## ROD RATIO - Frictional Internal Energy

There are many ways to improve an engine’s performance, such as: changing displacement, upgrading intake/exhaust, etc. – These are among the more obvious ways to do so.

If you have already leveraged these means, but wish to pursue even greater performance gains, one option is to change the Rod Ratio.

The “Rod Ratio” is the ratio of the connecting rod’s C-to-C to the stroke, as in:

Rod Ratio = Rod C-to-C / Stroke

Most OEM models have a rod ratio somewhere between 1.5:1 and 1.8:1. Aftermarket models can have different rod ratios depending on the application.

Consider this scenario: We have a crankshaft with an 80mm stroke, and the connecting rods matching it have a C-to-C of 150mm & 180mm respectively. What will be the difference between these two rods when they are paired with the same pistons and operating under the same RPM, displacecment, & stroke?

We can take a look at this diagram:

We can see that when the crankshaft moves from 0° to 90°, the distance which the piston moves is not equal to half of the stroke, but larger than half of the stroke. We can also see that the swing angles are different for each rod. The rod with a 150mm C-to-C has a swing angle of 15°, whereas the rod with a 180mm C-to-C has a swing angle of 13°.

The swing angle directly affects friction between the piston skirt and the cylinder wall. We can demonstrate this with a formula.

Fn = tanβ*M

Fn = positive pressure of piston skirt

β =  swing angle of connecting rod

M = gravity on the piston

f = μ*Fn

f = frictional force

μ = friction coefficient of cylinder wall

Fn = positive pressure of piston skirt

L = movement distance of piston

The larger the swing angle β, the greater the positive pressure Fn of the piston skirt on the cylinder wall. If the friction coefficient μ remains constant, the frictional force f = μ*Fn will also increase.

This frictional force f will constantly affect all four strokes of the piston. This can cause serious wear on the cylinder wall.

The friction also raises temperatures inside the cylinder, which raises energy loss to heat, thus reducing energy available for mechanical use.

Therefore, it is critical to choose the correct connecting rod ratio according to the actual working conditions of the engine application.