What is Octane?

When you pull into a gas station, you usually presented with four options: regular, plus, premium and ultra premium. But, have you ever wondered what those numbers- 87, 89, 91 and 93 mean?



Octane is the measure of how much compression a fuel can withstand before igniting. The higher the octane rating, the less likely the fuel is going to pre-ignite (i.e., explode unexpectedly) at higher pressures and damage your engine. That’s why performance cars with higher compression engines require higher octane premium fuel. In essence, higher octane fuels are compatible with higher compression engines that can increase efficiency and performance while potentially reducing emissions by combusting the fuel more completely.

As mentioned above, the higher the octane rating, the more the fuel can withstand greater pressures without pre-igniting. This allows for a variety of options that can increase power and efficiency while adding the ability to decrease emissions as well. These options include increasing the compression ratio, changing the timing of spark ignition, injecting less fuel into the cylinder, and more. Additionally, engines designed to work with higher octane also burn the fuel more completely- meaning less harmful emissions in exhaust. One MIT study even estimated that if higher octane fuels and engines designed for them became more widespread, the U.S. could cut our annual CO2 emissions by 35 million tons.

The standard octane rating that most car engines are designed for is 87. Anything lower and you risk engine damaging detonation or knocking. This ignition of fuel creates a shock wave throughout the cylinder as the burning and expanding fuel-air mixture collides with the piston that is still traveling towards top-dead-center. The resulting knock or ping is the sound of the pistons slamming against the cylinder walls.

The effects of detonation may be anywhere from arbitrary to severe. Prolonged and intense knocking can break the piston or the engine, though it is can also endure this slight issue for thousands of miles. Similarly, overheating may cause additional wear-and-tear on the engine, be relatively harmless or cause the engine to catch on fire and break.

Since 87 is the standard octane rating for the fuel most engines use today, the 89, 91 and 93 octane rated fuels are only different because they can handle more heat and pressure before igniting on their own. This, in turn, means they can run in performance engines designed to use higher compression ratios. It also means that while higher octane fuels won’t hurt engines designed to run on 87 octane fuel, they won’t provide a benefit either.

Some drivers may think that it's nice to give their cars "treat" by selecting a premium grade of gasoline, even though they only require regular octane gasoline. Unfortunately, this is a waste. It would actually be better to treat their cars to a nice detailing. At least they could see the results from that.

Here are Some Common Myths about Octane:

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 with high octane gasoline. If it does make a noticeable difference, your engine, or the engine’s electronic control systems, may need repair."

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

There is 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.

My Engine has a Knock Sensor, so I don't have to Worry.

It's true that modern engines are equipped with knock sensors that can detect detonation and modify engine timing to eliminate harmful knocking. However, 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 engine, something requires fairly immediate attention.

Summary- Best Advice

Car engineers know best. Buy the gasoline grade that your ride was designed for. In addition, buy Top Tier detergent gasoline (see previous article in the Challenger Maintenance forum) in the recommended grade and you'll be saving money in the long run.

Here are the recommended octane numbers for recent Dodge engines:

3.5L= 87 octane

3.6L= 87 octane

5.7L= 89 octane (auto.) and 91 (manual)

6.1L= 91 octane

6.4L = 91 octane

6.2L= 91 octane (Hellcat)

6.2L= 91 or 100 octane (Demon)