One set of USFS markings coming up:
http://www.google.com/products/cata...=X&ei=RUsPTvKHHYmDtgfE0sj5DQ&ved=0CGwQ8wIwAQ#
Sweet piece of work. 4-strokes can fatten the powerband by manipulating the pressure and sound waves in the exhaust, with little loss of peak power output. Such an exhaust assumes a megaphone shape. Tapering the outlet is a part called a reverse cone that slams the sound waves together, creating an echo that travels backwards up the pipe and arrives at the exhaust valve near the end of the valve overlap period, efectively stopping crossflow from a long overlap cam (necessary for high rpm power) and increasing torque at low rpm. At moderate rpm, the reflected sound arrives after overlap and has no effect. At high rpm, the reflected sound bounces off the exhaust valve just before it opens, creating a low pressure area in the port, aiding scavenging, and restoring most of the high rpm output lost to the restriction of manipulating the sound for low rpm torque. Every Japanese 4-stroke single motorcycle I've ever owned was fitted with such a tuned exhaust. The shorter and fatter the headpipe and megaphone, the higher up the powerband shifts. The longer and skinnier the headpipe and megaphone, the lower the powerband shifts. This is old tech, and was well-understood as far back as the early 1960s.
For instance, my first wife's 100cc flattracker didn't come on the cam until 7,000rpm and pulled to almost 13,000rpm with a straight pipe. This worked great on the 1/3 to 1/2 mile ovals we raced. Rpm of peak power could be adjusted by using different length pipes. Problem was, shorter tracks with sharper corners dropped the bike off the powerband, if it was geared for maximum speed on the straights. Some of the short tracks were little more than two piles of tires 100 feet apart. Switching to a reverse cone megaphone widened the powerband enough to have good pull out of the corners, though slightly less straightaway speed, but an overall quicker lap time.
Yes, a straight pipe will make more peak horsepower, and a good low-restriction mufflet won't have much effect, but a reverse cone megaphone will make a wider powerband, be easier and more laid back to ride, and in any situation except a wind-to-redline drag race, be quicker out of the corners. Different strokes for different folks.
