Product Feature: Octane
May 1968 Popular Mechanics

May 1968 Popular Mechanics

[Table of Contents] Wax nostalgic over early technology. See articles from Popular Mechanics, published 1902 - 2021. All copyrights are hereby acknowledged.

This "Product Feature: Octane" pamphlet was an insert in the May 1968 issue of Popular Mechanics magazine. As the subtitle claims, it summarizes "What it is, What it does, How to get your money's worth." Truth is it is really an infomercial from the Sun Oil Company (aka Sunoco), but there is some useful information about how octane levels affect engine performance. Back in the days before computers ran car engines, various factors like temperature rating of spark plugs, compressions ratio, condenser and ignition coil condition, timing (before top-dead-center, BTDC), ambient air temperature and humidity, altitude, and other factors made using the right blend of gasoline necessary to get top performance. Today's computers measure, monitor, and compensate for a lot of that, so most engines can run on just about any gasoline. An exception is high compression ration engines in expensive sports cars requiring high octane levels. As an aside, the current (2023) Sunoco logo is my favorite of the four historical logos. It looks very retro.

Octane: What it is, What it does, How to get your money's worth

By Mort Schultz

Once and for all, let's set the record straight about octane.

Important? You bet. When you buy premium or regular grade gasoline, you're buying octane - either high or low. But buying too much octane wastes money. Buying too little octane will give poor engine performance.

Here, then, are the facts about this important, but not too often understood subject.

Octane is a Measurement - Not an Additive

Octane is not a physical property of gasoline. Instead, it's a rating of a gasoline's ability to resist power-robbing detonation or "knock" in an engine's combustion chamber. Octane rating is accepted as a standard method of measurement and comparison by all manufacturers of gasoline and automobiles.

Every engine has a different octane requirement!

Octane Rating Should Meet Your Engine's Needs

Why? Because as a piston compresses the fuel mixture in a cylinder that mixture gets hot. The more it's compressed, the hotter it gets. If gasoline is too low in octane rating for the engine, heat of compression could spontaneously ignite the mixture before the spark plug fires. This detonation causes a sudden, extreme rise in pressure which you hear as knock and feel as loss of power.

The result is just the opposite when your engine uses a gasoline with the octane rating it needs. The mixture is ignited at the proper time by the spark plug. You get all the power the engine's built to deliver - smoothly, efficiently.

What does this mean to you? Obviously, that the gasoline you use must be of an octane rating which your car needs. It could be low octane or high octane. That depends on the engine.

Unfortunately, many people buy too much octane. This is a throw-back to the late 1950's when approximately half of the cars manufactured needed high octane gas for their high compression engines. In those days, the alternative would have been detonation and millions of knocking engines.

This is no longer true. Almost 67 percent of today's new cars are designed to run on low octane (regular) gas. Those who use high octane (premium) unnecessarily are spending 4¢, 5¢, even 6¢ a gallon more for no reason. The high octane is not giving their engines anything that lower octane gasoline can't give them.

As we've noted, an engine that requires high octane gasoline should not be operated on low octane gas. Detonation is the result.

So, how do you pinpoint the octane rating your engine needs? It's easy!

How to Select the Right Octane

1. Tune your engine to manufacturer's specification, making sure that ignition timing is on the nose. This will get you the mechanical efficiency intended by the manufacturer and will also bring you within range of his recommendation for grade of gasoline. But be careful. Here's where many people misinterpret the manufacturer's recommendation for "premium" or "regular" gas.

Variations in engine tolerances, differences in the way engines are adjusted, and even individual driving habits result in different octane requirements for engines that are identical in year, make and model. Even when properly tuned, these engines can differ in their needs by several octane numbers. A manufacturer's recommendation, then, is a guide only.

2. Select the major brand of gasoline that offers the widest choice of octane ratings. Obviously, you have a better chance of meeting your engine's octane needs if you can choose from several octane ratings rather than from the two or three which most stations offer. Of the major brands, Sunoco offers the widest choice-eight octane levels, about a penny apart in price.

3. Don't be misled by claims of additives. Virtually every major brand of "premium" gasoline and some "regular" brands contain additives to prevent corrosion, carburetor icing, dirty carburetors and cylinder deposits. Don't buy a "premium" gas just to get extra additives. Both the fuel and automotive industries agree that you gain riothing by using a "premium" fuel in a properly tuned engine which is designed to run on "regular".

4. Once you've decided on "regular" or "premium," try a tankful. If you get good performance, switch to a lower-octane rated, less expensive grade. If the engine still operates efficiently without knocking, there's no need to use the higher grade. If you try "regular," however, and hear a knock and suffer poor performance, move up in octane level until the knock disappears.

In any event, what you're doing is carefully customizing gasoline to your engine's needs for peak efficiency and performance at the lowest possible cost.

Posted September 4, 2023