| I've pondered what is more important - less weight in the reciprocating mass, or less weight in the rotational mass. In the interest of making more power with any given turbo setup and by only manipulating the weight distribution of the bottom end assmebly, what is going to be more advantageous? Here's my logic:
The rotational mass directly affects the energy required to make changes in the velocity of the rotation. Reciprocating mass also has this affect as the inertia of the rotating assembly is responsible for accellerating and decellerating the motion of the piston, but the more massive the reciprocating mass is, the more energy it will take to manipulate its speed. An interesting thing came to me when watching how the entire assembly is balanced. Balance is a combination of both rotational masses as well as reciprocating masses and each part plays a specific role of specific quantity. The reciprocating mass is only taken at a fraction of its total mass when balancing the rotational assembly. This is because the rotating assembly is not always 'feeling' the mass of the reciprocating assembly; its a sinusoidal effect and has a factor of 0.5 on the rotating assembly. In this case, it means that the effect of changes in reciprocating mass is only 1/2 of the effect in changes to rotational mass. So to make a difference of 3 lbs of weight change of the crank, you would have to take off 6 lbs of mass on the reciprocating assembly.. In this case there would be no percieved difference on an engine dyno between these two cases.. BUT, the problem comes down to actualities. You are not going to be able to remove a significant amount on the reciprocating mass simply because those parts are already lighter (aluminum piston, most rods are lighter on the small end than on the big end). You are going to be able to make more of a change in the rotational mass of the engine simply by form and it appears that it actually has 2x the effect of changing reciprocating mass.
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