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Old 12-29-2002, 04:41 PM
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Post exhaust sizing

How would one go about determing what size exhaust pipe is required? I have found engine calculators online that will calculate carb cfm requirements, but have not found one for exhaust sizing. Any help would be appreciated.

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Old 12-29-2002, 07:35 PM
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calculators do squat. talk to someone who builds racecars or an expert in the engine building field. give them all the info on your motor, and what you are wanting from it. they should be able to help. sometimes the only way to find out what is best is through trial and error.
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Old 12-29-2002, 08:15 PM
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This topic has been covered in depth several times on this board. Do a search and you will get your question answered.

http://www.hotrodders.com/cgibin/ubb/ultimatebb.cgi?ubb=get_topic&f=15&t=000789

[ December 29, 2002: Message edited by: willys36@aol.com ]</p>
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Old 12-30-2002, 04:37 AM
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I had read those threads previously, but they don't really answer the question that I'm asking. How would one go about determining what size exhaust pipe is required? What is the determining factor to use 2.5" pipe over a 2" pipe? I know the 2.5" pipe will flow more, but how do I determine how much exhaust flow is actually needed? Whether a person is using a stock manifold or header, what determines the pipe size the rest of the way out of the vehicle?

If a motor requires, lets say, a 600-650cfm carb. I would assume that the exhaust would have to flow the same 600-650cfm. How do you calculate the estimated cfm of a pipe?
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Old 12-30-2002, 06:00 AM
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The pipe diameter that corresponds to the circumference of the exhaust port is a good place to start. This assumes you start with a head that will service the piston size, cam, and RPM range you designed into the engine. Thus a 350cu in street engine with stock or mild performance heads will probably use 1.5" or 1.625" tubing whereas a full race 454 will need 1.75" or 2". Pressure loss due to friction will be minimal but velocity will be high enough for the headers to work properly. For performance headers on naturally aspirated engines it is better to be a little small in the range than too big. If you are running a blower, tuned headers are less important so size can be a little larger. Understand that most commercial headers are too short, too big dia., and unequal length so probably work worse than stock manifolds.

It sounds like you are interested in digging into the theory a little deeper than normal so I recommend you get a copy of paper No. 80-DGP-6, "Short Pipe Manifold Design for Four-Stroke engines: Part II". from ASME, 345 E 47 St., New York, NY, 10017. This is a very good summary of intake and exhaust manifold design. Authors recommend velocity of 200fps at design rpm as optimal. He cites several commercial products such as Edelbrock and Chrysler cross-ram manifolds as well as some exhaust headers that comply with this standard.

[ December 30, 2002: Message edited by: willys36@aol.com ]</p>
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Old 12-30-2002, 09:32 AM
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One of the magazines a while back did a series of test on this. I'll dig thru and see if I can find the article. I remember what they did find out was that on the particular engine (a slightly modifies 350) ran better with smaller tubes vs. bigger tubes. This was on the headers. The conclusion was that the engine needed a certain amount of back pressure. I know this doesn't answer your question but I'll see if I can find the article.

Kevin
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Old 12-30-2002, 11:53 AM
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Kevin; I know it is a common notion in the hot rod literature that 'back pressure' is the benefit of running small tube headers but that would fly in the face of all the facts. On second thought, that could be said of much of what is said in those publications. Back pressure is bad for performance, period. The benefit they see is from the high velocity of the gasses in the tubes. Designers must balance the desire for high energy, high velocity flow vs. the detrimental effects of friction losses (back pressure) from the higher velocities. As mentioned above, empirical and theoretical studies have determined that 200fpm is the optimal speed. Obviously, this speed can be achieved at only one rpm in a given engine. Below that speed, 'back pressure' will be low but at the expense of cylinder scavenging. Above that rpm, perormance will suffer mostly from high friction losses.
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