THE CARBURETOR SHOP / Octane use

The Carburetor Shop LLC

204 East 15^th Street

Eldon, Missouri 65026

OCTANE REQUIREMENTS FOR YOUR VEHICLE

We see many enthusiasts using the wrong octane fuel in their engines. Generally, too high an octane is used, which can be costly in the wallet. Seldom do we have a customer using too low an octane, which could cause engine damage. This article is meant to be a non-technical aid to understanding what the numbers mean, and what octane you should use.

UNDERSTANDING THE NUMBER SYSTEMS

There are three common “groups” of numbers, which you may see at the pump. These are: RON (research octane number); MON (motor octane number); and AKI (anti-knock index).

RON and MON are two different methods of measuring and rating octane of a given fuel. The AKI is a “politically correct” average of the two, possibly devised to eliminate argument over which method is better, or more accurate.

DIFFERENT METHODS ARE USED IN DIFFERENT COUNTRIES!!!!!!!!!!!!!!!!

The United States once used the RON, but sometime in the 1970’s this was changed to the AKI.

It is my understanding that Canada uses the AKI, but Europe uses the RON.

Checking owners manuals for octane requirements for U.S. built 1960’s muscle vehicles may suggest 100 octane…..this is RON, and not applicable in the United States today. Typically, RON will exceed MON for the same gasoline by 8~10 numbers. Thus (and this is an illustration, not an absolute) a typical “premium” gasoline might have a RON of 100 and a MON of 90. Averaging these two figures would yield an AKI of 95.

It is not within the scope of this article to explain the criteria for either RON or MON; or to present an argument that one is better than the other. What is important is that one reads on an automotive forum where someone in the USA says that he is using 93 octane on his/her 1961 Whizbang 500; and someone from the U.K. says that his Whizbang knocks profusely on 93 octane and 98 octane is required THAT THEY MAY BOTH BE USING THE SAME FUEL, just different rating systems.

WHAT OCTANE SHOULD YOU BUY AT THE PUMP

Regardless of the rating system used in your area, it is quite easy to determine which octane to use. Start with an octane that you (or your mechanic) think is too high for your engine. Fill the tank. Now, find a highway where you can drive the cruising speed (please, observe the local speed limit) that has a fairly long straight, followed by a moderate upward incline, warm the engine to normal operation and head for the highway. Drive at your cruising speed until you reach the upward incline, and then gently accelerate sufficiently ONLY to maintain your existing speed. LISTEN closely for “ping” or “detonation” in the engine. If there is no ping, continue to drive the vehicle as necessary until you have approximately ½ tank of fuel. Fill the tank with the next lower grade. You now have a grade between the two grades. Repeat the test. If there is no ping, continue to drive the vehicle as necessary until you are low on fuel, and fill with the same grade (remember before you had half a tank of higher grade for a mixture). Now you will have the actual grade that you bought. Again, repeat the test; and continue to repeat until you either hear ping or get to the lowest octane available at your station. Once you hear ping, back up one test.

WHY YOU SHOULDN’T USE TOO HIGH AN OCTANE

There is a technical definition for octane which this article will ignore; but for purposes of this article, thing of octane as the speed of burning of the fuel. THE HIGHER THE OCTANE, THE SLOWER THE FUEL BURNS! Octane says nothing about the available energy in the fuel. If your engine does not have sufficient compression or timing, some (or a lot) of the fuel will go unburned. This can lead to idle issues, as well as LOWER fuel economy. Thus using too high an octane will cost you at the pump, may cost you with reduced fuel mileage, and may create engine-running issues, especially rough idle.

One other consideration is that some of the higher-octane fuel is higher octane because of the addition of ethanol rather than more costly refining. Ethanol has a RON of 129 and a MON of 102. This gives an AKI of 116. But ethanol has a much lower energy content than gasoline, so the ethanol fuel of the same octane will produce less power and poorer fuel economy than gasoline without the ethanol.