Changing the weigh of the flywheel?

[Chris-PA]

also if a modded saw is turning 1000 rpm more that is a lot more air probably more air than needed for cooling the saw that you opened the muffler optimized squish and timing on all of which can help with operating tempatures.

That's a good point. I think the fan drag is non-linear - I had one of those fan trainer stands for my bicycle years ago, and you could not spin it faster than a certain speed.

When I built my 42cc Poulan I noticed the older flywheels had fewer fins, so I used one of them hoping that it would have less resistance.

 

[OldJack]

Power needed for a centrifugal fan = RPM cubed.
http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html

 

[Rx7man]

Power needed for a centrifugal fan = RPM cubed.
http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html

I didn't realize it was cubed, but I could totally *imagine* it is at least squared... but that leads us to another side-effect of higher RPM..

Not only is the parasitic loss of the flywheel moving the air increased dramatically, but the AIRFLOW increases greatly.. so despite the fact you're making more power and heat, you're also moving a great deal more air to cool with when you bump the RPMs higher.

A little bit of speculation on my part, but removing fins at high RPM may not hurt airflow very much at all... flywheel fins are also a restriction in the airflow, and I think at high velocities it becomes more apparent. The job of the fins is to 'spin' the air, the fanning action comes from the centrifugal force this generates.

 

[xxl]

Just throwing this out here didn't stihl go to a plastic flywheel on the 660? Becuae it was sheering the cranks?

 

[Rx7man]

I think it also has a lot to do with the torque curve of the engine... on a radical engine which probably has squat for bottom end torque means that with a heavy flywheel, it's going to have to chug for some time to get up to 11K+RPM and be happy. A lighter flywheel will reduce that time, making it more responsive.