maximum RPM/speed

Discussion in 'Performance Mods' started by jaguar, Oct 7, 2014.

  1. jaguar

    jaguar Well-Known Member

    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.

  2. Fabian

    Fabian Well-Known Member

    Does it? and what other carburettors are we comparing it to?

    So you are inferring that the violent mixture motion through the crankcase and transfer ports doesn't play any part in homogenising air and fuel droplets, and the majority of homogenisation happens in the carburettor venturi?

    Exactly how do you come to this conclusion when the NT carburettor (with a removable main jet for air/fuel adjustment) is a virtual copy of the Dellorts SHA.

    As has been discussed many times before, a single cylinder engine cannot be balanced.
    You will find the engine balance factor (as standard) is set for lower rpms, and for good reason.
  3. Zen builder

    Zen builder Member

    Jag, would you drill out the balance holes, and the wrist pin, or one or the other?
  4. Purple Haze

    Purple Haze Active Member

    The problem has always been that the crank is balanced for a 40mm piston setup (49cc). Getting the weight down on the 47mm (66cc) theoretically balances the engine, but as Fabian has stated, no single cylinder engine can be balanced for all rpm. In practice I have found that judicious use of a Dremel to remove material from the inside of the piston, inside of the pin, and grinding and polishing the top half of the rod will make a huge difference in midrange smoothness, without pulling the crank out of the case.