What Is Reversion

Ever heard the term 'reversion' when talking about your motorcycle? Shawn will talk you through what reversion is, why it matters, and how we deal with here at Rinehart.
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[@FFL-vg9ro]

What you are talking about - Exhaust Gas Reversion - is very rare, except in race engines with lots of valve overlap. What is far more common, especially in V-twins, is Pressure Wave Reversion. When the exhaust valve opens, a High Pressure Sound Wave travels down the exhaust pipe (at the speed of sound). In an open straight pipe, when that sound wave hits the open end, a Negative Pressure Wave reflects back up the pipe and when that pressure wave hits the exhaust port, it seriously interferes with efficient cylinder scavenging. That’s why short straight pipes on a Harley absolutely kill your low and mid-range torque. And also why the stock exhaust with baffles, which breaks up the initial pressure wave, and block any reflected return wave, have very good low and mid-range torque.
Take a close look at two-stroke expansion chamber exhaust design , and you will see how critical this sound wave management is. In the case of a two-stroke, the returning sound wave pushes the escaping fresh charge back up the exhaust port and packs the fresh charge into the cylinder - like a reverse turbo charger. But based on the length of the pipe, dime tio s of the straight section, and the angles of the cones, the pipe is tumed to a very narrow RPM range (based on the speed of sound). That’s where the expression “coming on the pipe” originates. When the two hits that target RPM range, the power hits like a freight train.