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Gasoline Octane Rating Systems

Octane Rating Systems

by Larry Cann

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.

Late model R models (2 valve) – BMW recommendation is for a RON of 91 and MON of 82. Under most circumstances, Regular Unleaded 87 pump octane (AKI) will meet your needs. However, due to production variations, tuning, etc. the recommended pump octane (AKI) may not be adequate.

K75 & K100 (2 valve) – BMW’s recommendation is 89 octane (AKI) with a RON of 95 and a MON of 85. We’ve found premium fuels are rarely required by these engines. I would recommend a high quality mid grade fuel.

R1100’s – BMW recommends a “mid to premium” grade fuel with a minimum of 89 pump octane (AKI) with a RON of 95 and a MON of 85. I find no knocking in these engines when we use a high quality premium as opposed to the mid grade fuels.

K100 & K1100 (4 valve) – BMW recommends a “premium” grade fuel with a minimum of 89 pump octane (AKI) with a RON of 95 and a MON of 85. These bikes like a higher pump octane (AKI). I wouldn’t recommend using anything other than a high quality premium grade fuel in these motorcycles.

My preference in all cases 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. Whenever you purchase fuel, be sure to select brands that have passed the BMW unlimited mileage test. Note also that some brands do not include cleaners in their mid and low grade fuels.

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.

Larry Cann
BMW of Orlando
Phone (407) 826-4BMW
FAX (407) 856-0568
Tue-Sat, 1st & 3td Sun

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