Specifications

What grade of fuel should I use - Octane
from 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,