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Looking at the following graph you can see that increasing the diffuser length (lessening the cone angle) causes a boost at the top of the powerband with a subsequent reduction of power at the beginning of the powerband. This example was for a 125cc reving to a max of 10,000 RPM but the long pipe gave it 500 RPM more. The mid sized diffuser (blue graph) was a bit erratic but also gave an extension to the powerband. The test diffusers all were of 3 cones and they maintained their effective ratios of area change between cones amongst all three diffusers so differences were not because of a different curve of area changes between one pipe and the others.  |