People often buy vehicles that specify premium fuel only, and later would like to save by using a lower-grade fuel. The term octane is a very misunderstood, and a great deal of misinformation is around. Is it really necessary to use the high-octane fuel in a vehicle that specifies it?
Premium Fuel Only
The label that reads, ‘Premium Fuel Only,’ serves two functions. It informs the driver of what the engineers feel is necessary for the engine and it limits the liability of the manufacturer. An explanation of the latter may be helpful, before a more thorough explanation of the former.
Manufacturers are responsible for the products they sell, within the terms of the warranty they express. When liability issues arise, courts consider if the driver knew the behavior that caused the damage, was wrong. The label that states, premium fuel only, is such a warning. If engine damage occurs from the use of the wrong fuel, the manufacturer will not be liable.
Why is premium fuel specified?
One thing an engineer uses to enhance performance is increased engine compression. Most people realize, that gasoline literally explodes in the combustion chamber and the energy drives the vehicle. Compression-ratio is how tightly they squeeze the fuel/air mixture before igniting it. The greater the volume of the engine cylinder, in relation to the size of the combustion chamber, the higher the compression-ratio and the greater the power output.
They rate most standard engines around 8.5-1 compression. For instance, with 350 cubic-inch, eight cylinder engines, the cylinder volume is about 43.8 cubic inches or 350 divided by eight. If the combustion chamber volume is 5.15 cubic inches, the ratio is 8.5-1. A high performance engine may use a smaller combustion chamber and achieve 9.5/1 or higher. This will produce more horsepower and often a more efficient engine.
Raising engine compression also has an undesired tendency. Increased cylinder pressure, raises the tendency of the fuel/air mixture to combust spontaneously. With an automotive gasoline engine, the piston compresses the mixture and just before it reaches the top of its travel, the spark plug ignites the mixture. Engines carefully control the number of degrees a spark plug fires, before the actual top-dead center. Ignition timing will greatly influence performance and emissions.
Should the mixture ignite too soon, the burning fuel tries to push the piston back down. This occurs when low octane fuel is subjected to cylinder pressure that is too high of when too much heat develops. Pre ignition produces a pinging noise often heard on acceleration. The sound comes from the shock waves of the exploding fuel, and is very damaging to the engine. The octane raises the point at which the fuel/air mixture will ignite, without a spark. This allows more performance, efficiency and lower emissions, without engine damage. Proper octane fuel burns smoothly, starting at the point of ignition, and provides even power. This is why they specify higher octane fuel in high-compression engines.
As ignition occurs the burning of fuel is not instantaneous. It starts at the point of ignition and travel across the combustion chamber. Consequently, the spark plug fires before the piston is completely up. The burning fuel also heats the unburned fuel in its path. This further increases the pressure. Combustion reaches a peak, as the piston is moving back down the cylinder.
What is engine detonation?
Detonation is another malfunction in the combustion process. With detonation, the spark ignites the mixture as normal. The combustion heats and compresses the unburned fuel, but it explodes rather than burning smoothly. This produces a knock in the engine and causes severe damage, similar to pre ignition. Higher octane fuel helps prevent detonation, by controlling the burn rate of the fuel.
The octane does not increase performance, it allows the things that do increase performance to operate properly.
Iso-octane may be added to fuel to increase the octane rating. A higher octane rating is NOT a measure of higher fuel quality. Low quality fuel can have a high octane and a high-quality fuel may have a lower octane. A higher octane rating will not increase the performance of an engine, which they do not design to use it.
In the past, lead was added to increase the rating. Ethanol added to gasoline also slows the burn-rate of the fuel. The additives, detergent-content and cleanliness determine the quality of gasoline, not the octane rating.
Modern engines use variable cam timing and direct gasoline injection to allow even higher compression, with the fuels that are available. These technologies can more precisely control the combustion chamber temperature, although they come at an elevated cost.
What happens when using lower octane fuel?
Modern engines use knock sensors to detect pinging and detonation. They will try to tune the engine to prevent these problems when they can. For instance, when knocking occurs, the computer may retard the ignition or valve timing. It may also try to add additional fuel or add exhaust to the intake air with the EGR system. These steps can help prevent damage, but also decrease performance and economy.
Using a lower octane fuel than specified defeats the design of the engine. It is similar to buying a lightning fast computer and connecting to the internet with a dial-up modem.
Older vehicles cannot adjust and may ping and detonate. If the driver hears the rattling sound of pre ignition on acceleration, damage is being done. Unfortunately, human senses cannot detect all pre ignition and detonation. If any doubt exists, using the higher octane is much less expensive than engine damage.
Other vehicles list premium fuel as an option. For example the owner’s manual may state 87-octane is okay but they prefer 91 or higher. With these vehicles a bit of math is handy. Checking the fuel mileage and performance with different fuel grades will give the miles per gallon. With our miles per gallon cost calculator, seeing is easy if the higher octane fuel pays for itself or not. The same thing can be done with a vehicle listed for 87-octane. An engine will sometimes get enough improvement in mileage to make the extra cost worthwhile.