It sorta makes sense to me. If you stayed at the cylinder diameter of a .21 motor, you would have to increase the stroke to increase torque. But, when you increase the diameter of the cylinder to the size of .26 motor, you have more force pushing on the piston, creating more torque.

I've never heard that you bore out a motor to increase the RPM. IMHO, you would decrease the stroke to increase RPM.

Doesn't HP = RPM * Torque.

I am NOT an engine guru by any means, this is just my thoughts on this.

Matt