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Discussion Starter · #1 ·
The main rule is that it’s mostly the pipe that sets the top RPM, and that the porting determines the RPM range of power emphasis.
The exceptions to that are
1. when the porting is for too low a RPM range which prevents the engine from reaching the RPM limit set by the pipe. This is more for small engines than large ones.
2. when the blowdown time/area is too little to allow the engine to reach the RPM limit set by the pipe.
3. when the porting is for higher than the pipe limit. This allows for plenty of over-rev.
How to figure the pipes RPM limit: For typical expansion chambers you can use my free spreadsheet calculator downloadable from https://dragonfly75.com/moto/various.html
To read more about porting go to https://dragonfly75.com/moto/blowdown.html
 

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Discussion Starter · #2 ·
Blowdown
Any chart you look at only applies to your engine if it has the same porting arrangement. Here’s some engine types:
1. old style with only one transfer on each side. Single exhaust port, non-bridged.
2. moderate porting with two transfers on each side, single or bridged exhaust port.
3. modern with 3 transfers on each side and a bridged exhaust or a single one with 2 auxiliary ports.
Why does it matter? The effectiveness of any port is a combination of its area and open duration (in degrees). So you can’t just say 30 degrees is good blowdown degrees for a 10K engine of any style. The more open area it has, the quicker the combustion gases leave and so the less blowdown degrees you need. Same goes for the transfers but in a different way. The higher/wider they are, the higher the RPM range is for the powerband influence from the porting.
 
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