How is RC Engine Size Measured?

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How is RC Engine Size Measured?
A reader asks, "how do you determine the cc of an engine if it is measured in so many different ways?" The confusion comes in the way the engine size is expressed by different RC manufacturers. Some might use something like 2.5cc or 4.4cc while others use a number like .15 or .27. How do these numbers compare to each other?

Answer: RC engine size or displacement is measured in cubic centimeters (cc) or cubic inches (ci). In terms of RC engines, displacement is the volume of space a piston travels through during a single stroke. A larger number, whether expressed in cubic centimenters or cubic inches, denotes a larger engine. Displacement is only one factor that determines performance of the vehicle.
The best way to determine the displacement for a specific engine and vehicle is to view the detailed specs for that engine which should list the displacement in either cubic centimeters or cubic inches (or both). However, if you don't have the specs handy for a specific engine, you can often figure out the approximate displacement based on the name, as explained below.

Typical RC Engine Displacements
Common RC engine displacements range from about .12 to .46 and larger. These numbers that begin with a decimal point are the displacement in cubic inches. Sometimes the abbreviation ci is appended to the measurement. But just remember that a .18 engine is actually .18ci or .18 cubic inches of displacement.
That same .12 to .46 range, expressed in cubic centimeters would be approximately 1.97cc to 7.5cc of displacement. You can use an online conversion tool to quickly convert from cc to ci or ci to cc. Here's a small reference list (cc is rounded) to give you an idea of how cubic inches compare to cubic centimeters:

.12ci - 2.0cc
.15ci - 2.5cc
.18ci - 3.0cc
.21ci - 3.5cc
.25ci - 4.1cc
.27ci - 4.4cc
.32ci - 5.2cc
.36ci - 5.9cc
.46ci - 7.5cc

Determining Size by Numbers in a Name
Studying the manufacturers specifications is the best way to determine engine size, but manufacturers will often include a number in the name of the vehicle or the name of the engine that represents the displacement. For example, the HPI Firestorm 10T is described as having a G3.0 engine. The 3.0 refers to the displacement of 3.0cc. That 3.0cc is the equivalent of a .18 engine.
The Supertigre G-27CS engine, found in the DuraTrax Warhead EVO is a .27 big block engine. It has a 4.4cc displacement. Traxxas often puts the engine size right in the name of the vehicle, to differentiate an earlier model with a different engine size. The Jato 3.3, the T-Maxx 3.3, and the 4-TEC 3.3 all feature the TRX3.3 engine. That's 3.3cc, which translates to something like a .19 engine when expressed in cubic inches.

RPM and Horsepower
In discussing the power or performance of a specific RC engine, displacement is only one indicator. RPM (revolutions per minute) and horsepower (HP) are also indicative of how the engine performs.
Horsepower is a standard unit for measuring the power of an engine. An engine with a .21ci displacement can typically produce between 2 and 2.5 HP at around 30,000 to 34,000 RPM. Some manufacturers might emphasize the horsepower of their engine. You'll have to refer to the individual specs to determine the actual displacement of a specific horsepower engine.