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Deleted member 12676
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How to maximize RPM/speed
Both the 48cc and 66cc (80cc) engine have around 141 degrees exhaust port open duration which is enough for a maximum RPM of 7000. But these engines only turn up to 5500. Why?
1) ignition timing is too advanced at high RPM. This basically serves as an RPM limiter. Some motocross bikes with digital CDIs stop retarding the timing at about 500 RPM before the max RPM the engineers want the engine to turn at. This causes the timing to be too advanced which causes the peak cylinder pressure to occur too early to do much good at contributing to engine torque.
2) engine compression is too low. All performance engines have at least 130 psi cranking pressure to maximize engine performance. The Grubee engine only has around 90 psi.
3) the cheap carburetor that comes with these engines does a weak job of atomizing the fuel. That causes a slower rate of combustion which is terrible for high RPM power. It is also basically non-adjustable except for the needle height which affects mostly mid range power. Without the ability to adjust the mixture ratio at wide open throttle settings then it is hard to have the mixture ratio optimal for optimal engine power at high RPM.
4) the crank flywheels aren't balanced adequately for the connecting rod and piston weight. The balance holes need to be enlarged with a carbide bit in a drill press. But drilling out the heavy piston wrist pin can be enough to get rid of the worse part of the stock vibration. That, and using a replacement CDI that has ignition timing at high RPM much more retarded than what the stock CDI has.
So if you want a 27% increase in RPM/speed (from 28mph to 36mph) I am here to tell you that it isn't necessary to change the port durations which is a scary task to most people. Just fix these four points and your bike will run stronger, faster, and with less vibration.
Both the 48cc and 66cc (80cc) engine have around 141 degrees exhaust port open duration which is enough for a maximum RPM of 7000. But these engines only turn up to 5500. Why?
1) ignition timing is too advanced at high RPM. This basically serves as an RPM limiter. Some motocross bikes with digital CDIs stop retarding the timing at about 500 RPM before the max RPM the engineers want the engine to turn at. This causes the timing to be too advanced which causes the peak cylinder pressure to occur too early to do much good at contributing to engine torque.
2) engine compression is too low. All performance engines have at least 130 psi cranking pressure to maximize engine performance. The Grubee engine only has around 90 psi.
3) the cheap carburetor that comes with these engines does a weak job of atomizing the fuel. That causes a slower rate of combustion which is terrible for high RPM power. It is also basically non-adjustable except for the needle height which affects mostly mid range power. Without the ability to adjust the mixture ratio at wide open throttle settings then it is hard to have the mixture ratio optimal for optimal engine power at high RPM.
4) the crank flywheels aren't balanced adequately for the connecting rod and piston weight. The balance holes need to be enlarged with a carbide bit in a drill press. But drilling out the heavy piston wrist pin can be enough to get rid of the worse part of the stock vibration. That, and using a replacement CDI that has ignition timing at high RPM much more retarded than what the stock CDI has.
So if you want a 27% increase in RPM/speed (from 28mph to 36mph) I am here to tell you that it isn't necessary to change the port durations which is a scary task to most people. Just fix these four points and your bike will run stronger, faster, and with less vibration.