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An Aluminum rod functions as a cushion for the crankshaft in a high Horsepower engine, thats why they're still used in Pro Stock engines.
They will also accellerate faster...since there is less weight to accellerate.
You could have two theoretical motors, and they both make the same HP, since the Weight of the rotating assembly shouldnt effect how much power it makes.
However one will accellerate quicker, the one with less rotating mass.
Also the story may change if its a Dyno that measures HP at an Accelleration rate.
The way a water dyno works, at least the one i have used, is that you will apply a load to the engine with the dyno pump, and that will be used to calculate your HP......however the problem is that its not at a set accelleration rate.
Engines will have a rate of accelleration in each gear change, and some of todays smarter dynos you can program that rate into them.
In that case, the Weight of the assembly "MAY" have an effect on the power number.
What should be the concern is the best rod for the application, without knowing the application its hard to specify exactly what i personally would use.
If you are looking to reduce rotating mass, there are many ways to accomplish that. It happens to be one of my current goals, and one of the next "big ticket" items on the list is a different Balancer with an Aluminum hub.
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