We answer your fuel questions, such as : Is there an advantage to using an octane rated higher than your vehicle? What is octane? and other questions that you’ve been asking. One day when combustion engines are obsolete, people will be asking questions like : Mom, did people really drive cars with gas engines? Did people really drive cars? Why?

Anyway, let’s get back to combustion engines and octane ratings.

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Gas — What you Need to Know

Gas is something we use all the time in our cars, generators, and all sorts of power equipment, like chainsaws and lawnmowers. Though few people fully understand one of the most basic principles of petroleum-based fuels, octane. The common misconception is that high octane equals more power. So, you may think that using an octane rated higher than your vehicle should yield benefits. But is this true? This article is going to uncover the facts related to fuel octane and how it affects your engine. What is octane? What’s the difference between regular and premium gas? It’s time for some answers.

What is Octane?

Using the correct octane rated for your vehicle, begins with understanding octane and how it affects engine performance. We should, therefore, start with internal combustion engine 101 in order to see how the gas generates the energy needed to propel your vehicle.

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Combustion and Compression

The key to understanding the octane rated for your vehicle is all about compression. A gas engine compresses air and fuel inside a combustion chamber and this is ignited by a spark plug, creating a small explosion. This explosion forces the piston downwards in the same way that a bullet is pushed forward when gunpowder explodes.

If the compressed gas and air mixture is compressed more than it should be, it will ignite from the pressure, before the actual spark occurs. This is known as pre-ignition. Commonly referred to as knocking, or pinging, pre-ignition causes an unbalanced running of the engine. This reduces the engine power and will, over time, damage the engine. So, the ignition timing in your engine is critical for efficiency and the durability of your engine.

High-performance engines have higher compression, so these engines need gas that ignites at a higher pressure. This means that different engines will use different compression to combustion ratios to ensure that the fuel ignition happens exactly when it should.

Fuel Octane ratings

Octane rating is the amount of compression that the fuel can withstand before igniting. A high-octane fuel will ignite at a higher pressure. In other words, an engine with greater compression will run better using a higher octane rating.

Octanes are hydrocarbons, a chemical structure made up of hydrogen and carbon with a specific structure. Gas consists mostly of hydrocarbons derived from crude oil. Biofuels are made from ethanol, which is also made up of hydrocarbons, but these are extracted from plant cellulose.

The octane number that we see on the gas pump is the percentage of iso-octane in relation to heptane. It’s the iso-octane that determines the combustion to compression reactivity of the gas. Fuel rated at 90-octane has a 90% iso-octane to 10% heptane ratio. Gas rated at 90-octane will require higher compression to ignite than an 85-octane rating.

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How is octane rating determined?

A specialized engine was developed for the testing of octane ratings. A Cooperative Fuel Research (CFR) engine, also known as a knock engine, is a modified internal combustion engine fitted with equipment to measure engine knock.

A CFR engine has a single cylinder and a standard carburetor. The engine head is height adjustable, allowing for the compression to be altered. The fuel is tested under predetermined conditions: air temperature at the intake needs to be 125.6°F, and manifold vacuum is maintained at 2 – 3 Hg. The engine head is then raised or lowered to test the fuel under varying compression ratings. The engine knock is measured at the different compression settings to determine knock levels.

There are two methods of octane testing in use. Motor Octane Number (MON) is measured at an engine speed of 900 RPM, and Research Octane Number (RON) is tested at 600RPM. This basically represents typical engine speed at idle, or average RPM when driving. Though, engine RPM can actually be anything from 400 RPM to 12,000 RPM. The 600 and 900 RPM figures are merely used to set a universal standard of measurement.

How will your engine be affected by…

— using an octane rated higher than your vehicle?

Since a high-octane rating won’t ignite prematurely, it will have no effect on the running of the engine. This will be true for high and low compression engines. The fuel will only ignite once the spark plugs provides the additional heat needed to complete the reaction.

Premium gas is usually more expensive than Regular gas. This means you’ll be wasting money using fuel with an octane that is rated higher than that specified by the vehicle manufacturer. Using an octane that is rated higher than your vehicle will not improve performance. Nor will it “burn your engine cleaner” as some gas suppliers like to claim.

Using an Octane Rated Lower than your vehicle

So, let’s do a quick recap before continuing. An engine designed to use high-octane fuel, has high compression. Conversely, an engine with a low-octane rating has a low compression engine. This is because low-octane fuel will self-ignite at a lower pressure.

Using a low-octane fuel in a high compression engine will cause preignition. Some of the gas in the combustion chamber will ignite as the pressure increases. Because the gas is exploding before the correct ignition timing, the combustion cycle will be unbalanced. You should never use fuel with an octane rating lower than what is recommended for your vehicle.

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Can the wrong octane fuel damage your engine?

So far, we’ve established that using an octane rated higher than your vehicle has absolutely no affect, there is no benefit. The only disadvantage is that you’re wasting your money on expensive fuel for no reason. Using an octane that is rated lower than your vehicle is likely to cause pre-ignition, or engine knocking. This is not good for your engine and reduces performance.

Fortunately, modern engines with electronic engine management systems, have knock detectors that detect when the fuel ignites prematurely. This means that the computer will adjust the ignition timing to prevent pre-ignition. So, it’s only older cars that may be harmed by using octane that is rated lower than your vehicle.

However, relying on your car’s computer to compensate for incorrect fuel octane will have a minor effect on the performance, resulting in lower gas mileage. Reduced performance means you’ll be spending more on gas.

The Final word on using the correct octane for your vehicle

Use the octane recommended by the vehicle manufacturer. It’s as simple as that. Some manufacturers may stipulate multiple octanes, you can use several octane ratings. I would recommend using the highest octane rating specified for your vehicle, this will provide the best performance and, therefore, the lowest fuel consumption. Though, exceeding the octane recommended for your vehicle will be of no benefit.

— Exceptions to the rule

I may be confusing the issue here, but there are certain instances where high-octane fuel is beneficial, and this is related to air temperature. Air pressure also affects the octane requirement for an engine, this is most relevant to lower air pressure at high altitudes.

Research Octane Number (RON), and Motor Octane Number (MON) are tested under precise, controlled conditions. The difference between the two testing relates only to engine RPM. The research method (RON) is conducted at low RPM (600), and MON is tested at a higher (900) RPM. However, the air pressure and temperature for both methods of testing are the same.

The engine oil temperature has to remain between 100°F and 129°F. More importantly, the air intake temperature has to be maintained at a constant 125.6°F. This is important because hot air expands, and this increases its pressure and the amount of oxygen that the engine receives. Similarly, the ambient air pressure will affect the volume of air entering the combustion chamber. For this reason, under test conditions, the manifold vacuum is kept within 2 – 3Hg.

Under normal driving conditions, these conditions cannot be controlled. In hotter climates, engines are likely to run hotter and the use of a higher octane gas can be beneficial for a hotter engine. At high altitudes, air pressure is lower, and this means using a lower octane fuel. You may notice the octane rating at the gas pumps in high-altitude areas being slightly lower than areas closer to sea level.

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The octane rating, at sea level, for regular gas is 87, mid-grade is usually 89-octane, and premium can be 91 – 93 Octane. At higher altitudes, regular gas can be 85 – 86-octane, sometimes as low as 83-octane. This is the recommended octane at these altitudes, and is needed to compensate for the lower air pressure. It is, therefore, perfectly fine to use a slightly lower octane fuel at higher altitudes.