Pick your poison!
Every time you pull up to the pump to fuel up your ride, you have to make a decision—which gasoline should I use? Look at most any gas pump and you will be presented with a broad selection of “octane ratings”, with numbers like 87, 89, 91 and 93.5 prominently displayed on the buttons used to “pick your poison”. With the dizzying array of available choices you should ask yourself, “What is an octane rating and what does it mean to me?”
The octane rating assigned to gasoline is a measurement of the fuel’s ability to be compressed without detonating. Low octane fuel will combust without a spark at a lower pressure than higher octane fuels. If the fuel in your engine ignites before spark is introduced, engine knock will be the result, which can be very bad for your car. Does this mean that you need to use high octane fuel in order to keep your car safe? Absolutely not!
Most cars are designed to run just fine on 87 octane gasoline. The use of a higher grade fuel in a car that runs without issue on 87 is usually a waste of money. High performance engines, however, will often require a high octane fuel and the use of anything less than the recommended grade of gasoline may cause severe damage. Any car that requires high performance fuel will typically have this noted next to the fuel cap or in the owner’s manual.
Interestingly enough, the octane rating is not a measurement of how much octane is actually in the fuel. A common misconception is that gasoline with an octane rating of 87 contains 87% octane. The octane grade is not a measurement of how much octane is in the fuel but rather a measurement of how the fuel performs as compared to pure mixtures. Gasoline with an octane rating of 87 exhibits the same compression and detonation characteristics as a mixture of 87% octane and 13% heptane. The gasoline you purchase at the pump may have very little actual octane in it.
Many pumps prominently display the formula for calculating the octane rating. Most people have no idea or interest in what (R + M) / 2 actually means. Just in case you are an inquisitive person that really wants to know, here is a brief explanation of the octane rating formula.
(R + M) / 2 is nothing more than an average of 2 methods of measuring octane rating.
R represents the Research Octane Number (RON) method. RON is calculated by running fuel in a variable compression test engine and comparing the results with pure octane/heptane mixtures.
M represents the Motor Octane Number (MON) method. MON is calculated by running a preheated fuel mixture under load in a variable compression engine at higher RPMs than used in the RON method. The MON method is regarded as being a more accurate measurement and will return lower octane ratings then the RON method.
The rating you see on pumps in the U.S. is an average of these two methods and is represented by the formula (R + M) / 2; it is also commonly referred to as the “Anti-Knock Index” (AKI).
With a better understanding of octane, you should be able to make the best decisions for you and your car when standing at the pump. Do your research and drive safely.
Andy Sheehan is a blogger, aspiring novelist, and relentless hoon. He plans to will his 2002 Subaru WRX Wagon to his firstborn, plans his daily commute around the swoop of its roads, and doesn’t plan to ever buy an automatic. A cool-car omnipath, he loves the common Mustang or Chevelle, but hunts for the weird and wonderful Velorexes and Cosmos of the autoverse. And when he can afford a garage, he’s going to turn an MX-5 into a race car.