Rick's Free Auto Repair Advice

Best gas

Which company makes the best gas?

I often hear people say that Chevron, BP, Mobil, etc. make the best gas or that one brand of gas provides the most power or best gas mileage. Baloney! Gasoline isn’t what you think; it’s a blend of over 150 different components and each manufacturer have their own recipe. Plus, the recipe changes based on the market price of raw materials and the season. I’ll explain the differences in gas and why one brand may perform better in one particular vehicle;but that doesn’t mean that same gas will improve performance or MPG in another vehicle.

What is gasoline?

There is no such thing as pure gasoline. Gasoline is formulated differently by every company to cover a wide range of operating temperatures, altitudes, and climate conditions.  Gasoline is a combination of over 150 different chemicals and each brand has their own proprietary “recipe” for which chemicals they use and in what proportions. In addition, each company changes their blend as raw material prices fluctuate. Plus, they change their blends based on the season and the altitude where it’s sold. Winter blends must have higher volatility to enable vaporization at cold temperatures. So the makeup of each company’s gasoline is changing constantly. In other words, there is simply no best gas that works well in all makes and models, engines and in all seasons.

In technical terms, gasoline is a fuel made from hydrocarbons with one to fifteen carbon atoms along with many other components and additives. The exact makeup depends on the type of crude oil available, which refinery equipment is available in that particular market area, and the local product demand. Here’s an example of what type of hydrocarbons are used to make gasoline.

The base components are:

N-Parrafins Alkanes 15%: butane, pentane, hexane, heptane, octane, nonane, decane

ISO-Parrafins 30% to provide higher octane values:  methyhexane, dimethylpentane, trimethylbutane

Cycloparaffins up to 12% very high in octane value: cyclopentane, cyclohexane, methylcyclopentane

Olefins and Diolefins (Alkenes) up to 8% to reduce ozone/smog and reduce engine deposits, gums, lacquers

Aromatics (Arenes) 25%-35% for energy content and high octane rating: toluene, benzene

Alcohol up to 10% to boost oxygen and octane but lowers energy content

Additives: anti-knock, anti-oxidants, metal deactivators, anti-rust, deposit control, anti-icing, upper cylinder lubricants, detergents and dye.

best gas contains a variety of alkanes

What is octane rating all about?

The common misconception about the internal combustion engine is that once the spark plug fires to ignite the air/fuel mixture, the combustion process is an explosion. Nothing could be further from the truth. Explosions are uncontrolled and erratic burns that reduce power and destroy engine components.

A proper combustion process, on the other hand, proceeds in an orderly and controlled fashion, starting with the spark event and spreading in smooth waves throughout the combustion chamber as it burns the entire air/fuel mixture. It’s like spreading ripples in water after tossing in a rock. A controlled burn provides a steady and smooth downward push on the piston.

Abnormal combustion is caused by detonation and pre-ignition

Gasoline can ignite in abnormal ways, causing an normal combustion with best gas octaneexplosion or uncontrolled burn in the combustion chamber. Abnormal combustion is caused by detonation or pre-ignition. Unfortunately, the two terms are so closely related that it’s hard to distinguish between them by sound alone. Yet each can lead to major engine damage.

What causes detonation?

Detonation is ignition caused by a rapid rise in cylinder pressure which causes the fuel to auto-ignite and burn in an uncontrolled fashion, causing “knock”.  The detonation from low octance gasknock is the sound of the violent explosion and can cause piston and ring damage, ring groove wear, cylinder scoring, and damaged head gaskets.

Detonation can be caused by:

Low octane fuel, Lean fuel mixture, Carbon deposits, Lugging, Improper ignition timing, modification of the compression ratio

What causes pre-ignition?

Pre-ignition is the ignition of the air/fuel mixture by a heat source other than the spark plug. Glowing carbon buildup, an overheated spark plug insulator or the glowing edge of a head gasket ring causes a flame front to begin before the spark plug fires. The combination of flame front ignited by a heat source and a second flame front started by the spark plug results in two colliding flame fronts. Like detonation, pre-ignition causes noise (pinging), loss of engine power and engine damage.

Pre-ignition can be caused by:

Carbon deposits in the combustion chamber, incorrect spark plugs that have too hot a heat range, spark plugs not properly seated or torqued, sharp edges in the combustion chamber, improper valve opening due to wear, overheated engine, crossfire caused by spark plug wire induction.

What part does octane play in reducing detonation knock?

As mentioned above, the primary cause of detonation is the ignition and uncontrolled burn of the air/fuel mixture caused by a rapid rise in cylinder pressure and auto-ignition of the fuel prior to the spark event. Using a gasoline with a higher octane rating prevents detonation.

