Gasoline Octane Rating Systems & the V-Max

by Jerry Ferguson <jerry@vmaxoutlaw.com>


As consumers, we use the pump octane and manufacturers recommendation to determine which gasoline to buy. Octane is a general term used to indicated a gasoline's ability to resist engine knock. The pump octane is also referred to as the Anti-Knock Index (AKI). AKI is determined based on an average of the Research Octane Number (RON) and the Motor Octane Number (MON). The formula is RON+MON/2 normally abbreviated as R+M/2 on the pump.

Octane is tested in a single cylinder octane test engine. The MON is a measure of the gasoline's ability to resist knock under sever operating conditions. MON affects high speed, part throttle and performance (under load such as in passing). The RON on the other hand, is a measure of gasoline's ability to resist knock under less sever conditions. RON affects low to medium speed knock and engine run-on (dieseling). For a given AKI, RON is typically 8-10 points higher than the MON. As an example, 87 AKI (pump octane) fuel would have a MON of 82 and a RON of 92.

What your engine requires to operate knock-free, is referred to as the Octane Number Requirement (ONR). The ONR for an engine is affected by design factors and real world conditions. Engineers must balance performance, economy and environmental concerns in their design. Compression ratio, ignition timing, air/fuel ratios, temperatures and combustion chamber design all have an affect on the ONR. Compression ratio has the most significant impact on the ONR and engine efficiency. The higher the compression ratio, the higher the ONR and combustion efficiency. Retarded timing, rich or lean air/fuel ratios, lower combustion temperatures and high swirl combustion chambers all work to reduce an engine's ONR.

In the real world, there are other factors that affect these designs including barometric pressure, temperature and humidity. Increases in barometric pressure or temperature, increase the ONR. Increases in humidity or altitude (lower barometric pressure), reduces the ONR. Combustion chamber deposits increase temperature and compression thereby increasing the ONR.

Yamaha states the following in the Vmax owners manual:

"Your Yamaha engine has been designed to use regular unleaded gasoline with a pump octane number ([R+M]/2) of 86 or higher or research octane number of 91 or higher. If knocking or pinging occurs, use a different brand of gasoline or premium unleaded fuel. Unleaded fuel will give you longer spark life and reduced maintenance cost. If unleaded gasoline is not available, then regular leaded gasoline can be used.

Gasohol

There are two types of gasohol; gasohol containing ethanol and that containing methanol. Gasohol containing ethanol can be used if ethanol content does not exceed 10%. Gasohol containing methanol is not recommended because it can cause fuel system damage or vehicle performance problems."

Many Vmax owners have reported good performance using standard unleaded gasoline of 87 or so pump octane number. Although this seems to be adequate for the vast number of Vmaxes, some, depending on location and modification levels, seem to require gasoline with a slightly higher pump octane number. I personally have several Vmaxes and in my location (Arizona) I've noticed that my 1996 model particularly seems to require higher octane in order to reduce detonation possibility. However we are blessed with additives in the fuel which tend to reduce it's overall performance, which is also a contributing factor.

My preference is Chevron due to the Techron additive in their fuels. This additive is also available under different names, however, I don't have to guess at the Chevron station.  Note also that some brands do not include cleaners in their mid and low grade fuels.

The key point of all this is you should use the minimum pump octane (AKI) fuel that will run in your engine without knocking. You're wasting your money on higher octane fuels if there aren't needed to control knock. The two most common myths regarding pump octane (AKI) are that it will increase performance, and result in better fuel mileage. You may see improvements in your bike due to the cleaners in higher grade, higher quality fuels. However octane by itself will not have any effect.