# 2.5 vs. 3.0?



## dustyminpin (Jun 19, 2006)

I've got LS3 heads, Fast LSXR intake, custom Comp cam 228/232, .595 lift, 114+2 LSA, Kooks signature series headers with Kooks catted connecting pipes, and Borla exhaust. The Borla is 2.5" and has a DEEP rumbling sound, like bass in a car, which I like, but I've heard some GTOs at cruise-ins with a stock motor and Spintech exhaust or Loudmouth exhaust (1 or 2, whichever is louder) and you can barely hear my car idle next to these things. I love the DEEP throaty sound of the Borla, but was thinking about moving up to a 3 inch system since the Kooks headers to go a 3" collecting pipe anyways. Any pros or cons going bigger all the way down? I was thinking about Kooks Street Screamer new 3" cat back system. Has anybody heard what this sounds like? I will miss my 5" Borla tips, but if it sounds better and more importantly gives me more HP, then I'll make the switch. All questions, comments, and threats are appreciated.


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## 6QTS11OZ (Mar 27, 2005)

Too large of a diameter might hurt your horsepower and torque numbers more than help it. Plus you may lose some torque in the process. Exhaust backpressure is sometimes missunderstood. Sure you'd like to have less of it as possible but you still need to maintain a certain amount exhaust velocity to be efficient. If sound is what you're really looking for I say play around with the mufflers first.


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## svede1212 (Nov 1, 2005)

You really don't want ANY back pressure. The issue is velocity. When you go too big the gasses don't move fast enough and the gas cools down, becomes denser and goes even slower down the pipe. That "backs" things up. You want the largest size that doesn't slow down the velocity. Higher displacement and higher RPM warrant bigger pipes.

If it's all about sound then none of the above matters.


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## 6QTS11OZ (Mar 27, 2005)

Some info from Magnaflow;

http://www.magnaflow.com/07techtips/faq/question10.asp



> MagnaFlow's Frequently Asked Questions
> 
> Question: Why ISN'T a bigger pipe always better?
> 
> ...


Based on your hp/tq output a 3" exhaust should be fine for a dual exhaust.


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## jpalamar (Jul 22, 2008)

I think 3" would work best with a big ass n/a build or FI. I think 2.5 is more then enough for 90% of the cars on the road.


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## goat400 (Aug 18, 2009)

when i bought my 2.5" spintech website said 3" is for 500+ horse.


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## 87GN06GTO07IRL (Aug 10, 2009)

Your fine with a dual 2.5 at your power level. 3 inch will be beneficial if your 600+


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