Gordon Jennings Pipe Calculation Formulas
Here's my assessment of the formulas proposed by Gordon Jennings way back in the early 70's. They were just beginning to investigate back then and so were starting from scratch.
Jennings main shortcomings;
0. He assumes the same temperature and sound wave speed inside all pipes.
1. Because of his temperature error his calculations for the tuned length were off by 3.5" compared to a Maico 440 pipe, and 1.75" compared to a Maico 250 pipe. (both pipe specs along with full engine data, horsepower data, and EGT were given to me for analysis.)
2. He uses formulas to calculate everything but belly length, letting it take up the remaining distance. This is totally wrong because the belly length should be set to allow a slight overlap of the diffuser and baffle waves. That set length is optimally almost always below 4" (100mm).
3. He had no idea whatsoever about the return waves the exhaust pulse causes, their return time or their time duration. His ideology was not based on the reality of what was happening inside the pipe but just on scant trial and error data.
4. He calculates the header length as a multiple of its diameter. It's correct length is that which locates the secondary waves in correct relation to exhaust closing time at top RPM.
5. He calculates the belly diameter as a multiple of the header diameter although the header can be too narrow or too wide. Proper sizing depends on the exhaust pulse pressure.
6. He calculates the stinger diameter as a fraction of the header diameter whereas it should be calculated by the exhaust pulse pressure.
7. He calculates the stinger length as a multiple of its diameter whereas in reality the same back pressure can be had by a fat long stinger as by a skinny short stinger.
8. He recommends not going over a combined baffle angle of 30 degrees (15 from centerline) although many successful pipes use much larger angles. According to physics there is no limit to the angle. It's just that the shorter the baffle, the shorter its return wave. The return wave length should match the RPM, the timing between transfers closing and exhaust closing, and the desired RPM spread of the power band. And its length is dependent on the length of the exhaust pulse.
9. He was calculating for a single coned diffuser which gives weak results compared to a multiconed diffuser of gradually increasing angles.
10. He calculated for a 2 to 1 ratio of angles between the baffle and diffuser which is too limiting. Any way when you go to a multiconed diffuser there is no single angle of overwhelming importance to compare the baffle angle to. So in modern pipes there is no ratio formula.
Well he was a pioneer 46 years ago but anyone who uses his formulas now is way behind the times. We have progressed so much farther than his formulas and now have computers to simplify the complex process. (But thank you Gordon for helping along pipe research in the early days. RIP)
Click here for a free download of a pipe calculator based on his formulas. Click here for an extensive read about 2 stroke expansion chambers.
