KDX Pipe Design


Any pipe design needs to be so that its powerband is centered around the RPM range the engine uses from one upshift to the next. The following info is derived from the Excel file "KDX gear ratios" at http://www.visualize.co.nz/kdx200/

With 13/47 sprockets here are the mph/RPM thru the 6 gears. In 1st gear the bike goes 18mph at 6600 RPM and 25mph at 9000 RPM. (The SR is the same except for a lower 1st gear.)

RPM  6600 7000 7600 9000
gear .       MPH     .    RPM
1st  18   19   21   25 (1000-9000)
2nd  25   26   28   34 (6600-9000)
3rd  32   34   37   44 (7000-9000)
4th  40   42   46   54 (7300-9000)
5th  47   50   54   65 (7600-9000)
6th  57   60   65   77 (7600-9000)

The last column shows the beginning RPM if you shifted up from the lower gear at 9000rpm. So the engine powerband needs to be from 6600 to 9000 in 2nd gear, 7000 to 9000 in 3rd gear, 7300 to 9000 in 4th gear, and 7600 to 9000 in 5th and 6th gear. Normally only 3rd gear and up are considered for pipe design so a 2000RPM spread is needed. My exhaust pipe calculations using my Excel file "ECcalc" show a normal expansion chamber (with 10 degree baffle) causes the powerband to focus on high RPM power instead of more even power throughout the powerband. (The stock KDX port durations are 185 degrees for the exhaust and 123 degrees for the transfers.)  Please read all my pages about expansion chambers to understand this topic of pipe design. (warning: not for the weak minded) The new pipe I've designed will have the dimensions shown in the drawing below. The blue pipe is the internal stinger which greatly reduces exhaust noise without reducing power.

Look at the graphs below to see how the pipe power is evenly spread out over its whole powerband whereas the same pipe with a 10 degrees baffle creates a rather peaky powerband. (These aren't graphs of engine power but only of the additional power the pipe imparts to the engine.)




How does the powervalve figure into this?The KIPS valve opens up gradually from 6000 to 7000 RPM to increase exhaust duration by 27 degrees (from 158 to 185) which causes the pipe powerband to extend 900 RPM lower. 

 
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