A special test engine used by the Cooperative Fuel Research to determine the octane rating of a hydrocarbon fuel. A CFR engine has a variable compression ratio, and it can be adjusted to cause any fuel being tested to detonate, or explode, rather than burn evenly. The fuel being tested is run in the CFR engine and the compression ratio is adjusted until the fuel detonates.

With the engine adjusted to cause detonation, the fuel under test is shut off, and a reference fuel, which is a mixture of iso-octane and normal heptane, is directed into the engine. The ratio of the iso-octane to heptane is changed until the reference fuel detonates the same as the fuel being tested.

The percentage of iso-octane in the reference fuel having the same detonation characteristics in the CFR engine as the fuel being tested is the octane number given the fuel.