Oversquare vs Undersquare

A closely related topic to rod ratio is that of bore and stroke. If the bore and stroke dimensions in an engine are the same (say a 4.00 inch bore with a 4.00 inch stroke), the engine is said to be “square.” If the bores are larger than the stroke, the engine is “oversquare,” and if the stroke is longer than the bore diameter it is said to be “undersquare.”

If you divide stroke by bore, you get a numerical value for the stroke/bore ratio. Many production passenger car engines have a stroke/bore ratio between 0.8 to 1.1. Truck stroke/bore ratios are typically higher (1.0 to 1.4) to improve efficiency and low speed torque. The higher the stroke/bore ratio, the less RPM the engine can safely handle, but the more low end torque it will produce.

The 2017 Ford GT 350 has a 5.2L engine with a flat-plane crank that redlines at 8,250 RPM. It has a 3.7-inch (94 mm) bore and 3.66-inch (93 mm) stroke, making it slightly oversquare. By comparison, a C7 Corvette with a 6.2L LT1 engine has a bore and stroke of 4.06 x 3.62, which is quite a bit oversquare, yet it redlines at

6,600 RPM (due to hydraulic lifters). Both are excellent engines with lots of performance potential, but the Ford revs higher because of its overhead cam heads, and makes more horsepower (526 vs 460).

As with rod ratios, the geometric relationship between bore and stroke can also affect an engine’s power and RPM potential. Even so, such generalities often don’t hold true across the spectrum of production engines or engines that are purpose-built for racing.

As a general rule, large bore, short stroke engines are high revving, high power engines good for road racing and circle track applications. Pro Stock racers also like this combination for drag racing as do NASCAR engine builders. Small bore, large stroke engines, on the other hand, are better for low RPM torque, street performance, towing and pulling, but have limited RPM potential.

Formula 1 engines have an extremely short stroke, only 1.566 inches. The bore size is limited to a maximum of 3.858 inches. This is a very oversquare design, but one that allows these engines to rev to an incredible 20,000 RPM and squeeze 800 horsepower out of 2.4 liters of displacement! One of the reasons they are able to rev so high is the extremely short stroke. The pistons are not moving up and down very far in their bores. The stroke/bore ratio is only 0.4, which is less than half that of a typical passenger car engine. At 20,000 RPM, the relative piston speed in a Formula 1 engine is 5,248 feet per minute. Formula 1 engines also use a pneumatic valve system that is far faster than any mechanical valvetrain.

By comparison, a 358 cubic inch NASCAR engine with a 4.185 bore and 3.58-inch stroke (still oversquare, but not as oversquare as a Formula 1 engine) redlines at 10,000 RPM with a piston speed of 5,416 feet per minute.

A 500 cubic inch Pro Stock drag motor may be running a bore size of 4.750 inches with a crank stroke of 3.52 inches. At 10,000 RPM, the piston speed in one of these motors is about the same as a NASCAR engine. If they are running a smaller bore with a longer crank (say 3.75 inches), pistons speeds may be as high as 6,250 feet per minute.

High piston speeds not only increase friction and ring wear inside the engine, it also increases loads on the connecting rods dramatically. Using longer rods with shorter, lighter pistons can help reduce the stress on the rods in these applications.

Determining the best rod ratio and bore/stroke combination for a Pro Stock motor depends a lot on the breathing characteristics of the cylinder heads, intake runners and plenum. Some say shorter rods work best with heads and intake systems that can flow big CFM numbers. Longer rods are better for heads and intake systems that don’t flow as well. The rod ratios that seem to work best in Pro Stock drag racing years ago was around 1.8, but today it’s more in the 1.70 to 1.65 range according to some sources.

There is no magic formula for building a race-winning engine. Rod ratios and stroke/bore ratios can vary quite a bit. Rules that limit maximum engine displacement in certain classes may also restrict maximum bore diameter and stroke length, but within those rules is often some leeway to experiment with different combinations – and that’s the real secret to finding the right combination of parts that will create a truly competitive engine.