06-01-2012, 07:52 PM
Join Date: Jan 2012
Location: Chesapeake, VA
What I Drive: 2012 Black Ford Focus SE Hatchback
FF Reputation: 3
Buy-Sell-Trade Rating: (3
Originally Posted by Bwill9886
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.