A recent AAA report found that American motorists wasted $2.1 billion in the last year buying premium gasoline for engines designed to run on regular gas

The reasons why are likely due to the following misconceptions about premium gas:

  • Contains higher energy content (increasing power and fuel economy)
  • Formulated with higher-quality additives (increasing engine cleanliness)

What is Premium Gas?

When motorists see premium 91-octane gas at the pump, they may assume it contains higher energy content compared to regular 87-octane gas. After all, “high-octane” is often synonymous with increased power and performance. The 91-octane gas should, they think, provide improved fuel economy and power.

In fact, octane has nothing to do with energy content or quality – it’s a measurement of the gasoline’s ability to resist engine knock. Higher octane denotes greater knock control.

What is Engine Knock?

On an engine’s intake stroke, the piston travels down the cylinder, allowing air/ fuel to fill the available space. Assume the cylinder holds 900cc when the piston is at bottom dead center. The piston then travels up the cylinder, compressing the fuel/air in preparation for combustion. Assume cylinder volume is reduced to 100cc when the piston is at top dead center. The relationship between the two volumes is known as the compression ratio. In this case, 900:100 is reduced to 9:1. The compression ratio indicates cylinder pressure, and more pressure equals more power and greater efficiency. That’s why high-performance cars and heavy-duty diesels typically have higher compression ratios than standard cars or trucks.

While higher compression seems like all up-side, it can invite negative consequences. Compression heats the fuel/air mixture, allowing it to burn more efficiently. If compressed too much, gasoline can ignite too early, causing uncontrolled and early ignition. This leads to a knocking or pinging sound, robs the engine of power and can lead to engine damage. Typically, the engine’s computer will detect engine knock and adjust timing and the air/ fuel ratio accordingly. Although this protects the engine from damage, it can substantially reduce engine performance and efficiency.

Most high-compression gas engines require use of premium gas to better resist engine knock and prevent the computer from detuning the engine to protect against knock-related damage. Using premium gas in a clean, mechanically sound engine not designed to use it, though, provides no benefit.

In engines with carbon buildup on pistons or in the combustion chamber, however, premium gas can provide some benefit. Deposits can reduce cylinder volume at top dead center, effectively increasing the compression ratio. This alone can lead to engine knock. The deposits can also become hot spots that preignite the mixture, leading to engine knock.

In these cases, a higher octane fuel helps resist engine knock and allows the engine to operate closer to its normal conditions rather than detuning to prevent engine knock.

For best performance, use the fuel recommended in your vehicle owner’s manual.

Higher Octane Doesn’t Mean Higher Quality

The other popular misconception is that premium gas contains a higher concentration of cleaning agents and other performance-improving additives.

While many formulators market a highquality premium gasoline, such as Shell* V-Power* Nitro+ or ExxonMobil* Synergy*, the premium gasoline at your local filling station may not be formulated to improve performance in any aspect other than octane rating. Quality can vary from brand to brand and station to station.

This scenario provides an opportunity to sell AMSOIL P.i.® (API) or AMSOIL Quickshot® (AQS). They provide excellent detergency to help clean dirty injectors and carburetors for maximum fuel economy and operability. Once your customers understand the truth about premium gas, they’ll be more inclined to purchase an additive that provides the benefits they’re seeking.

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