A lot of people who believe that higher octane gas is “better” for your car than lower octane gas, as if the word premium next to the higher ratings actually mean the gasoline is better for your car. It isn’t. If you believe it is, don’t be dismayed, you’ve simply become accustomed to reading the words “premium” next to the label and believing the great marketing machine that helped create over $10B in 4th quarter profits for companies like Exxon-Mobil. The octane rating of gasoline is a measure of how much you can compress the gasoline before it ignites, not how “good” it is.

Before doing a detail analysis of pros and cons of higher octane gas, let’s first understand what does octane number actually mean.

All About Octane Number or Octane Rating

Now that you know a something about octane number, let’s move on to our original topic.

If you know a little about auto engines, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

Higher octane gasoline resists knocking much better as compared to lower octane gasoline. Because knocking can damage an engine and rob it of power, gasoline formulations have been developed to minimize the problem. Gasoline containing relatively large amounts of straight-chain hydrocarbons (such as n-heptane) has an increased tendency to knock, whereas those containing branched-chain forms (such as isooctane) burn more smoothly. In addition to isooctane, other compounds also reduce engine knocking. By using an index called octane number, it is possible to compare the antiknock properties of gasoline mixtures. A higher octane number indicates that a mixture has the equivalent antiknock properties of a gasoline containing a higher percentage of isooctane.

If your engine requires Premium fuel, for the sake of the car use premium fuel. If you use a lower octane, the car cannot “change the compression ratio”… this cannot happen, its like asking the cars computer to change the diameter of your wheel rim. The computer cannot change the shape of your engines internals. What it does do is pull ignition timing. It detects the engine is knocking and retards the timing (this is called Knock Retard) What this does is cause the spark plug to ignite earlier than it would have. Ever 2 degrees of ignition timing is equal to 5 HP. On a car that requires premium, like a turbo/supercharged or high compression car, you can lose in excess of 20 degree of timing and that could be a noticeable drop. Also, igniting the fuel mixture earlier causes the piston to push down on the crankshaft earlier than it should, causing excess wear on your connecting rod bearings, which are very expensive to replace (and can cause complete engine failure if left unchecked) not to mention, that extreme cases of KNOCK can cause a lean condition or a “chipped” piston, when a chunk of the piston actually fragments and beats the inside of the combustion chamber until it looks like the moon.

On any car, the higher octane you run, the more timing advance you can run, so as long as your cars computer can command it, and your fuel can support it, you can get more timing + and more HP. (boosted, high compression cars especially)

Higher octane will not burn hotter unless it contains more ethanol. The more ethanol in your gasoline the hotter it burns, and less Horse power. Also, regardless of the “octane” rating of ethanol, the required Air fuel mixture is 5 times that of gasoline. Higher octane should burn cooler.

Conclusion

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.

Using a premium fuel when your engine does not require it or can’t take advantage of it, is not money well spent, especially at today’s prices. Octane is added to a fuel to prevent the fuel from self-igniting when subjected to higher amounts of compression (reason high compression engines require high octane fuel). Using high octane fuel in a low compression engine, or an engine that does not have the ability to advance its timing, is a waste of money. It is actually more harmful to the environment.

Please use the fuel recommended for your car, and leave that decision to the people designing cars, because honestly, most people don’t know anything about what’s going on inside their car.

As a general rule, always read and follow the owner’s manual.

Impact of Higher Octane gas on Horsepower

By now, we all know that a premium motor fuel will often be formulated to have both higher octane as well as more energy. Now how much impact a higher octane gasoline or premium gasoline can have on horsepower?

Most people will say “not much” or some of them wouldn’t even agree that it could. But auto combustion is a science rather than an art and we should have some real world data to examine if it is a science.

As few of you already know that Dinan is known for its high performance tuning of BMW cars. Dinan engineering did extensive testing on octane effect on high compression, high performance engines usually found in BMW M cars. Here is a good example from a white paper published by Dinan engineering.

Dinan did their testing using two high performance models from BMW (M3 and M5). If you’re a car enthusiast, you probably know what M cars are capable of. M3 and M5 engines are highly sensitive to temperature and octane rating. To prove the point they used an E46 M3(3.2L 333HP) for dyno testing.

The E46 M3 is an excellent example of an engine that is sensitive to octane. It has a very high volumetric efficiency as well as a very high static compression ratio of 11.5 - 1. The engine being tested was a stock M3 engine. It was first warmed up and stabilized using the standard method used for Dyno testing, running 91-octane fuel. When engine was warmed up and standard testing procedures applied, the stock M3 produced 280 hp (RWHP). 91-octane fuel was then replaced with 93 (available in most parts of the country). The M3's computer was so quick to determine that the fuel had been improved that it only took four dyno runs for the timing to adapt to the increased octane and raise the power up to 291 hp. A gain of 11 HP with just 2 points of octane! The M3 engine is equipped with a very good ram air system. While the large dyno fan is pushing large volumes of air into the ram air duct, the volume and velocity of air seen by the car on the dyno is still less than the engine would see on the road. The power output drops off after 7350 rpm because Dinan simply couldn’t duplicate the airflow the car would receive on the road in the dyno room, resulting in a loss of power. BMW claims peak power @ 7900 rpm. They concluded that if they could accurately reproduce the ram air that the M3 would receive on the road, our peak power would move up from 7350 rpm to 7900 rpm.

Read the complete white paper here -

Dinan White Paper on Octane Rating and HP