What is meant by the antiknock performance of gasoline?

The antiknock performance of a gasoline is usually expressed as an octane number that measures its ability to resist knock. "Knock" is a sharp metallic noise resulting from uncontrolled combustion. Knock is just loud enough to be heard may be displeasing, but rarely causes engine damage. Intense, sustained knock may damage pistons and other engine parts. If heavy knocking occurs, a higher octane gasoline should be used and/or spark timing should be adjusted to reduce or eliminate the knock.

Use of a higher octane gasoline to prevent knock can also help prevent "after-run" (dieseling) that is usually caused by piston head and combustion chamber deposits from incomplete fuel burning.

What gasoline antiknock performance does my car require?

Use the recommendation in your car’s owner’s manual as a starting point for selecting the proper octane grade of gasoline. If sustained engine knock is encountered and engine adjustments are correct, try higher octane Super-M® [93 (R+M)/2]. If knocking or dieseling persists, have the engine checked for mechanical problems.

I see an (R+M)/2 octane posted on the gas pump. What is it and what does it mean?

Neither the Research Octane Number ("R") nor the Motor Octane Number ("M") of a gasoline adequately relates to the actual anti-knock performance required for a multicylinder automobile engine. The arithmetic average of the Research ("R") and Motor ("M") Octane Numbers, (R+M)/2, is more closely related to actual engine requirements. This is the octane rating that is posted on service station dispensing pumps. This posted octane number measures the resistance of the gasoline to premature detonation in the cylinder, which can be heard as knock. Super-M®’s (R+M)/2 of 93 is more resistant to knock than Marathon’s 87 octane Milemaker® gasoline.

Should I assume that Super-M® is a "cure-all" for any case of severe knock? Are there other concerns?

A tankful of Super-M® is the first thing to try to prevent severe knock. However, other mechanical conditions may exist that require a mechanic’s attention. If you are still having knocking or other performance concerns, a problem with the spark timing, emission controls, or other systems may exist. You may want to have your engine inspected by a qualified mechanic.

I seem to hear sporadic knocking 2 to 3 times a year during certain seasons. Is there a logical explanation? What can I do to alleviate this?

Climate conditions affect the "octane appetite" of a car’s engine. Knocking is more likely to occur during warm, dry weather, particularly when the barometer is high. These weather conditions often occur on beautiful summer and fall days when the temperature is high and the humidity is low. On these days, you can try Super-M® to prevent the temporary knocking condition.

Is octane the only gasoline quality to be concerned about?

No! All grades of Marathon gasoline ensure:

• Seasonizing - The proper volatility for good starting and overall driveability.
• Engine Cleanliness - Marathon’s PBA-4 cleans dirty fuel injectors and intake valves.
• Quality Assurance - Marathon’s handling procedures stress keeping our gasolines free of contamination from water, dirt, or incompatible fuels.