I have been working on a small problem for a little while, and I so happen to find this forum. I was wondering if anyone can point me to the right direction with this. I am trying to calculate very accurately how much the rod/piston assembly in an engine will weigh at a given RPM. I have been studying ARP's (Automotive Racing Products) equations from their website, but the numbers do not sound correct at all ( 228,742 kilo's @ 1000RPM? ). So I am kind of skeptical about their math or the way I am doing it. this is the equation and graph My thoughts about this is that the piston and rod are not changing direction instantly, but the rod slowly changes direction in about a 180 degree rotation of the crankshaft, but the change peaks at the bottom and top of the strokes. So I believe there has to be a much more detailed math to describe this. I have an excel sheet i made that has all the work already there. So if anyone could take a look at it and steer me in the right direction with this, that would be cool.