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for Racing (for maximum hit although it produces a nasty pre-powerband power dip unless there is a power valve set to obliterate that dip)
Header : flare it out as much as 2 degrees (from centerline) which reduces the diffuser angle to reduce the return suction wave (only recommended if the reed valve is ample) and increase the return baffle wave (the plugging pulse)
Diffuser : make it a single angle cone
Belly : make it 50-100mm long for a strong hit although this is counter to a long powerband
Baffle : make it 100-150mm long (this short increases hit due to the increased angle which makes the return wave stronger) although short reduces the RPM spread of the powerband (which needs to be long enough so you don't fall out of the powerband when you upshift)
Stinger : make it narrow enough to produce 1 to 1.5 psi inside the belly at top RPM (the higher you make it, the stronger the diffuser wave needs to be)

for Street/Trail (for less hit, less pre-powerband power dip, and more RPM spread)
Header : make it parallel walled, not flared
Diffuser : make it a multi-section cone with increasing angles for a strong peak of the return wave
Belly : make it 100-150mm (Long reduces hit but extends powerband)
Baffle : make it 150-300mm (Long reduces hit but extends powerband)
Stinger : make it wide enough to produce 0.5 to 1.0 psi inside the belly at top RPM

For my full explanation of expansion chambers go to Expansion Chamber Theory
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