Its not "creating power" that wasn't there, just making more of the existing power available for use. For all practical purpose these are the same thing.

If a dyno reads 100hp with one crank, and 100.1hp with a lighter crank, I consider that an "increase" (a tiny one) for my purpose. Although you can play semantics and say "you didn't 'increase' it, it was already there before just stored in the heavier flywheel".

 

Lighter rods can be balanced with a lighter crank, therefore maintaining (and probably even raising) the max rpm.

And yes HP = (torque * RPM)/5250 is how a dyno calculates HP numbers. But the physics behind the actual power that a machine produces is a little more complex than that, and its effected by mass and time.

This is grossly over-simplifiying it but,

Power=Work/Time

Work=Force*Distance

Force=Mass*Acceleration

of course energy transferred and stored in the rotating mass, heat transfer, and so many other things come into play in an internal combustion engine... but we dont really need to look that far into it to see that the mass effects power.

 

I'll agree with that.

 

Yes, and no. It's like the question is the blonde better looking? Depends on the criteria. An engine with a light set of rods will accelerate to speed faster, so it is better in sprint type applications, drag or track racing. Quicker acceleration is rewarded with track position. Heavier rods would give the virtually the same power at a given rpm, all other considerations the same. What changes though could be higher safe operating rpm equaling more power, or a lighter rod could mean stronger pistons for same reciprocating mass. OTOH, rotating and reciprocating mass allows for smooth launch and smoother running, moderating the power pulse of a cylinder firing in a street driven motorcycle. In the end, I would think that light rods in a low rpm, long stroke engine like a Harley, in street application would be an extravagant bench race worthy modification with almost no measurable return.

 

All absolutely correct, but you nailed it the first post citing the laws of physics.

The heavier the rods, the more energy required to overcome their inertia and momentum. That energy is power lost.

 

Another thing to consider is the amount of extra energy needed to accelerate (and decelerate) the heavier pieces which equates to a power loss.

 

How much power an engine makes is dependent on how much fuel and air it takes in. Rotating mass affects how the power is delivered.

 

The energy is not lost, it is stored in the moving mass, just like your bike, the energy is stored in the moving mass and returns in the form of say, brake heat when you stop.

 

The heavier an engine is, the more power it takes to run just itself. This leaves less power to be transfered to the drive train.

A because of this, a heavier engine is more sluggish, and a lighter engine more responsive.

 

If it takes more energy to push one rod up versus another rod, that is more energy being stored in that rod that is not being used to move the bike. In that sense, the energy is lost. Perhaps wasted would be a better choice of words.