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Latest post Wed, Nov 25 2009 12:06 PM by Xea. 0 replies.
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  • Wed, Nov 25 2009 12:06 PM

    • Xea
    • Top 10 Contributor
    • Joined on Tue, Apr 20 2004
    • Posts 5,093

    Pipe physics

    Locked Reply Contact


    Couple of things:

    1) Flow: you want to be able to flow as much exhaust gas as is practical. This means low-restriction.

    2) Scavenging: you want the flow to have as high a velocity as possible so that it properly scavenges the exhaust gases out of the head when the exhaust valve opens. This places a practical limit on the inner diameter of the pipe that can be used with a given engine.

    3) Reversion: when the pressure wave from the exhaust pulse reaches the end of the pipe, it will be reflected. At certain engine speeds at which the whole system (exhaust AND induction) are in the proper resonance, this pressure wave (reflected as a pulse of rarefied air), will reach the head at the precise moment of overlap in which both the exhaust valve and the intake valve are open, producing a partial vacuum that helps to draw air into the cylinder. In general, this will only happen over a narrow rpm range. For a Harley, there will only be one such range on the safe side of redline. At all other engine speeds, a compression pulse blows through to the carburetor and is carbureted once on the way out, and again on the way in, producing a rich stumble and a resultant loss of power at that rpm range. In general, the "sweet spot" where the reversion pulse helps will be well over 5000rpm, so pure straight pipes do NO GOOD AT ALL on street machines.

    Generally, you will want some mechanical device (usually baffles of some sort) to break up the reversion pulse so that the rich stumble goes away. You trade off the high end power to gain low end and midrange since you do not want multiple carburetion of the exhaust pulse when it blows through the carb and gets sucked back in. The way to prevent this is by using baffles of some kind. This "breaking up of the reversion pulse" is often confused with "backpressure."

    Repeat after me, folks: BACKPRESSURE HAS NOTHING TO DO WITH ANYTHING. We are dealing with an acoustic phenomenon here, as well as pure wave mechanics. The purpose of the baffles, besides keeping the bike quiet, is to break up the reversion pulse so that it doesn't screw up your carburetion. Period.

    In general, flow and scavenging should be matched to the capabilities of the induction end of the system, as you have wave mechanical difficulties if one end of the system can flow more than the other. Generally, you should pick the pipe AFTER you've picked the carb, cam, and headwork. Just slapping on a pipe because you like the sound will hurt overall performance rather than help it.

    Ideally, you'd have laminar flow through the system, except for the turbulence you need in the combustion cycle to get a complete burn. By matching the capabilities of induction, head, cam, and exhaust, you will most closely approximate laminar flow (laminar meaning "ordered". As an example, when people line up, they can get through a doorway much faster and more easily than when a crowd tries to push through the door. The lined up crowd is laminar flow; the pushy crowd is turbulent flow).

     Faster, Faster, Faster! Till the trill of speed overcomes the fear of death......

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