I just finished reading a very long thread in one of the news groups I follow, about answering the all time question; "What octane should I use in my……….?"

I guess there are a lot of people out there who have no clue just what octane is and how it affects their engine or its performance. Reading through the thread it reminded me of several conversations I have had with other riders and my own experimentation. So if you're one of those folks who don't know or you think you know here are some facts.

Several friends who ride Harley's where also confused about this, they assumed the higher cost meant better fuel, and thought they were buying the best quality they could. But a stock EVO Harley with its 7 to 1 compression ratio; burning all of that expensive fuel did nothing for mileage or performance. Some sport bikes will run on regular grade fuel 87 octane, my little Honda CBR 600F3 with it's 12 to 1 compression ratio requires 91 octane but seems to run just fine on the lower 87 grade, however my Yamaha R1 also requires the 91 octane fuel and will run poorly on less.

I ran my Fat Boy on mid grade or premium grade fuels because it pinged under acceleration, I also advanced the timing about 5 degrees. It never seemed to lack for power. Gas mileage (MPG) was never what it should have been. During a ride up to the Blue Ridge just before the part where the road starts climbing I stopped and filled the tank with 93 octane fuel. All the way up the mountain the engine pinged under load and not until we fueled up the next day did it stop. In this case it was the quality of the fuel not the grade which was at fault.

Ok you just got the bike and don't have an owner's manual because you bought it used. You could try calling any of the dealerships for the brand you bought and ask the question, or you can fill it up with 83 octane and ride it if it pings step up a grade for the next fill up. Most cars and motorcycles are designed to run on regular grade fuel, only the most exotic cars and bikes will require premium fuel.

OK, OK, I'm not a petro chemist so don't take my word for it read on…..

How is gasoline made?

Gasoline is manufactured from crude oil through a series of physical and chemical processes in an oil refinery. The end product, a blend of several refinery streams containing hundreds of chemical compounds called hydrocarbons, must meet over 20 different requirements to be considered a suitable gasoline for use by a wide range of vehicles.

Rigorous quality control tests are conducted prior to releasing the gasoline into the market place. In some cases, additives are included to improve specific properties of the gasoline.

What is octane?

A gasoline's octane number is a measure of its ability to resist knocking as it burns in the combustion chamber of an engine. A spark from the spark plug starts normal combustion. The flame travels across the combustion chamber rapidly and smoothly until all the fuel is consumed. Abnormal combustion occurs when part of the fuel/air mixture ignites spontaneously and burns very rapidly, causing the pressure to rise suddenly. This results in a metallic knocking or pinging sound. A gasoline's ability to resist knocking is called its anti-knock quality. The octane number of a gasoline indicates the anti-knock quality of the fuel. A gasoline's octane number is determined by comparing its resistance to knocking to the performance of reference fuels in a test engine.

What octane does my vehicle need?

To determine your vehicle's octane requirement, look at the manufacturer's recommendation in your owner's manual. Most auto manufacturers recommend 87 octane gasoline, as measured by the (R + M) / 2 method on a test engine under defined operating conditions. If the vehicle knocks on the recommended grade, a higher octane grade should be selected. Some foreign vehicle manuals recommend a Research Octane Number (RON) instead of the more common octane rating that appears on most gasoline pumps. As a rule of thumb, the recommended octane rating can be determined by subtracting four (4) from the recommended RON number. A vehicle that calls for "91 RON" should use 87 octane gasoline (as measured by the (R + M) / 2 method). Using a higher grade than is required will not usually increase performance. However, if the vehicle is equipped with knock sensors, as many late model vehicles are, a higher octane grade may enhance performance.

Why does my vehicle require a higher octane gasoline than is recommended in my owner's manual?

There are two reasons. First, the engine may be at the upper end of the octane requirement range for the particular model. It may, therefore, knock during periods of heavy engine load. The octane requirement for each engine in vehicles of the same make is different because of variations in manufacturing tolerances. Industry testing has shown that this difference can range between two and five octane numbers. While you will not hear knock, the spark timing will be retarded, resulting in loss of power and performance.

The second reason for using a higher-octane gasoline than recommended is because the equilibrium level of combustion chamber deposits is higher than average resulting in a higher than average octane requirement for the engine. Industry testing shows that the octane requirement increase (over the initial 20-50,000 kilometres) can be as much as nine octane numbers. This means that a vehicle that was originally designed for 82 octane fuel may eventually require 91 octane fuel to perform satisfactorily.

Petro-Canada's premium gasoline, SuperClean*, contains a higher level of Tactrol*, our exclusive deposit control additive, than our other grades and will help clean up deposits left by lesser gasoline brands.

Will higher octane improve a vehicle's fuel economy?

Generally, higher octane will not improve fuel economy. In fact, the difference in composition or heating value of different grades of gasoline plays a small role in fuel economy. Good fuel economy is a result of vehicle design and weight, good vehicle maintenance and driving style.

How can I improve my fuel economy?

