I have a theory question. If underdrive pulleys are smaller than the stock ones, wouldn't this make it harder for the engine to turn them? It seems a larger pulley would be easier to turn, sort of like it is easier to turn a wrench with a long handle rather than turning it right at the bolt: ie the larger diameter pulley has a greater torque placed on it (torque=F*d) making it spin more readily. I'm sure I am wrong somewhere in my reasoning, but just not sure where.

 

It all has to do with wieght. The more weight there is, the more power you lose when he engine goes to turn the pulley. Whats harder to get moving....a 5 pound dumbell, or a 100 pound bar bell?

 

i thought it had to with it being smaller its less parasitic drag on the motor to turn them therefore more rwhp?im running a crank pulley myself.

 

The best way to describe it is to look at how a mountain bike works. Most bikes usually have 2 or 3 gears attached to the pedals. When you put the bike in the lower gear (the smaller one), it's easier to turn the pedals. But, the bike chain doesn't move as fast, which means the bike goes slower. This is what an underdrive pulley does on an engine. The crank is going to operate between 700~6,000 RPM's no matter what kinda pulley is attached. So, the smaller pulley is less of a load because it doesn't spin the belt as fast as the bigger stock pulley.

 

You can also explain this by using the moment of inertia. The moment of inertia is smaller for the underdrive since both the radius and also the mass is smaller. Moment of inertia being I = .5MR^2. Basically just gives you so backing for why the bike example works the way it does. Just learned about this in class.