In this writeup I am comparing aluminum with gray cast iron, common material of engine blocks.

Getting back to science, the problem comes from the relative spring rates (moduli of elasticity) of the metals involved. In relative terms, aluminum is a 10, gray cast iron is a 20, and nodular iron and compacted graphite iron are 30. So it is easy to see that using aluminum would give some problems with stiffness.

HOWEVER, another factor jumps in here and that is how stress applies to bending. We call it different things, but flexing or bending is a way that power is lost in an engine block. Stress is what produces strain, or motion. Stress in a beam in bending is proportional to the moment (bending force) and proportional to the distance from the neutral axis. It is inversely proportional to the moment of inertia. Moment of inertia almost always involves a dimension to the fourth power. So if, for example, you have a main bearing web 1" thick, and you increase it to 2" thick. the distance from the neutral axis increases by factor of two, while moment of inertia increases by factor of 16, 2 to the 4th power (2x2x2x2). You have a 2 in the numerator and 4 2s in the denominator so the increase in resistance to bending is actually increased by a factor of 8, 2 to the 3rd power, or 2x2x2.

If you followed me this far and think it through, it means that to increase the thickness of a beam to compensate for the aluminum extra springiness, you don't have to increase the dimension by 2, you have to increase it by factor of cube root of 2, which is 1.26. So your 1" thick main bearing bulkhead is now 1.26" thick and equally as stiff, and if you want to make it stiffer it doesn't have to be much thicker. Another 0.24" and it is now 1.69 times as stiff as the 1" cast iron.

This only applies in bending, not in straight tensile or compressive stresses. But bending or flexing adds friction and reduces ring seal so it is more important.

R.