10 Things You Always Wanted to Know About Engines

This article is copied from the September 1999 issue of Hot Rod Magazine

By Ray T. Bohacz

    It is our opinion that stroke/rod ratio (SRR) needs to be examined with a definite goal in mind. As the length of the connecting rod increases in relation to the stroke, the ratio becomes greater. The lower the ratio, the greater the angularity of the connecting rod, which increases thrust-side bore wear and internal friction. Since engine airflow is scaled against piston velocity, the poorer SRR allows for greater instantaneous piston velocity and rates of piston acceleration, thus allowing the engine to pump more air and increase VE.

    A numerically high SRR decreases angularity, is easier on bores, and requires less force to rotate the crankshaft, thus allowing for more power to be produced from the same amount of fuel. Another benefit of a high SRR is that the piston dwells at TDC longer, keeping the combustion region smaller for more of the crankshaft’s rotation. This helps to quench detonation and allows for higher cylinder pressures before the piston begins to move.

    With this in mind, we cannot make a blanket statement about the stroke/rod ratio proposition. It is generally accepted that a SRR of 1.6:1 is a good compromise. For longevity with added octane tolerance, the higher SRR is the way to go. If you are not concerned with 100,000-mile life cycles and your cylinder head has large valves and ports, the lower numerical SRR may be your ticket for increased pumping action. So it’s either less friction and longer squish time or additional wear and friction for more airflow. You decide.