70cc bicycle engine vs. 50cc bicycle engine

After looking around I've seen that they have a 50cc motor with the spark plug at an angle, which may work better with my drop-down tank. Is there a noticeable difference between the two motors. Physical size or output? Does anyone have both? I'm aware that the actual cc's are generally smaller than advertised.

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The 50cc are supposed to be slightly smoother, and rev quicker, though with less torque. The 70-80cc are really 67or 69cc's so there is not a huge difference in size.

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If it ain't broke, and you mess with it long enough, it will be.

I have both and I preffer the 48cc engine. although the 67cc engine will haul you and a little trailer too.

Could always switch heads if you really feel the need and even grind and polish it up a bit to improve size and power.
That is interesting about the smaller engine being smoother, as well as revving up faster I'd never heard that before.

If it has a shorter stroke and the same bore, it would do that.

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If it ain't broke, and you mess with it long enough, it will be.

second that. mine even has a flat piston instead of a rounded one. different factories I guess.

Ill THIRD that. My 48cc is much smoother and rev's more freely than the 66cc one (aka 80cc). I found power is pretty similar, but the 66cc has a nosehair more torque and a few mph higher speed.

What the other guys said!

I'll add that the 67cc has more potential, and here's why. Seems to me the 49cc units have the same carb, intake and exhaust, etc as the 67cc. 49 cc thrives on this set-up, as the 67cc is limited by it. So the potential is there to get a better exhaust, clean up the intake, ports, etc......and in reality just a little work on the 67cc makes a HUGE difference. Ghost0 is a larger fellow than myself - our bikes have the same gearing - however I have done essentially nothing to my 67cc, while he has tuned his and added a good pipe. He pulls by me at partial throttle like I am standing still!

Nothing beats cc's (if all else is equal )

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I've always heard the only thing better than cubic centimers is cubic meters..................

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Any motor will always spin up faster compared to another if the rotating mass and therefore inertia is lower.

This is the idea behind lightened forged rods, cranks, pistons and etc in racing engines.

The idea being that the less weight induced inertia in a moving mechanical system the less the pumping losses required to overcome it. This means faster revving and smoother power delivery although by itself it doesnt really give a power increase or increase torque more than the difference in inertial and/or pumping losses (being the amount of the total output required to overcome inherant friction and engine compression).

In the case of an engine 'seizing up' what actually happens is that forever reason increased friction (ie inertia) and pumping losses get to a point where the total output of the engine is less than the energy required to move the piston and therefore the engine will not run. This is the reason why a siezed engine will often respond to a sharp tap on the piston with a drift & hammer and free off because the effective one time power of a clout with a hammer is more than the power of the motor and can exceed the inertial moment.

With any engine, specially ones with power ratings such as these, there is very little difference between motors. It depends more on how the available power is put down as to performance. Have a look on the nacc buzzing archive for the Fantic TI motorcycle - it demonstrates the point perfectly...

Jemma xx