Here's an excerpt from hotbikeweb.com...
http://www.hotbikeweb.com/tech/0805_hbkp_exhaust_systems/buying_considerations.htmlIf header diameter is too large, exhaust gas velocity will be low, thereby weakening the scavenging wave and reducing its effect during valve overlap. As such, it is important to note that as blowdown pressure declines, there is an increased dependency on the exhaust system to scavenge cylinders of spent exhaust gases. Ideally, you want a balance between backpressure and velocity. Headers made of 1-3/4-inch pipe work well with stock and mildly modified V-Twin engines. To maintain the proper backpressure/velocity balance with a street engine, it is suggested not to use 2-inch diameter or larger header pipes unless your engine is at least 100 cubic inches and preferably larger. But be aware that even in the case of a large engine there are tradeoffs, because a 2-inch pipe will bleed off some bottom-end torque for top-end horsepower. For comparison's sake, to optimize high-rpm power with a 120ci to 130ci race-only engine, you should start with a 3-step straight-pipe design having 2-inch, 2-1/8-inch and 2-1/4-inch pipe diameters, and then tune from that baseline.
Larger diameters and shorter length header pipes optimize high-rpm operation, while smaller diameters and longer pipe lengths favor low-end power.
Although a stepped header generates more low-pressure waves than a non-stepped design, its waves are weaker. Steps help maintain a higher average gas velocity over the total length of header pipe. A stepped header won't necessarily make more power than a non-stepped pipe, but it can broaden the engine's torque curve by widening the scavenging wave's effect, which increases the time of negative depression. This can result in a win-win situation: High torque at low rpm while maintaining high horsepower at high rpm. Since the engine views a stepped header as a tapered pipe, the greater the number of steps in a length of pipe, the greater the taper angle and the stronger the pressure waves. But as the pipe angle increases, the rpm band it affects gets narrower. On the otherm hand, a longer pipe with fewer steps results in a narrower angle, thereby widening the working rpm range but lessening the strength of the pressure wave. Stepped headers are most beneficial when used on large-displacement and/or high-rpm engines.
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Topic: head pipes? (Read 10872 times)