The Displacement Game. Fact or Marketing?


Engine displacement is, perhaps, one of the most misunderstood terms when it comes to an engine. Especially when comparing one engine’s power to another engine’s power.

On this web site, we often mention displacement, or make comparisons between our engines and a competing manufacturer’s product. Displacement is important, and can give you a general idea about an engine’s overall power, but it is not the only factor that determines power output.

First off, what is displacement anyway? Quite simply, displacement is the volume of air displaced by the pistons in the cylinder when the engine is operating. The more air displaced, in theory, the more powerful the engine. For those with an interest, the formula for determining displacement is here:

 mbox{displacement} = {piover 4} times mbox{bore}^2 times mbox{stroke} times mbox{number of cylinders}

So why is an engine with a larger displacement automatically more powerful?

In the old days, before the advent of turbochargers, superchargers, and whatnot, displacement was a pretty reliable determinant of power. However, engine power is a product of combustion, and there are many factors that determine combustion.

For instance, our 6.6 litre VGT series is much, much lighter than other engines made by our competitors. However, we produce far more torque and are at a much lighter weight than any other competitor on the market. Why? Others have the same displacement, but do not produce the same power or torque. The answer is that displacement is only a single component of the compression ratio. Our VGT turbocharger can be altered in order to change the amount of air let into the combustion chamber, and at what force.

Likewise, there are many 500 horsepower engines sold on the market for workboats, for instance the Scania DI13-80M. It produces the same 350 mhp as our VGT 350, but at a much lower RPM (1,800 vs 3,500), and produces far more torque. It also weighs over twice the weight of our VGT 350. The Scania engine has a displacement of 12.7 litres, versus our VGT 350’s 6.6 litres displacement. For small tugs requiring lots of bollard pull, the Scania engine is a far better choice than our VGT. Likewise, on a fast RHIB, the VGT 350 is a much better choice.

Over the years, marketing departments have used the quick to define displacement as a way to show high power. Think TV commercials that say something like, “With a whopping 5 litre displacement power!” and so on. It is best to base engine choices on the manufacturer’s rating, rather than simple rules of thumb. As in our example above, if you own a small tug, you want to look for engines with lower RPM and higher displacement. The CAT C7 or C9, with higher displacement, or the Hyundai Seasall offerings.