Octane Myths

It's nice to think that you're giving your car a "treat" when you pull up to the pumps and select a Premium grade of gasoline. Unfortunately, the fact is that you're better saving your money and treating it to a nice detailing. At least you can see the results from that.

While Old Wives didn't really know anything about octane, they seem to get blamed for a number of incorrect tales involving gasoline. Let's look at some of them.

High octane gasoline increases power

According to the Minnesota Department of Commerce, "If your car is designed to run on 87 octane gasoline, you shouldn’t notice any more power on high octane gasoline. If it does make a noticeable difference, your engine, or the engine’s electronic control systems, may need repair."

An Octane rating is an indicator of a fuel's ability to resist premature ignition, or "ping." Other factors can also impact the likelihood that an engine may ping, including ambient temperature and humidity, engine timing, cooling system effectiveness and more.

A requirement for the use of higher octane fuel may come from engine modifications such as higher compression ratio and advanced ignition timing. When an engine pings, power is lost because ignition usually doesn't happen at the right part of the cycle.

The flame front initiated by premature ignition can actually work against that from the spark plug in two ways. First, it consumes fuel that would have been available to the primary flame front. Second, because it is "premature" the increased cylinder pressures work against the rotation of the engine, slowing it down somewhat.

High octane gasoline improves mileage

The mileage you experience is mainly determined by the energy content of the gasoline, which has nothing to do with its octane rating, and the efficiency with which it is burned by the engine. Ethanol, for example, has a significantly higher octane rating than regular gas, but a lower energy content per gallon. As a result overall gas mileage when running E85 is lower than with E0 or normal gas.



High octane gasoline gives quicker starting

As it turns out, there is just no correlation between starting ability - which is affected largely by the light (or volatile) hydrocarbon content in the fuel blend - and octane rating, which is predominantly determined by selection of anti-knock additives. There is no significant or predictable impact of one on the other.

My engine has a knock sensor, so I don't have to worry

Modern engines equipped with knock sensors that can detect detonation and modify engine timing to eliminate knock. If the condition is persistent, running the engine on retarded timing will have an undesired impact on both power and fuel economy.

Knock sensors work by listening to engine vibrations, particularly in the range of six to eight kilohertz. When detected, the sensor raises a signal to the control computer which reacts as described. Certainly, knock sensors can fail, so if you do hear pinging in your modern engine, something requires fairly immediate attention.

At the End of the Day

Buy the gasoline grade that your ride was designed for. Buy good quality product in that grade and you'll be saving money in the long run.