Why change the header pipe? The Harley Dyna is 48 cubic inches per cylinder. Our Savage is 40. 20% difference. Bigger is not better in exhausts. The rule is that for more power at high rpms (and give up power at lower rpms) one wants a bigger diameter and shorter length pipe. For more power at lower rpms (and give up power at higher rpms) one wants a skinnier, longer pipe. We need to keep in mind that we're not dealing with steady flow; we're dealing with pulsating hot gas.
"The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s."
http://ssheaders.com/header.htm"The most important part of a header's design is to get the tube diameter selected correctly - whether the engine is used in a daily driver situation or used for all out racing - because that diameter establishes the gas velocity inside of the header which then establishes the primary scavenging characteristic of the header. If the header tube diameter is selected correctly, the engine then produces a noticeable increase of power over a wide rpm band which makes correct size selection almost more beneficial (almost more important) for a street car than it is for a a race car. If the header tube diameter is too big, the top end power might go up a a little but the bottom end and mid-range power will actually be reduced by a far larger amount. (We've seen engines experience a 5-10% loss in power in the mid-range to get less than a 1% gain on top end just by oversizing a header by ONE tube size!) "
http://headersbyed.com/xfaq.htmMore:
http://burnsstainless.com/TechArticles/Theory/theory.html Design of multicylinder exhaust is more complex than a single cylinder system, but the principles are exactly the same. Try the bigger pipe. If you feel a loss of low end power, the pipe is too big in diameter. If you feel a gain in power
in the range you want, you've done good. Do your best to identify a gain in noise level from a gain in real power.
Turning the idle mixture screw counterclockwise richens the mixture. With the engine fully warmed, turn the screw in clockwise until the rpms drop. Note this point (turn it all the way in to the stop and back out to this point counting the turns so you know where you are.) Turn the screw counterclockwise until the rpms drop again, counting the turns. Repeat this a couple of times to find the points that give these actions. Go halfway between the two points, ride, and fine-tune the screw setting to get the response you like. Some folks feel a difference in 1/8th turn changes. I don't. It takes more change for me to feel a difference. Let us know what your engine likes--and there is no one setting that is just right for all engines.
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