Q. What is octane?
A. Octane, or Octane number, reflects a gasoline's antiknock quality. It's a measure of the ability of gasoline to resist knocking when it is burned in an engine. Laboratory testing that determines a gasoline's octane number involves burning the fuel in a single-cylinder engine under different conditions to yield a Research octane number (RON) and a Motor octane number (MON).
Q. My car owner's manual says that I can use regular unleaded gasoline. Is it true that a car engine's octane requirements can increase over time?
A. Yes, and Chevron Supreme and Plus gasolines have high octane that can help maximize performance in cars with
increased octane number requirements. Over time, deposits can accumulate in your car's combustion chambers.
These deposits can cause knocking and pinging, or reduced performance, particularly if your car has driven over 24,000 kilometers.
A higher octane number gasoline can help you reduce or eliminate these problems by improving your engine's combustion process.
While some of these engines may be satisfied by the octane offered by our Plus grade, others may require the even higher octane of Chevron Supreme or Supreme Plus. So if combustion chamber deposit build-up is affecting your car, Chevron Plus, Supreme, and Supreme Plus with Techron® can help.
- Use a gasoline with a higher antiknock index (AKI)
- Treat the gasoline with a bottle of Techron® Concentrate Plus. If there are combustion chamber deposits in the engine, the treatment will reduce them, which, in turn, may lower the octane number requirement of the engine. This may decrease the engine's tendency to knock until these deposits reform.
Q. How much octane do you need?
A. If you have an older car or a high performance car, you may need a higher octane gasoline to help prevent engine knocking.
Your driving conditions can increase your car's octane requirements. For example:
- Do you make a lot of short trips?
- Do you drive in high temperatures?
- Do you carry or pull heavy loads?
- Do you drive in a low altitude area?
If you answered yes to any of the above, your vehicle may perform better with a higher grade of Chevron with Techron®. If your engine is knocking or losing power on a lower octane fuel, in most cases, switching to a higher octane fuel can help.
Q. What octane gasoline should I use in my vehicle?
A. For starters, use a gasoline with the octane number (AKI) recommended by your owner's manual. Using gasoline with an antiknock rating higher than that required to prevent knock will not improve a vehicle's performance, including its power, unless the vehicle is equipped with a knock sensor and the car's computer is controlling the engine to reduce or eliminate knock on the lower octane grade fuels. Many late model high-performance engines fall into this category.
There are two reasons why your vehicle might knock on a gasoline with the AKI recommended by the owner's manual:
- The engine is at the upper end of the octane number requirement range.
- Combustion chamber deposits in your engine are higher than usual.
Q. What will happen if I use the wrong octane gasoline in my vehicle?
A. Using a gasoline with an AKI lower than required by your vehicle can cause the engine to knock or lose power. If the engine is equipped with a knock sensor and the car's computer is controlling the engine to reduce or eliminate knock, using a gasoline with an AKI lower than that required by your vehicle will decrease the vehicle's power and acceleration. Using a gasoline with an AKI higher than that required by your vehicle will not improve its operation.
Q. What determines my car's octane requirements?
A. Your car's octane requirements are mainly determined by its basic design. In addition, variations in engines due to manufacturing tolerances can cause cars of the same model to require a different octane of several numbers. Also, as a new car is driven, its octane requirement can increase because of the buildup of combustion chamber deposits. This continues until a stable level is reached, typically after about 24,000 kilometers. The stabilized octane requirement may be 3-6 numbers higher than when the car was new. Premium or mid-grade fuel may be advisable to prevent knock or loss of power.
Other factors also influence your car's knocking characteristics:
- Temperature – Generally, the hotter the ambient air and engine coolant, the greater the octane requirement.
- Altitude – The higher the altitude above sea level, the lower the octane requirement. However, modern computer-controlled engines adjust spark timing and air-fuel ratio to compensate for altitude changes, and thus the effect of altitude on octane requirement is smaller in these vehicles.
- Humidity – The drier the air, the greater the octane requirement. The recommendations that vehicle manufacturers give are for normal- to low-humidity levels.
- Engine spark timing – Octane requirements increase as spark timing is advanced. Both the basic setting of the spark timing and the operation of the automatic spark advance mechanisms are important in controlling knock. In some computer controlled engines, the spark timing can only be changed by replacing modules in the computer. If they are equipped with knock sensors, these computer controlled engines have the ability to retard the ignition temporarily when a sensor detects knock. This temporarily reduces the octane requirement and may also temporarily reduce vehicle performance.
- Method of driving – Rapid acceleration and heavy loading, such as pulling a trailer or climbing a hill, may result in a greater octane requirement. Stop-and-go driving and excessive idling can increase octane requirements by causing the buildup of combustion chamber deposits.
- Malfunctions of emission control systems – An improperly functioning emissions control system can affect the octane requirement by changing the air-fuel mixture or by not providing dilution gases through the exhaust gas recirculation (EGR) system. If a malfunction occurs, your vehicle should be taken to a qualified vehicle service mechanic. Some problems are indicated by warning lights on the driver's instrument panel.