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Using the painstakingly accurate method of determining the crankcase compression ratio I outline at http://www.dragonfly75.com/motorbike/PCR.html I discovered that my reed valved 55cc has a ratio of 1.2 and that a meager ratio of 1.11 is existent with my 48cc piston port engine. This ratio is the ratio of compression of intake charge before its released into the cylinder thru the transfer ports. A higher ratio indicates there is more pressure at the time of transfer opening. 1.2 is for low rpm engines and up to 1.5 for high rpm engines. The reasoning is that at low rpm you don't want the intake charge blasting (via high pressure) into the cylinder too fast and looping around and exiting the exhaust port. But at high rpm you do want a fast transfer since there's so little time for the movement of charge to take place. A ratio of 1.1 is around 1/2 the pressure that 1.2 produces and is too low.
So how can we increase the ratio? By reducing the space that contains the intake charge being compressed. This space includes the space between the two halves of the flywheel, the space under the piston, and the space in the transfer channels. To reduce the space between the flywheel halves we'd have to take apart the flywheel and bolt on aluminum spacers. Too much work for me baby! JB Weld can be used in the other two areas but increasing the piston weight would probably imbalance the crank and cause more vibration unless you drill out the center of the piston pin to counteract. JB Weld can be safely put along the walls of the transfer channels since the channel cross sectional volume is much more than the area of the transfer ports when opened.
What effect can this have? It can increase the delivery ratio of intake charge into the cylinder which increases power throughout the whole rpm range.
Since the total volume that can be added to the transfer channels is small and I don't want to add flywheel spacers, then I'm going to also add JBWeld under the piston and let you know how it goes. (I am well aware that temperature is pretty high under the piston crown which may disconnect the JBWeld there, leaving the side adhesion in effect.) This isn't new by the way. I have read of others doing this. Here's a graph showing the relationship of compression ratio to psi pressure:
So how can we increase the ratio? By reducing the space that contains the intake charge being compressed. This space includes the space between the two halves of the flywheel, the space under the piston, and the space in the transfer channels. To reduce the space between the flywheel halves we'd have to take apart the flywheel and bolt on aluminum spacers. Too much work for me baby! JB Weld can be used in the other two areas but increasing the piston weight would probably imbalance the crank and cause more vibration unless you drill out the center of the piston pin to counteract. JB Weld can be safely put along the walls of the transfer channels since the channel cross sectional volume is much more than the area of the transfer ports when opened.
What effect can this have? It can increase the delivery ratio of intake charge into the cylinder which increases power throughout the whole rpm range.
Since the total volume that can be added to the transfer channels is small and I don't want to add flywheel spacers, then I'm going to also add JBWeld under the piston and let you know how it goes. (I am well aware that temperature is pretty high under the piston crown which may disconnect the JBWeld there, leaving the side adhesion in effect.) This isn't new by the way. I have read of others doing this. Here's a graph showing the relationship of compression ratio to psi pressure:
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