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Old 05-17-2013, 11:17 PM   #9 (permalink)
Ryland
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Quote:
Originally Posted by jeff88 View Post
That's what I'm looking for! What is that math? How can I determine what is best based on the variables of a particular vehicle? I know different people have different rules of thumb, but is there an "equation" of some sort to help determine it? I know I can go to a muffler shop and ask an expert, but I wanted to be able to figure it out on my own.
Exhaust shops have experts in bending pipe and welding, they also have experts in selling fancy fart cans, math does not tend to be their strong point.
There are entire books on exhaust design, but for the most part if you find an online exhaust length calculator that will work, I've found a number in the past but the links die after a few years for one reason or another, so it will take some searching on your part, exhaust diameter is the critical part but length is important too most people don't have a choice in the length tho.

Quote:

I'm not sure I get you here. So back pressure vs. momentum. Do I want it as free flowing as possible, minimizing the back pressure as close to 0 as possible?
You are never going to get -0- back pressure... well you could in theory without any exhaust pipe at all, but at that point you get an engine that will hardly run either because the exhaust exits and has no momentum!
If, like I said, you put a pressure gauge on where the o2 sensor screws in, and you look at the pressure, you will have a small amount of pressure from the moving exhaust gases, if you want that pressure to be -0- you'd need to hook a strong vacuum up to the exhaust port at the head, the next best thing you could do is have a proper sized exhaust pipe that carries the pulsed slug of exhaust gas out and creates a slight negative pressure wave as the next pulse exits, because they are a moving wave they will show up on a pressure gauge as a slight "back pressure", but exhaust is moving in pulses not a steady flow like water in a hose.
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