Rick's Free Auto Repair Advice

Can I get Better mileage with higher octane?

Will premium gas provide better MPG and power?

People usually ask this question when they’re hoping to get better mileage with higher octane even though their engine is rated for regular gas. If the engine is in good condition, with no carbon deposits or knocking, the answer is a resounding NO. You will NOT get better MPG or more power.

If your car requires premium and you fill with premium, you’ll get the performance the car was rated for.

High octane or “premium gasoline” is one of the most misunderstood fuels. So I’ll explain the myths and legends surrounding it.

Premium is usually listed on the pump as 91 or 93. Whoever coined the phrase “premium” was a marketing genius because it implies you’re getting a better gasoline, or gas that’s more powerful. It’s neither.

In order to understand the differences between regular and premium grades of gas you first must understand these two basic concepts; gasoline is a blend of different substances, and what causes engine “knock.”

Understanding what causes engine knock

Every engine compresses an air/fuel mixture and ignites it with a spark. The resulting combustion expands rapidly, thereby pushing the piston against the crankshaft to create power and engine rotation.  The spark is timed to fire slightly BEFORE the piston reaches the top of its travel on the compression stroke so the combustion reaches full force right AFTER the piston reaches the top of travel (called top dead center TDC). That system works fine in engines that are in good operating condition with no carbon deposits.

Now a short physics lesson

Any time you compress a gas, you create heat. A high compression engine compresses the air/fuel mixture more than a lower compression engine so it creates more heat. When you use a regular grade gas in a high compression engine, the heat of compression can ignite the gas BEFORE the spark plug fires. That creates a problem.

Regular fuel in a high compression engine creates detonation

When a fuel begins combustion before the piston reaches the top of its travel and before the spark plug fires, the gases expand and start pushing down on the upward moving piston. This early ignition caused by the heat of compression is called DETONATION. Then, when the spark plug fires the rest of the air fuel mixture ignites, creating a second flame front. The two flame fronts collide in a violent action, creating an uncontrolled burn and extreme downward pressure. This creates an audible knocking sound called, ah, KNOCK. The whole process of detonation and knock robs the engine of power, and if allowed to continue, can destroy the engine.

What is an octane rating

A fuel’s octane rating represents the fuel’s ability to resist DETONATION and ignite only from the spark plug. When used in a high compression, engine premium high octane fuel ignites on time and provides a controlled burn. Here’s an example:

When an engine fires the spark plug, the combustion begins as a kernel and the flame front moves smoothly across the combustion chamber, traveling at 45 to 90-mph. Of course, the speed depends on the air/fuel mixture and the design of the combustion chamber. Late model engines with high-swirl combustion chamber designs may have faster airflow and thus faster flame spread. However, when you use regular grade gas in a high compression engine and detonation occurs, the gases expand rapidly and the flame front can move over 700-mph. That kind of rapid expansion increases combustion temperatures and can damage pistons, gaskets, valves, and cylinder heads. The metallic knock you hear is the force of the combustion slamming against the piston while the piston is still moving upwards on its compression stroke.

Then there’s pre-ignition. It’s caused by hot carbon deposits in the engine that ignite the air/fuel mixture. The result is the same as detonation–it causes knock. But it’s not caused by ignition due to the heat of compression.

How does an engine compensate for detonation and pre-ignition

If you own a late model car or truck (1996 or later) the engine management system most likely uses a knock sensor. The knock sensor detects the vibrations caused by metal-to-metal contact and informs the engine control module (ECM). The ECM then retards ignition timing so the spark event occurs closer to the top of the piston’s travel. So it delays the formation of a second flame front to cause the least amount of knock and damage.

Retarding spark timing reduces knock, but robs the engine of power. It also reduces gas mileage. However, there’s a limit to how much the ECM can retard spark timing. If the ECM still detects knock once its reached its limit, it will turn on the check engine light.

If your vehicle is older (pre-1996) it mostly likely doesn’t have a knock sensor, so the ECM can’t adjust timing due to knock.

Premium and mid-grade gas can eliminate knock and pre-igntion in engines with carbon deposits

If your engine has severe carbon deposits, the ECM has reached the limit of how much it can retard spark timing, the check engine light is on and the code is related to continuing knock, filling with a mid-grade or premium grade fuel is one way to eliminate knock and restore power and MPG.  It doesn’t solve the problem of the carbon deposits, but it does eliminate the side effects.

