Why are two ratings of Octane?

The correct name for the (RON+MON)/2 formula is the "antiknock index",and it remains the most important quality criteria for motorists.

The initial knock measurement methods developed in the 1920s resulted in a diverse range of engine test methods and conditions, many of which have beensummarised by Campbell and Boyd.

In 1928 the Co-operative Fuel ResearchCommittee formed a sub-committee to develop a uniform knock-testing apparatus and procedure.

They settled on a single-cylinder, valve-in-head, water-cooled, variable compression engine of 3.5" bore and 4.5" stroke.

The knock indicator was the bouncing-pin type. They selected operating conditions for evaluation that most closely match the current Research Method, how ever correlation trials with road octanes in the early 1930s exhibited such large discrepancies that conditions were changed ( higher engine speed, hot mixture temperature, and defined spark advance profiles ), and a new tentative ASTM Octane rating method was produced.

This method is similar to the operating conditions of the current Motor Octane procedure. Over several decades, a large number of alternative octane test methods appeared. These were variations to either the engine design, or the specified operating conditions.

During the 1950-1960s attempts were made to internationally standardise and reduce the number of Octane Rating test procedures. During the late 1940s - mid 1960s, the Research method became the important rating because it more closely represented the octane requirements of the motorist using the fuels/vehicles/roads then available.

In the late 1960s German automakers discovered their engines were destroying themselves on long Autobahn runs, even though the Research Octane was within specification. They discovered that either the MON or the Sensitivity ( the numerical difference between the RON and MON numbers ) also had to be specified.

Today it is accepted that no one octane rating covers all use. In fact, during 1994, there have been increasing concerns in Europe about the high Sensitivity of some commercially-available unleaded fuels. The design of the engine and vehicle significantly affect the fuel octane requirement for both RON and MON.

In the 1930s, most vehicles would have been sensitive to the Research Octane of the fuel, almost regardless of the Motor Octane, whereas most 1990s engines have a 'severity" of one, which means the engine is unlikely to knock if a changes of one RON is matched by an equal and opposite change of MON.

Note that the Research method was only formally approved in 1947, but used unofficially from 1942.