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Hey guys, who here is running 3 inch piping w/ boost, did you feel a big difference before the 3 inch? Any pics? Thanks!

 
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with a 3" straight through exhaust, i was able to gain about 15 whp over a 2.5" exhaust with a resonator and muffler.

 
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how much hp does a cat take away on stock vs turbo???

 
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depends. You could be losing 2 hp or you could be losing 50.

on my 85 vette i gained almost 35 whp when i cut the cat off. But that was because they put a 2" cat on my dual 2.5" exhaust (bastards!)

 
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what about on d17
non-built engine-stock vs turbo
damn 35 whp with no cat-------------SWEET!!!
vettes aslo SWEEETTTTTTT!!!!!!!!!

 
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you can gain a bit. I have an LX, so i never had a cat to worry about (took it off in order to install the turbo)

 
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Hey Mike, what about the gains of a 2.5" to 3" downpipe?

 
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it depends. If you are running relatively low boost, you wont see much of an improvement.

Im going from a 3" to a 4" downpipe. i will probably gain a few hp. maybe 10

 
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It also depends on what turbo you are running. If you have a t25 you arent gunna wanna put 4" exhaust and downpipes on your car. turboengnr has a sick setup and can benefit from it. I have a t3/t04e and the outlet on the turbo is only 2.5 inches and I am running mandrel bent 2.5 DP and piping with no cat all the way back. Maybe I could go bigger than 2.5 but at 8-10 pounds I dont think thats where I need to focus at the moment. Its all relative to what you are doing. My friend put a 3" DP and exhaust on his turbo hatch and gained almost 20 whp. Also if you take the cat off while you are N/A usually you can expect to lose some mid and low range power in favor off the top end, even more if you have no resonator or muffler.