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Discussion Starter #1
Essentials to know first:
The peak power and length of powerband are mostly controlled by the design of the expansion chamber.
The engine porting determines whether high RPM or mid range RPM is where the engine harmonizes the most for the most power (relatively speaking).
The carb size favors high RPM power or mid range power. Bigger favors high RPM.
Ignition timing typically is retarded for high RPM power and advanced for mid range power.

What can be done:
1) You can lower the top RPM by lengthening the header or belly. How much? I’d start with 3 inches (8.5cm) and see how you like it. The amount of RPM per inch differs with top RPM and engine port durations. Use 1mm thick sheet metal (not galvanized).
2) You can lower the amount of pipe “hit” and increase the over-rev by lengthening the belly.
3) You can lower the hit with a longer baffle cone (the rear cone) with a less steep angle. This also increases the RPM range of the powerband. Also it reduces the loss of power right before getting “on the pipe”. Of course you can install a homemade boost bottle to completely correct that power dip by using a good boost battle calculator to tune its size to resonate right at that same RPM range of the power dip.
4) Lowering the powerband means the power also lowers and may need some help with a steeper angled diffuser cone so a stronger return suction wave increases the amount of air/fuel into the crankcase thru the reed valve. The cone angle can be increased by making a straight header section to replace the flared header. The flared header is only advantageous to high RPM engines. Also you can increase the engine compression but first you need to use a squish velocity calculator to know how much metal you can shave off the head before the squish velocity starts to be excessive (which can cause detonation). Actually it is more beneficial to shave metal off the cylinder base since that will also lower the ports a bit for smaller open durations (which favor mid range power). You can use a good porting calculator to do virtual changes on the computer before doing them in real life.

To read an in depth study of how expansion chambers aid engine power go to: http://dragonfly75.com/motorbike/ECtheory.html

Why fiddle with such projects in uncertain times?
Because fiddling is a good hobby and a good distraction from the nonsense of the world. Some of my best times ever was had with just me, my motorcycle, and good music.
Don’t worry, just do what makes you happy.
 
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