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I just got a custom 2.5" cat-back with an Arospeed muffler (have to update my sig) and when I had it installed the guy said that the muffler had a 2.25 inch inlet. I just wanted to get it on so I had him expand the inlet.

I was wondering how much hp, if any I was losing off this. I know it's not much but I'm kinda anal about this stuff. Thanks.
 

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It depends on the inner tube diameter. Some mufflers and resonators use inner tubes larger than the inlet, outlet diameter, basically using the same muffler and just tacking on different inlet and outlet sizes on. If your whole system is 2.5" and your exhaust is the only thing 2.25", then you shouldn't notice anything. If on the other hand you had 2.25" piping with a 2.5" exhaust you would notice a difference because resistance is a function of diameter and length. The good news is that our 2.5L V6 doesn't need more than the stock 2.125" diameter piping it came with, so long as you stay NA. The real bottlenecks in the exhaust are the chambered stock muffler and the 2" stock cat. Go with a larger cat, larger resonator, and straight through muffler, and you won't even need to touch the piping.
 
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