Mathematical relationships have been developed whereby excellent correlations are obtained between the road antiknock performance of conventional volatility gasolines and their Research and Motor octane numbers. Correlation equations for current and experimental high compression ratio engines at a given speed are about the same as in older cars of about 7:1 C.R. Although the road antiknock performance of fuels varies greatly from car to car, good correlations with laboratory octane ratings are obtained in cars causing extremes in fuel octane performance as well as in the average car. Fuel depreciation (Research O.N.-Road O.N.) is shown to be a poor measure of the relative effects of Research and Motor O.N. on road antiknock performance in many instances. Neither can fuel depreciation be used as a good measure of relative engine severity. A new definition of engine severity is proposed.
The importance of Motor O.N. has increased rapidly during the past few years due to an increasing average speed of knock. In experimental 10:1 C.R. engines, the data indicate that the Motor O.N. requirements are higher than the average Motor O.N. of fuels which will probably be available, thus requiring spark retard from maximum power settings at high engine speeds or the manufacture of less critical engines.