I work for an oil company and literature from our company's engine lab
basically states that you should use the minimum octane gasoline that
doesn't cause your engine to knock. Octane is just a measure of
resistance to knocking, so if you car is knocking excessively, then go
up a grade of gasoline. As the car gets older the octane requirement
will creep up due to tolerance changes and deposit build-up.
For a more detailed write up on octane requirements here is a reprint
from an article from the American Petroleum Institute.
A Consumer's Guide:
Gasoline Octane for Cars
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, 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.
> ----------
> From: Thorn[SMTP:tthorn@comp.uark.edu]
> Reply To: Thorn
> Sent: August 31, 1998 5:48 AM
> To: spitfires@autox.team.net
> Subject: gasoline for 1500
>
> I've been told by a local Spit mechanic that regular unleaded gasoline
> (87 octane) is "okay" for my unmodified 78 Spit. Right or wrong????
>
> harve
>
> TThorn@comp.uark.edu
>
>
|