Vehicle type, driving style, good vehicle maintenance and driving conditions have the greatest effect on fuel economy. Driving for best fuel economy involves:

  • Smooth, steady acceleration rather than "jackrabbit" starts
  • Maintaining moderate speeds
  • Avoiding heavy loads
  • Avoiding a luggage rack or towing a trailer unnecessarily
  • Avoiding using an air conditioner or heater unnecessarily
  • Not idling the engine when it could be switched off (generally not longer than 10 seconds), and
  • Avoiding short trips where the engine does not have the opportunity to fully warm up.

Maintenance factors include:

  • A properly tuned engine
  • Clean air filters
  • Aligned and balanced wheels
  • Tires with the correct air pressure
  • Periodic changing of the fuel line filter (in older vehicles)

Driving conditions that can reduce fuel economy include:

  • Cold temperatures
  • Head winds
  • Driving up hills
  • Water
  • Slush
  • Snow on the road

How significant is the impact of vehicle type, driving style, good vehicle maintenance and driving conditions on my fuel economy?

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.

View More Questions and Answers from API

Click here to order API Publication 1580, Sixth edition, Gasoline Questions & Answers for Your Car.

The facts on High Octane Gasoline

Do you buy a high octane gasoline for your car because you want to improve its performance? If so, you should note: the recommended gasoline for most cars is regular octane. In fact, in most cases, using a higher octane gasoline than your owner’s manual recommends offers absolutely no benefit. It won’t make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner’s manual.

The only time you might need to switch to a higher octane level is if your car engine knocks when you use the recommended fuel. This happens to a small percentage of cars. If it was designed for regular gas, but knocks, often that means you need a tune up.

Unless your engine is knocking, buying higher octane gasoline is a waste of money, too. Premium gas costs 15 to 20 cents per gallon more than regular. That can add up to $100 or more a year in extra costs. Studies indicate that altogether, drivers may be spending hundreds of millions of dollars each year for higher octane gas than they need.

What are octane ratings?

Octane ratings measure a gasoline’s ability to resist engine knock, a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders. Most gas stations offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane) and premium (usually 92 or 93). The ratings must be posted on bright yellow stickers on each gasoline pump.

How is octane rating determined?

Gasoline is subjected to two testing methods to establish its octane rating: one, called the motor method, runs the gasoline in an engine under load; and a second, the research method, runs the gasoline in a free running engine. The research method gives slightly higher ratings, and the octane number displayed on the pumps is an average of the two methods.

What’s the right octane level for your car?

Check your owner’s manual to determine the right octane level for your car. Regular octane is recommended for most cars. However, some cars with high compression engines, like sports cars, old cars and certain luxury cars, need mid-grade or premium gasoline to prevent knock.

How can you tell if you’re using the right octane level?

Listen to your car’s engine. If it doesn’t knock when you use the recommended octane, you’re using the right grade of gasoline. Knock occurs when cylinder pressures are high. It is normal for an engine to ping a little at full throttle because cylinder pressures are very high at full throttle. Engine knock, however, should not be ignored since it can result in serious damage to the engine. High octane gasoline burns slower than low octane gasoline. The slow burn prevents engine knock when cylinder pressures are high.

If your engine runs well and does not knock or ping on low octane gasoline, there is no advantage in switching to higher octane gasoline.

If your engine knocks or pings, it does not necessarily mean something is wrong with the gasoline. It could be a problem with the car's ignition timing or exhaust gas recirculation. On high mileage engines, a carbon build-up in the cylinders can increase cylinder pressures and cause knock.

Will higher octane gasoline clean your engine better?

As a rule, high octane gasoline does not outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car’s engine. In fact, the U.S. Environmental Protection Agency requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your car.

Should you ever switch to a higher octane gasoline?

A few car engines may knock or ping - even if you use the recommended octane. If this happens, try switching to the next higher octane grade. In many cases, switching to the mid-grade or premium-grade gasoline will eliminate the knock. If the knocking or pinging continues after one or two fill-ups, you may need a tune-up or some other repair. After that work is done, go back to the lowest octane grade at which your engine runs without knocking.

Is knocking harmful?

Occasional light knocking or pinging won’t harm your engine, and doesn’t indicate a need for higher octane. But don’t ignore severe knocking. A heavy or persistent knock can lead to engine damage.

Is all "premium" or "regular" gasoline the same?

The octane rating of gasoline marked "premium" or "regular" is not consistent across the country. One state may require a minimum octane rating of 92 for all premium gasoline, while another may allow 90 octane to be called premium. To make sure you know what you’re buying, check the octane rating on the yellow sticker on the gas pump instead of relying on the name "premium" or "regular."

Does high octane gasoline improve mileage?

In general, if your car is designed to run on 87 octane gasoline, high octane gasoline will not improve mileage. If switching to high octane gasoline does improve mileage, you might find that a tune-up will give you the same improvement on 87 octane gasoline.

Does high octane gasoline achieve quicker starting?

No, it doesn't.

Does high octane gasoline increase power?

If your car is designed to run on 87 octane gasoline, you shouldn't notice any more power on high octane gasoline. Again, if it does make a noticeable difference, you may need a tune-up.

Is high octane gasoline more refined -- is it just a better product?

Additional refining steps are used to increase the octane; however, these additional steps do not make the gasoline any cleaner or better. They just yield a different blend of hydrocarbons that burn more slowly. The additional steps also increase the price.