I was trying to decide on what diamater to use to build my exhaust, and I have a question. I'm going to be using an OBX header which has a 2.5" mouth at the cat end, then there is the cat (which I have yet to measure), and then the exhaust. Would using a 3" exhaust be pointless if it was a 2.5" mouth from the header?

t.i.a.,
Jake

 

you can actually have a muffler shop make you a piece to connect it. they just take a piece of 2.5" pipe and flange the end to 3". it's almost the same as going from the stock header to a 2.5" exhaust, right? (assuming the stock exhaust is 2.25")

 

I don't know if the following is correct, but it sounded good to me. Consider your system in it's entirety and how much exhaust gas (think water if gas is too hard to visualize) it can hold for a given length of pipe. The length of the pipe is determined by how far it is from the header to the point you want to release the exhaust gasses and in conjunction with how much back pressure you need. So the best bet would be to have the maximum volumn in that distance without sacraficing the back pressure. So if the exhaust piping squeezes down to a smaller diameter for a short bit, it is only going to affect that area and overall just a little. The gasses will accellerate and decellerate going into and out of that sqeezed areas respectively, creating a vortex of sorts. It is sure better than having the entire length squeezed down to that smaller size. Like I said I don't know if this is correct, but it sounded good to me when explained that way. Please, anyone, correct me if I am wrong.

 

Thats makes sense to me, but the problem is how much back pressure is needed? This is my first F/I car, and I know that a lot of turbo guys run test pipes, with little or no muffler, but I don't know if the same "biggest exhaust possible" theory applies to a S/C car.

Also, would adding a 3" pipe to a 2.5" header really allow you to push any more gas? If I keep my mouth mostly closed and blow out of it, it doesnt matter what size tube Im blowing into because the amount I can blow is regulated by how wide I have my mouth, not how big of a tube im blowing into (unless its smaller).

I have a feeling that using a 3" exhaust with a 2.5" header would just be creating a different exhaust tone, and not really increasing the flow over a 2.5" exhaust.

 

Ok, what you are asking has almost no affect on the power, since the diameter difference between the header and exhaust is too small. Going with 3" diameter exhaust will provide you a louder system, which is fine. It is a common practice, nothing wrong with that.

Berk

 

You may weaken the scavenging in the header slightly but you may gain the difference at WOT. But you aren't using a WOT header (4 into 1) So if your not looking for louder & deeper tone ,stay smaller & enjoy your across the rev. range torque gain.