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Old 06-01-2012, 06:52 PM   #55
greg hazlett
Focus Enthusiast
 
Join Date: Jan 2012
Fan#: 93486
Location: Chesapeake, VA
What I Drive: 2012 Black Ford Focus SE Hatchback

Posts: 378
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Quote:
Originally Posted by Bwill9886 View Post
Correct on the front one (40842S)
My rear is 40364.



While back pressure is important, I've always thought the bigger concern with exhausts is what I like to call the "flow vs. velocity ratio." Simply put, the best combination of flow and velocity = the best gains. You are able to move the most exhaust gases you can at the highest rate.

Go too big, you have the ability to move a lot of exhaust gases, but not at an optimal rate. Go too small, and you're getting the exhaust gases out more quickly, but at a lower volume than you could.

I bet there are a couple reasons most companies went with 2.25" and I suspect cost, familiarity (other stock, N/A 4 cyl. use that size, why not this?), and potentially higher stock gains play the biggest roles. It's all speculation though until someone asks. I'd love to know and I could very well be proven wrong.

I would, however, bet heavily that in a competition between 2.5" and 2.25" with a header, intake, and a tune that 2.5" will net you more power (whp AND I'll go ahead and bet wtq as well). I of course could be proven wrong and that's fine, but looking at other newer N/A 4 cylinders in similar "situations" tells me otherwise.
I agree with the theory; I have to believe that Ford and the aftermarket companies know what they are doing and that is why they went with the 2.25 diameter tubing; I can't say as I agree that 2.5 tubing would gain any more power as tuning a modern day NA car comes down to timing, a/f ratio and fuel, none of which I think would be affected from a 2.5 exhaust.
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