There are other ways to restore performance in an engine with carbon deposits.

• Buy gasoline from a station that sell Top Tier gas. Top Tier gas contains the best detergent additives and can help clean fuel injectors and remove some combustion chamber deposits.

• Add a fuel system cleaner (Chevron Techron is one very good product) to your tank. chevron techron fuel system treatmentFollow the instructions on the bottle. Make sure the fuel system cleaner you buy contains polyisbutylamine (PIBA) and polyetheramine (PEA), the only two proven cleaners.

• Pay a shop to perform a de-carbonization procedure.

Understanding the AntiKnock Index (AKI)

You get optimum engine performance by filling your vehicle with a gasoline that ignites only from the ignition spark and spreads in a predictable way. You want to avoid compression-induced and combustion-chamber deposit-induced ignition and out of control flame spread that result in knock.

Refiners follow two testing methods to determine a fuel’s anti-knock rating. These are referred to as the Research Octane Number (RON) and the Motor Octane Number (MON). Refiners determine the RON and MON results, add them together, and divide by two to get the AKI. The RON number affects knock that occurs at low to medium speeds, and engine run-on after shutting off the engine. MON affects knock at higher speeds and part throttle conditions. If the MON rating is too low, you’ll experience knock during power acceleration during passing or climbing hills.

When a car maker specifies a particular “octane rating” they’re talking about RON+MON÷2. So a car equipped with a low compression engine and a high swirl combustion chamber design may specify a “regular” fuel rating of 87, while a different high compression engine may require a fuel rating of 93 to prevent knock.

But the octane rating has NOTHING to do with the fuel’s overall energy content or its performance. In other words, using a higher octane gas in an engine designed to run on regular gas will NOT get you more performance. You may however, see some slight MPG differences between two different gasoline brands with the same octane rating. That’s because each refiner’s recipe differs slightly to attain their gasoline’s octane rating, and some of those chemicals may result in a slightly higher btu content. The difference in btu content from one refiner’s 87-octane gasoline to another 87-octane gasoline is small and may account for an MPG increase of 1-MPG at max.

gas pump signage telling the octane rating of each fuel



Premium gas MYTHS

• Premium gas doesn’t provide more power than regular. The available energy may vary slightly from brand to brand due to the brand’s proprietary gasoline formula. But within the brand, regular and premium have the same energy.

• Premium gas doesn’t have better additives or better detergents. Adding premium will NOT clean out your engine better than regular.

• Premium gas ISN’T always ethanol free unless you buy it from a station that specializes in selling non-oxygenated gas. Over 95% of all gas (regular and premium) sold in the U.S. contains 10% ethanol.

•Premium gas burns slower—nope.

• Premium makes it harder to start when used in an engine designed for regular—nope. A lower compression engine doesn’t produce enough heat during the compression stroke to ignite premium gas. But the spark always provides enough energy to ignite both regular and premium gas.

What is gasoline

Gasoline is not a single substance. It’s made of over 150 flammable hydrocarbon liquids including pentane, hexane, heptane—all refined from crude oil. In addition, all gasoline contains anti-knock additives like Benzene, xylene, and toluene and alcohols: ethanol, methanol, tertiary butyl alcohol (TBA). In winter blends the refiner may also add propane and butane. However, they reduce those two additives in summer because they contribute to hot-weather driveability problems.

Gasoline also contains other additives like;

Detergents to eliminate or remove system deposits

Anti-icers to prevent fuel line freeze up

Fluidizer oils used along with other deposit control additives to control intake valve deposits

Corrosion inhibitors to minimize fuel system corrosion

Anti-oxidants to reduce gum formation in stored gasoline

Metal deactivators to reduce the effects of metal-based components that might occur in gasoline

Lead replacement additives to reduce valve seat wear

Additives amount to a very small percentage of the total product. In fact, refiners add as little as 100-lbs of deposit control additive to treat 20,000 gallons of gas.

Can any car get better MPG with premium?

No. In fact, the EPA mandates that engines certified to run on regular CANNOT show a fuel economy increase of more than 3% by switching to premium. That ruling is designed to prevent car makers from setting factory spark timing too late so that adding premium results in advanced spark timing. In car makers were allowed to alter their software this way, it would create an unnecessary high demand for premium gas.


gas pump sticker showing 93 octane

©, 2015 Rick Muscoplat




Posted on by Rick Muscoplat

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