Best Rod Ratio?

Essentially, there is no “best” rod ratio for any given engine. Some say to use the longest rods that will fit the engine to make the most mid-range and peak RPM power while others say it doesn’t really matter. Smokey Yunick was one of the early proponents of long rods, and they worked well for him in NASCAR. Even so, some engines that have lower rod ratios will out-perform engines of the same displacement that have higher rod ratios. How can this be? Because of differences in the design and porting of the cylinder heads, different valve sizes and valve angles, different camshaft lift and duration, different intake systems and different tuning.

For example, a BMW M3 has a rod ratio of 1.48, which doesn’t sound very good based on the number alone. But the M3 engine also makes 2.4 horsepower per cubic inch (with the help of a turbo), which is nearly twice the power ratio of a typical street performance Chevy 350 or small block Ford. The point here is not that turbos make lots of power (they do), but that rod ratios don’t really affect performance one way or the other very much.

Some people put way too much emphasis on rod ratios and worry excessively about how their engine’s rod ratio will affect performance. Our take on the issue is that rod ratio is just a number that may or may not make much difference depending on the situation. In some cases, it can make a slight difference and in others it seems to make no significant difference whatsoever. Peak horsepower and torque depend on too many other variables.

The maximum achievable rod ratio is always going to be limited by the physical dimensions of the block (deck height, tall or short), the longest rods that are available to fit the engine (off-the-shelf mass produced rods or custom made), and the shortest pistons that will work with the rod, block and stroke combination. The combined weight of the rod and piston has more effect on momentum and throttle response than the rod ratio. Also, moving the wrist pin higher up in the piston and using a shorter piston may create some piston wobble and instability issues if you go too far. Because of this, excessive rod ratio may actually be detrimental to engine performance.

These relatively long and skinny Ford rods (4.6L left and 5.2L GT 350 right) have a rod ratio of 1.68, which is a little less than a 350 Chevy with 5.7-inch rods, but a little more than a Chevy 350 built with longer 6-inch rods.