Rick's Free Auto Repair Advice

Is premium gas better

Is premium gas better for my car?

I often see forum members state that they add premium gas to their cars to get “better performance.” Sorry, but adding a higher octane premium gas to an engine that’s designed to run on regular 87-octane, does not give you better performance. The assumption people make is that premium gas has more “power”. NOPE.

The difference between premium gas versus regular gas is just about it’s octane rating

One of the laws of physics is that you create heat when you compress a gas. In a high compression engine, higher compression creates quick heat buildup that can cause a regular gas to ignite before the spark and create a problem called detonation. Detonation, occurs when the air/fuel mixture ignites BEFORE the spark plug fires. The combustion begins and the burning gases expand and exert downward force on the piston as it’s being forced up by the crankshaft. This happens before the piston reaches the top of it’s travel. Then the spark plug fires, creating a second flame front. The result is a metal-to-metal collision that makes a knocking sound. If it continues, knocking can destroy your engine.

The fuel’s octane rating represents the fuel’s ability to resist early combustion and begin only when lit by the spark. 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 regular grade gas is used 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 or ping you hear is the force of the combustion slamming against the piston while the piston is still moving upwards on its compression stroke.

Higher octane is what prevents detonation

Gasoline is a blend of over 150 different ingredients in different proportions based on the brand’s secret formula. The exact chemicals and proportions can vary from brand to brand, but the fuel’s available energy for combustion is relatively the same between brands. So the higher octane rating in gasoline 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 minor MPG differences between one gasoline brand and another.

To prevent detonation in early engines, refiners added tetraethyl lead as an antiknock additive. That’s now illegal. So they use other detonation reduction products like:

Benzene, xylene, and toluene

Alcohols: ethanol, methanol, tertiary butyl alcohol (TBA)

Propane and butane The increase in volatility caused by the added propane and butane often leads to hot-weather driveability problems.

When blended into gasoline these chemicals create a final product with more bonds between carbon atoms and it’s those extra bonds that reduced the fuel’s tendency to ignite from the heat of compression.

What is gasoline?

Gasoline is not a single fluid. It’s a mixture of different fluids and additives, each with different chemical properties and purposes. The different fluids ensure that gasoline flows freely in different weather conditions, ignites properly with a controllable flame front, doesn’t form deposits, doesn’t foam or vaporize, and cleans fuel injectors.

Gasoline is made with Hydrocarbons like:

Methane (1 carbon atom), Ethane  (2 carbon atoms), Propane (3 carbon atoms), Butane  (4 carbon atoms), Pentane  (5 carbon atoms),

Hexane (6 carbon atoms), Heptane (7 carbon atoms), Octane (8 carbon atoms),

And Additives like:

Detergents/deposit control additives to remove fuel deposits, Anti-icers to prevent fuel line freeze up, Fluidizer oils to reduce or prevent deposits on intake valves, Corrosion inhibitors to prevent gas tank, fuel link, injector, and engine corrosion, Anti-oxidants to slow gasoline degradation and prevent the formation of gum and varnish, Metal deactivators to prevent the movement of metal ions, and Lead replacement additives to reduce exhaust valve wear.

Each refiner uses their own recipe to blend gasoline. And, they change the makeup of these blends in the summer and winter. Summer grade gasoline typically ranges from 113,000 British thermal units (btu) to 116,000 btu. Winter grade gasoline must be more volatile to ensure proper startup and prevent cold wear drivability problems, so it has slightly lower energy content, ranging from 108,500 btu to 114,000 btu. Combined with the fact that a cold engine needs a richer fuel mixture to start and warm up, the lower btu content of winter results in lower mpg.

The differences between one brand of gas and another

Depending on the area of the country, a major brand may operate their own local refinery, buy their gas from a locally owned and operated refinery, or buy their gas from a terminal supplied by a pipeline from a far away refinery. If you have a local terminal and see trucks with many different brand logos on the sides of trucks entering and leaving, they’re all getting the same gas. What’s different, however, is the brand’s special additives that are blended into the gas as the truck is filled. The quality of the additives varies by brand, with Top Tier qualified brands adding the best additives, and lower quality gas receiving the lowest quality additives.

MPG differences between brands

Lots of people claim they get much better gas mileage by using one brand over another. I question that claim because the exact difference in energy content between brands is minor. Generally speaking, those differences in energy content only account for 1-MPG in a best case scenario. Even if you do see in increase in MPG in your vehicle, you cannot make broad claims that your experience works can be applied to other vehicles.  First, the brand that works best in your engine and provides more MPG , may not provide more MPG in a different engine design. Also, the brand you buy in your city may be made by the brand’s local refinery, while that same brand in another city made be provided by a different refinery using a different formula–or it may be provided by pipeline from a far-away refinery. Finally, MPH depends on many different factors l like engine design, engine condition, time of year (winter or summer blends), and an individual’s driving habits that no-one can make a broad claim that one brand gets better MPH for everyone in every engine and under every driving condition.

Does premium gas do a better job of cleaning your engine

No. That’s a myth. The cleaning additives between regular and premium gas is exactly the same. It’s different between brands but not between regular and premium of the same brand.

Does Premium gas contain ethanol

Yes. 95% of all gasoline (regular and premium) is required to contain ethanol. Unless you buy your gas from a supplier that specializes in selling non-oxygenated gas, you’re getting ethanol in your premium.

Can I use regular in my car that requires premium?

You can, but you’ll see reduced performance. Your engine requires premium because it has a higher compression ratio. When you fill with regular gas, your engine will experience detonation. The knock sensor in late model cars detects the knock and the engine computer alters spark timing so it occurs closer to the top of the piston’s travel to prevent a second flame front from colliding with the front ignited during detonation. Retarding spark timing reduces pedal responsiveness, performance, and gas mileage. But every engine control computer has a limit as to how much spark timing they can provide. If you fill with regular gas and your car’s computer has retarded spark timing by the maximum amount yet the engine still experiences detonation and knock, the computer will set off the check engine light and record a trouble code. Once you refill with premium, the computer will slowly advance spark timing to achieve better performance and MPG.

Recommended gas versus required gas

If a car maker recommends premium gas and you fill with regular, you’ll experience performance and MPG loss. However, if the car maker requires premium gas and you fill with regular, you’ll not only experience performance and MPG loss, but incur a check engine light as well.

Want to learn more about gasoline? Download this article

Want to know what happens when you add 87-octane gas to an engine that requires 93-octane? Read this article

Getting back to Octane

* If the engine has significant carbon buildup and that buildup causes pre-ignition in an engine equipped with a knock sensor, the computer will retard timing or adjust air/fuel mixtures to minimize knock. That can reduce overall power. In that case, a premium higher octane fuel could provide better performance. However, that only occurs because the carbon buildup inside the engine has reduced it’s performance.

©, 2013 Rick Muscoplat

Posted on by Rick Muscoplat

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