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Is All Gasoline the Same?

Gasoline filler hole and nozzle inserted into a car for fueling

Gasoline (or petrol, for my British readers) is an energy-dense hydrocarbon fuel used to power most cars and light trucks, consumer-grade power equipment, and piston-engine aircraft.

Most gasoline is made from petroleum using a process called fractional distillation. Specifically, it is a blend of molecular chains C7H16 through C11H24.

Gasoline can also be made from other hydrocarbon-rich feed stocks such as coal, natural gas, and biomass; but those processes are not economically viable as of the time of this writing.

Some motorists are extremely picky about which gasoline they use in their cars, even to the point of planning their travel around the locations of gas stations that sell the brand and grade of gas that they prefer. Other drivers will use gas from whatever station happens to be closest without giving it much thought. Still others believe that all gas is the same and just use the cheapest gasoline they can find.

The truth is that there are very significant differences in gasoline. The gas you choose can in fact affect your car's performance, reliability, maintenance costs, and lifespan.

Let me explain it to you.


Differences in Quality of Gasoline

People who believe that all gas is the same often argue that different gasoline brands draw their gasoline from the same trucks and pipelines. To an extent, they're right. The base gasoline that constitutes the bulk of the finished gasoline product that we pump into our tanks is pretty much the same. Different brands of gas differ more in terms of their additive packages than in terms of the base gasoline itself. That much is true.

The mistake many people make is underestimating how different those additive packages are and how important they are to the engine's performance and lifespan. To say that they're "just additives" ignores the seriousness of the problems the additives are designed to prevent or reduce.

For example, detergents are among the most important gasoline additives. They are added to gas to reduce its tendency to form harmful deposits, especially in the engine's combustion chamber and on its pistons and rings. Carbon deposits around the piston rings can cause lost compression, oil burning, and increased emissions. Deposits on top of the pistons or elsewhere in the combustion chamber can lead to detonation (knocking), preignition, increased stress on the connecting rods and crankshaft, premature wear, and in severe cases, catastrophic engine damage.

Since 1995, all gasoline sold in the United States has had to meet minimum federal standards for detergency known as the "Lowest Additive Concentration." The problem is that the minimum federal standards are way too low to properly protect an engine from harmful deposits. That's why we have:

TOP TIER™ Gasoline

In 2004, a group of auto manufacturers got together to devise a more effective fuel detergency standard than the lame regulations the government had implemented in 1995. Their work resulted in a superior set of standards called TOP TIER™. The use of TOP TIER™ gas is recommended by many car manufacturers due to its proven, dramatic benefits in reducing deposits that can rob your engine of performance and efficiency, accelerate wear, and increase maintenance costs.

TOP TIER™ gas isn't a marketing gimmick. TOP TIER™ gas does in fact cause far less deposit buildup. How much less, you ask? A study conducted by the American Automobile Association in 2016 concluded that "non-TOP TIER™ gasolines caused 19 times more engine deposits than TOP TIER™ brands after just 4,000 miles of simulated driving."

Yes, you read that correctly, 19 times the deposits, not 19 percent.

The take-home is obvious: If you care at all about your car, then pay the few pennies extra for TOP TIER™ gas whenever possible. Your car's engine will thank you.

If TOP TIER™ gasoline isn't available where you drive, then using a good polyetheramine (PEA) based fuel system cleaner like Techron Complete Fuel System Cleaner or CRC Gasoline 1-Tank Power Renew no less often than every 3,000 miles is a must.


What About Octane

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Another decision motorists have to make when buying gas is what octane rating to use. In the United States, "regular" gasoline typically has an octane rating of 87, "mid-grade" usually has an octane rating of 89, and the various "premium" grades have octane ratings of 90 or more. What's the difference, other than the price?

The octane rating is a measure of a gasoline's tendency to detonate, or "knock," rather than burning evenly in the combustion chamber. Detonation causes severe stress on an engine's internal parts and can cause catastrophic failure, either suddenly or over time.

A gasoline's "octane" rating, therefore, is more accurately expressed as its anti-knock rating. The use of the term "octane" dates back to when fuels were rated by comparing them to test fuels consisting of a mixture of iso-octane and normal heptane. A fuel's octane rating was determined by comparing a test engine's performance to one running on a known percentage of iso-octane.

Nowadays, gasolines are much more complex than they were back then; but the octane rating has survived as a useful way to rate the anti-knock qualities of various fuels. The higher the octane rating, the less likely the fuel is to detonate. Gasoline should never detonate, or "explode," in an engine. Gasoline is supposed to burn evenly, not detonate, inside the engine's combustion chambers.

The tendency of gasoline to detonate increases with both an engine's compression ratio and with the load on the engine. Higher-compression and high-performance engines (such as most turbocharged engines) therefore require gasoline of a higher octane-rating.

