future upgrades, why buy a 2.5" only to buy a 3" when you do finally go FI?

[My_First_Subaru]

Thanks for input. I'm thinking of going 3". Although there is that thought in the possibility in a loss in case I do not fi. Lol. I'm a pretty indecisive person. Money wise it's a small difference between the two but If I do get 2.5 then fi it would be a bother to have to get another exhaust.

[industrial]

Originally Posted by reardrv

Remember exhaust gasses aren't stagnant in a chamber, they are rapidly expanding as they travel through an exhaust. And what happens when gasses expand, a temperature drop. Just like when you crack the valve on a tank of compressed air, the air will come out cold, sometimes even producing small ice chunks. Inversely when you compress a gas, like in forced induction applications, it gets warmer.

If the gasses expand too rapidly, they cool too rapidly and lose velocity, increasing back pressure. This is why an exhaust that is too big, especially in the beginning can hurt performance.

However since you can not stop the gas from expanding and cooling, you do want to give it more room as it travels down the exhaust.

A good example would be a high end motorcycle exhaust. The best performing exhausts from company's like Akrapovic, Arrow, or Yoshimura have very carefully selected pipe diameters that taper out as they move towards the end. The old school Kerkers and Vance & Hines exhausts use the same principal, which is one of the reasons they made big power in those early bikes.

Now on our specific car, I would say that despite a 2.0L displacement due to the specific output of the motor there may be a benefit in going 3". Remember exhaust volume is a factor too. And the volume produced by our 200hp, high revving, high compression motor will be greater than that of most other cars of similar displacement.

Personally I think that the jury is still out on whether a 3" exhaust helps or not. If we are talking a forced induction application, 3" all day long. Maybe bigger if you want a giant turbo, huge horsepower setup. But in my opinion if you plan on future upgrades, why buy a 2.5" only to buy a 3" when you do finally go FI?

Too large of an exhaust could ruin exhaust velocities because they act as expansion chambers. The exhaust is cooled from expanding rapidly. This is a concern with a NA car because of the exhaust scavenging effect. This is why you may lose some power going too large on a NA setup. Backpressure is probably more of a function of the cats and merge angles more than anything with our car.