Higher octane resists detonation, so gasoline’s octane quality is rated on an Antiknock Index (AKI) established by ASTM. The ASTM establish two measurement methods; the Research Octane Number (RON) and Motor Octane Number (MON). The two methods are added and divided by 2 (R+M)/2. The car maker determines the proper AKI number based on the engine design.

The RON method relates to low to medium engine speed knock and gas pump octane symbolengine run-on (engine continues to run after ignition is turned off—often called dieseling. The MON method relates to high speed and part-throttle knock. RON is typically 8 to 10 number higher than MON, so a gasoline with an AKI of 87 might have a MON of 82 and RON of 92. Since drivers typically do both low-to-medium speed and high speed and part-throttle driving, (R+M)/2 is a pretty good indicator that the fuel’s AKI will works to prevent detonation in all driving conditions.

A fuel with a higher octane rating or AKI will NOT provide more power or high MPG than the fuel AKI recommended by the car maker.

Octane is nothing more than a measure of the fuel’s anti-knock quality

To reach the recommended AKI, each company is free to use whatever blend of raw materials they wish, so one brand of fuel may have a slightly higher energy content in one part of the country based on prevailing temperatures and altitude, but you cannot and should not extrapolate the engine performance of that particular brand to apply to all vehicles or that brand of fuel in other parts of the country.

Can you get better gas mileage with high octane? Read this

How does a fuel’s volatility affect performance?

Gasoline is a liquid but to burn properly it must be atomized and then vaporized. Carburetors and fuel injectors convert the liquid gasoline into fuel atomized fuel droplets, but the atomized fuel must then vaporize in order to burn properly. The fuel’s ability to vaporize at different temperatures is referred to as its volatility.

To vaporize quickly in winter, winter fuel blends have higher volatility. To achieve that volatility, the fuel contains more of the butanes and components that vaporize quickly. However, if that same combination of highly volatile components is used in summer, it can cause starting and performance problems.

If gasoline volatility is too low:

The engine won’t start when cold and will have poor warm up performance. It will also have poor cold weather driveability (not responsive to acceleration, etc). It will also build up more crankcase, combustion chamber and spark plug deposits. In carbureted engines, low volatility can result in unequal fuel distribution to the cylinders. Finally, low volatility in cold weather can result in increased exhaust emissions.

If gasoline volatility is too high:

Too much of the gasoline will vaporize in the gas tank and quickly fill the charcoal evaporative emissions canister. That can cause refueling problems and rough engine operation during the purge cycle. High volatility can also cause vapor lock and decreased fuel economy.

Seasonal gasoline blends

Fuel volatility must vary during the seasons based on the prevailing temperatures and altitude where it is to be sold. So you may conclude that one brand qualifies as the best gas during summer operation, but when that brand changes to a winter blend, you may no longer feel it’s the best gas for your vehicle.

So which company makes the best gas?

If you’ve learned anything so far, it’s that each company has their own recipe and that recipe changes. So you may find the Chevron (for example) works best in your 2010 Toyota Camry because it provides the best starting, pick-up and MPG. Great. But that same gas in a different engine may not work the same way. And, when Chevron changes to a winter blend, it make not work as well in your Camry. In other words, there is no one best gas for all cars and trucks. If it works best in your particular car or truck, then it’s best for YOU and you alone. However, this leads us to the question of whether you should use Top Tier gas. To learn more about that, read this post

Factors that affect Fuel economy

From the Office of Mobile Sources, U.S. EPA, Fuel Economy Impact Analysis of RFG, August 1995

(Document EPA 420-F095-003).

Outside temperature: Fuel economy drops by 5.5% to as much as 13% when outside temperatures drop from 77°F to 20°F.

Idling/warming up your car before driving can decrease fuel economy by up to 20%.

Driving into a head wind can reduce fuel economy by 2.3% to 6%.

Uphill driving (7% grade) can reduce fuel economy by 1.9% to 25%.

Poor road conditions (gravel, curves, slush, snow) can reduce fuel economy by 4.3% to 50%.

Congested traffic reduces fuel economy by 10.6% to 15%.

Highway speed driving (70-MPH versus 55-MPH) reduces fuel economy by 25%.

Hard acceleration reduces fuel economy by 11.8% to 20%.

Low tire pressure reduces fuel economy by 3.3% to 6%.

Air conditioning can reduce fuel economy by 21%

©, 2017 Rick Muscopat

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Posted on by Rick Muscoplat


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