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Gasoline (petrol) Guide
(Comsumer and Racing Gas)
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A Consumer’s Guide: Gasoline Octane for Cars – Gasoline Questions & Answers for Your Car – API Publication 1580, Sixth edition, January 1996
Q. What is octane?
A. Octane is a measure of a gasoline’s ability to resist knock or pinging noise from an engine. In older vehicles, knock may be accompanied by engine run-on, or dieseling. Knock is the sharp, metallic-sounding engine noise that results from uncontrolled combustion. Severe knocking over an extended time may damage pistons and other engine parts. If you can hear knocking, you should have your engine checked to make sure it is calibrated correctly and does not have a mechanical or electrical problem, or use a higher octane gasoline.
In most vehicles no benefit is gained from using gasoline that has a higher octane number than is needed to prevent knock. However, in some vehicles equipped with a knock sensor (an electronic device installed in many modern engines that allows the engine management system to detect and reduce knock), a higher octane gasoline may improve performance slightly.
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 15,000 miles. The stabilized octane requirement may be 3-6 numbers higher than when the car was new. Premium or midgrade fuel may be advisable to prevent knock.
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. Modern computer-controlled engines adjust spark timing and air-fuel ratio to compensate for changes in barometric pressure, 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.
Your engine’s spark timing – The octane requirement increases as the 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.
Q. How many grades of gasoline are available?
A. Most places that sell gasoline offer three octane grades of unleaded gasoline–regular at 87 (R+M)/2, midgrade at 89 (R+M)/2, and premium at 93 (R+M)/2. In high-altitude areas such as the Rocky Mountain Region of the U.S., the (R+M)/2 number may be lower by one or two numbers. After January 1, 1996, no leaded gasoline may be sold for highway use.
Q. Which octane grade should I use in my car?
A. Use the recommendation in your car owner’s manual as a starting point for selecting the proper gasoline. If you notice engine knock over an extended time and your engine is adjusted correctly, try a higher octane gasoline. Also, higher octane may provide a performance benefit (better acceleration) in cars equipped with knock sensors. Many late model and high-performance (turbo-charged and supercharged) cars fall into this category.