You should never use gasoline of a lower octane rating than the car's manufacturer recommends. At best, it will harm performance. At worst, it can seriously damage the engine.

Manufacturers' octane recommendations assume a vehicle will be operated in a "typical" manner. Because engines are more likely to knock when under heavy loads, such as when driving in mountainous areas or when pulling a trailer, it's not a bad idea to fill up with gas one grade higher than what's recommended for your car when driving in those conditions.

Another time when you may want to use a higher-octane fuel than recommended would be when an older engine's combustion chamber has heavy carbon deposits. The deposits reduce the volume of the combustion chamber, effectively increasing the compression ratio and making the engine more susceptible to knocking. Having the pistons and cylinders cleaned (or trying a quality combustion chamber cleaner like Berryman 2611 Intake Valve and Combustion Chamber Cleaner) would be a better approach to the problem; but if that's not an option, then moving up a grade in octane may help restore your car's performance and reduce knocking.

Octane Rating and Fuel Economy

As mentioned, gasoline engines are most prone to detonation when under heavy load. When an engine starts to knock under high-load conditions, the car's ECM will intervene, typically by enriching the fuel-air mixture, retarding the spark, and down shifting if the car is equipped with automatic transmission.

The ECM's intervention is necessary to protect the engine. Knocking can cause catastrophic engine damage if it's not corrected. But the interventions also dramatically reduce the engine's fuel efficiency while they're in effect. Because of that, using a higher-octane fuel than the minimum recommended by the manufacturer may result in better overall fuel efficiency if you frequently operate your vehicle under high-load conditions.

Whether the increased efficiency will offset the higher cost of the fuel depends on local pricing. But higher-octane gas definitely will reduce the engine's tendency to knock under load. That, by definition, is what high-octane gasoline is for. Using a higher-octane gasoline that reduces the tendency to knock helps reduce the frequency of fuel-wasting anti-knock interventions.

If you never operate your car under high-load conditions, on the other hand, then using a higher-octane fuel than the manufacturer recommends will most likely be a waste of money.


Ethanol and Gasoline

For mainly political reasons, most gasoline in the United States contains a certain percentage of ethanol (an alcohol usually made from corn). At the time of this writing, the most frequent percentages are 10 percent (E10) and 85 percent (E85). Gasoline with 15 percent ethanol (E15) is also allowed, but very few stations carry it. Most consumers don't want it, in large part because using gasoline with more than 10 percent ethanol content would void the warranties of many gasoline-powered vehicles.

Other experts believe that increasing the ethanol content of gasoline will play havoc with older cars' fuel systems and increase engine wear in both vehicles and power equipment.

Ethanol advocates, many (if not most) of whom either are corn farmers or represent corporate agribusiness interests, claim that adding more ethanol to gasoline will reduce air pollution. Scientists, however, aren't too sure about that. Some, like Mark Jacoobsen, believe that E85 may actually produce more harmful pollutants than straight gasoline.

Ethanol also shortens the storage life of gasoline and increases its tendency to undergo phase separation. Phase separation is when ethanol and water separate from the gasoline and sink to the bottom of the storage vessel (the gas tank or gas can), leaving a layer of low-octane, low-quality fuel on top of the ethanol and water layer. If your car starts at all on separated gas, it will run extremely poorly.

In rare and extreme cases, phase separation can cause catastrophic engine damage due to hydrolock. This can happen when a car is left for such a long time that what is being drawn into the fuel pump is mainly water that settled to the bottom of the tank. Hydrolock can occur if that water makes it as far as the combustion chambers, especially if there was enough "good" gas left in the line for the engine to initially start.

Finally, because ethanol contains about 33 percent less energy by volume than gasoline does, adding ethanol to gas also decreases fuel economy. At 10 percent ethanol, you will burn about 4 percent more fuel per mile. At 15 percent ethanol, you will burn about 5 percent more fuel per mile.

Avoiding Ethanol-Related Problems

Due to federal laws requiring that most gasoline contain ethanol, consumers who wish to avoid ethanol-related problems have only a few options. One is to find a place that sells ethanol-free gas, which will usually be premium-grade and will always be more expensive. You can find a database of ethanol-free gas stations here. It's not always up-to date, but it's the best one I know of as of this writing.

The other option is to use a fuel stabilizer like STA-BIL. Fuel stabilizers don't remove the ethanol from the gas, but they do reduce its phase-separation tendency. Most also contain additives to reduce the damage done by ethanol to fuel systems and engines.

I personally add the "maintenance" dosage of the Marine flavor of STA-BIL to the gas tank every time I fill up with gas that contains ethanol, and the "storage" dosage to all gasoline that I store for use in power equipment (including ethanol-free gas). The Marine version is more concentrated and requires a lower dosage per gallon, making it both more convenient and more economical.