You may have noticed that there is a lot of online debate in car/truck enthusiast groups about whose engine is better. You buddy might be bragging about their powerful oversquare engine or how their V8 is so much better than your inline engine. Want to come to your next online debate armed with the facts? Read on!

Shape:

When describing an engine’s shape, Stroke-to-Bore or Bore/Stroke Ratio (ratio of the diameter of cylinder bore [inside of engine cylinder] to the distance piston head must travel from the top of the cylinder to the bottom [piston stroke]) is commonly referenced.

An engine is described as oversquare or short-stroke if its cylinders have a greater bore diameter than its stroke length, giving a bore/stroke ratio greater than 1:1. Oversquare engines are very common, because they allow for higher RPM (revolutions per minute), which generates more power. Examples include both Chevrolet and Ford small-block V8s and Duramax diesel engines.

On the other hand, undersquare or long-stroke engines have a smaller cylinder bore (width, diameter) than its stroke (length of piston travel), giving a ratio of less than 1:1. At a given engine speed, a longer stroke increases engine friction, since the piston travels a greater distance per stroke. It also increases stress on the crankshaft due to the higher peak piston acceleration. The smaller bore also reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. Because these factors favor lower engine speeds, undersquare engines are most often tuned to develop peak torque at relatively low speeds. Many inline engines utilize an undersquare design, because the smaller bore allows for a shorter engine. An example of an undersquare engine would be the Cummins ISB series of diesel engines.



Configuration:

There are three common types of engine configurations: in-line, V, and flat. For engines with in-line configurations, the cylinders are arranged in one row. In a V configuration, cylinders are arranged in two rows that are at an angle to one another. When you look at it from the side, they appear to form a V. Flat engines can also be known as a boxer or called horizontally opposed. In this configuration, the cylinders are positioned parallel to the ground and in two banks on either side of the engine.

There is no inherently “better” type of engine configuration. If you built an engine in each configuration with the exact same specifications (displacement, same intake, exhaust system, etc.), they would perform similarly. Car manufactures choose the type of engine that goes in a vehicle for a variety of reasons. Often they consider manufacturing cost, space available, power-to-weight ratio, engine cooling, and other factors. For example, an in-line engine can be mounted transversely (engine's crankshaft axis is perpendicular to the long axis of the vehicle) so that the car can have a shorter hood. However, an in-line engine can be harder to cool if it is air-cooled.

So, what does this all mean?

Ultimately, there is no hard and fast rule as far as which shape or configuration is better. Engine performance is based on a variety of factors that cannot be handled in sweeping generalities. It really depends on what you want your car or truck to do.

 

Written by:

Beckie Bean

Digital Content Coordinator at Country Truck & Auto

December 8, 2016

Sources: How Stuff WorksAchates PowerCar GurusCummins Hub

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