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Old 01-24-2003, 02:26 PM
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Post Question about exhaust

I have heard some engines need backpressure... so with a duel exhaust on a smallblock 400 would gong 3 inch pipes be too big??? would 2 3/4 inch pipes be better? Would it still rumble okay?

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Old 01-24-2003, 02:55 PM
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the object of exhaust is to let the burned gas get out of the engine, back preasure will slow down the flow, cause heat and restrick the amount of fresh fuel that can be drawn into the cyl. NO BACK PREASURE ON AN AMERICAN HOT ROD, pipe should be as big as carb, if it cantget out you cant put it in.
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Old 01-24-2003, 03:08 PM
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Bigger is not always better, you got to detrmine what is best for your engine, cam size and running RPM, like an intake the exhaust relys on velocity to help get the best effeincy, small pipes will give greater velocity which would cause, the exhaust to excape faster and act like a siphone to suck out more gas, big pipes, pressure may not build up and some gas will go slowly through the pipes causeing back pressure, the gas may also bounce off the walls, its all a mix for the aplication, like the runners on a intake manifold, u need a mix for your aplication,
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Old 01-24-2003, 04:24 PM
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unless your running TUNED HEADERS the bigger the better only tuned headers have a siphon effect other wise it has to free flow.
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Old 01-24-2003, 04:40 PM
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Toy mobile in that case y wouldnt all cars have 4 inch exhaust, why do race headers have big primary's, and stock engines have small primary's, i think its cause there is more velocity in small pipes, so it flows out faster wiht less back pressure, but there is less flow if the pipes are to small so you half to find a mix
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Old 01-24-2003, 04:47 PM
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okay so for a fiarly good street 400 then would 2and 3/4 inch pipe be good enough??? I know going too big just leads the thing to run like crap in certain rpm ranges
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Old 01-24-2003, 04:49 PM
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Small block 400? Right? I don't think any small block no matter the cid should go over 2 1/2 as I have said many times before. Just get a free flowing muffler and some good headers. Steve
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Old 01-24-2003, 04:54 PM
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hot roders be en doing it for years detroit got the message in 1998 started using 3in exhaust from front to rear on trucks, and then we have NASCAR, not that those guys know any thing about horse power, but they run 4in exhaust with out tuned headers, which are illegal buy the way , but i guess thats why they sell 1in pipe for more veloicty, had to say that.
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Old 01-24-2003, 05:16 PM
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Toy mobile you are right nascar uses big pipe's, nascar engines run at 6,500 RPM, at that speed you can get velocity out of a 4 inch pipe, the reason is the ammount of exhaust, if they use a 5.7 liter engine, and lets say they get the piston filled up almost 1/3 the way with gas each revolution. Lets do the math so it deplaces 1.9 liter of air and fuel each other revoution, so lets say it does the 6,500 RPM, that means each second the engine does 108 revolutions per second, so that engine acts like a pump half the time so that is 54 revutions per second, lets multiply that buy 1.9 liters which is what it pumps out. So every second it pumps 102 liters or air out the exhaust, thats 6156 liters of air and burnt fuel a minute, that is so much that nascar can use 4 inch exhaust and still get velocity out of it.
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Old 01-24-2003, 05:29 PM
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you threw a bunch of no. about NASCAR, what about detroit, do you think they would use 3in exhaustifthe trucks didnt needit, they count .005% for each one so rest asured if the engneers didnt think it was nessary the trucks, that have to run in all rpm rangrswouldnt have 3in pipe,cat,muffler, tail, all the way.
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Old 01-24-2003, 05:43 PM
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YAh trucks use 3 inch SINGLE exhaust, a have yet to come buy a stock truck with daul 3 inch pipes
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Old 01-24-2003, 08:03 PM
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Stereo you ain't been to my town
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