RON, MON and AKI. MGB's with high compression engines were listed as requiring Premium 98 octane fuel or above. However, this cannot be directly compared with the octane number found on the pumps in North America, which is rated differently. In Britain, the Research Octane Number (RON) is used to indicate a fuel's octane. This number is derived from tests on an engine running at a constant speed of 1500 r.p.m., conditions that do not closely relate to those found in real situations. In order to address this disconnect with reality, the Motor Octane Number (MON) system was developed using a tougher test carried out at higher engine speed and temperature. In the US and Canada the two systems coexisted for a while -- causing much confusion. To resolve the matter the U.S. Cost of Living Council (CLC) index, better known as. Anti-Knock Index (AKI), was introduced and is simply the arithmetical average of RON and MON; so AKI = (RON + MON)/2 and is the number now familiar to motorists in the USA and Canada. Yet another measure may sometimes be referenced: Road Octane Number (RdON).This is considered the best predictor of octane but it is determined by running a vehicle on the road or using a chassis dynamometer, and it is too dependent on the actual engine under test. RdON is so close to AKI that they are often used interchangeably. There is no direct correlation between RON and MON, different formulations for fuels with the same RON number might well produce different MON's. You can make a rough determination, however: for the high octane fuels we are concerned with, the difference between RON and MON, or the "sensitivity" as it is known, is about 10. If you know the AKI therefore you can add or subtract 5 to determine the approximate RON or MON respectively.
Taking Shell as an Example. In the USA Shell 93 (AKI) octane FormulaShell Premium is about the best found at the pump and is 98 RON. In Canada, Formula Shell Gold: 91 (AKI) is actually 95 RON, so Canadian "B" owners with vehicles requiring 98 RON could well need to address a knock problem.
