One of the most frequently asked technical questions we get at VP 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 or R+M/2 Octane Numbers.
To answer these questions, we need to first explain the machines that do the testing. These machines, both the Motor Octane Machine and the Research Octane Machine, 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 technology fuels or engines, but they are all there is for testing the octane of 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:
As the comparison above shows, the Motor Octane Machine is run at a higher RPM, hotter temperature and with more timing. The Motor Octane Machine will put more stress on any fuel and more accurately represents a racing engine. VP Racing Fuels always used the Motor Octane Number when advertising our fuel or in our VP Tech Bulletins because our fuels are primarily used 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 sample. This number is sometimes used by some fuel companies to trick the racer into thinking he or she is getting a 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 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.