One of the most frequently asked technical questions we get at VP involves the difference between Motor, Research and R+M/2 Octane Numbers. The next most frequently asked question is why some fuel companies represent their fuels with Motor Octane Numbers, while other companies use Research or 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 1930s 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 tech fuels or engines, but they the only means available for testing fuels. These machines are one-cylinder engines that have an adjustable head 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:
As the comparison above shows, the Motor Octane machine runs at a higher RPM, hotter temperature and more timing. This machine will put more stress on any fuel and more accurately represents a racing engine. VP Racing Fuels always uses 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 a better fuel. The R+M/2 Octane Number is the average of the Research and Motor Octane numbers and is the number displayed with yellow labels on retail level gas pumps.
When comparing fuels for racing purposes make sure to compare Motor Octane Numbers because these are the ones that count in your racing applications.