One of the most frequently asked technical questions we get at VP Race Fuels is about the difference between Motor, Research, and R+M/2 Octane Numbers. The next question often asked is why do some fuel companies represent their fuels with Motor Octane Numbers and other companies use Research of R+M/2 Octane Numbers.
To answer these questions, we need to first explain the machines that do the testing. These machines were made in the 1930's, and were designed to test for octane numbers from the 0-100 range. Any number above 100 is an extrapolation. Both of these machines are dinosaurs and are not adequate for today's high technological fuels or engines, but they are all we have for testing fuels. These machines are one-cylinder engines that have an adjustable head on them that can move up or down to increase or lower the compression ratio while the engine is running. The Motor, and Research machines are the same in this respect, but they differ in several other characteristics. The following is a comparison of the two machines used for testing octane numbers:
Motor Machine Research Machine
RPM 900 600
Intake Temp. 300 Degrees F 120 Degrees F
Timing Variable Basic Setting Fixed At 13 Degrees (Does Not Change)
As the comparison above shows, the Motor Octane Machine is run at a higher RPM, hotter temperature, and more timing. This machine will put more stree on any fuel and more accurately represents a racing engine. VP Racing Fuels always uses the Motor Octane numbers when advertising our fuel because our fuels are used primarily for racing applications. The Research Octane machine will always produce a higher number for the obvious reason that it does not put the same amount of stress on the fuel. This number is used by some fuel companies to trick the racer into thinking he/she is getting better fuel. The R+M/2 Octane number is the average of the Research and Motor Octane numbers and is displayed on the retail level of gasoline pumps.
When comparing fuels for racing purposes make sure to compare Motor Octane numbers because these are the ones tha count in your racing applications.