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Demystifying Octane Ratings

It’s Monday.

Your GPS shows traffic beginning to back up and you’re in a hurry to make your way out the door. The pull of coffee is intense, but you’ve got to have it with just a hint of sweetness. In a frenzied rush - you accidentally grab the salt instead of the sugar shaker. Quickly, you dash it a few times and give the morning brew a stir. A minute later, you’re in the car and the piping hot liquid hits your lips.

The next moment hits you like a bad dream.

Think of that cup of coffee as a high performance engine and the salt as lower rated gasoline.

Regardless of the type of engine you have in your vehicle – it requires the right fuel in order for it to work efficiently.

At the pump, you’ll typically see 3 or 4 fuel types which have names, “Standard, “Plus” or “Premium”. These monikers are assigned based on their octane rating, and have a number ranging from 87 to 91.

Octane ratings simply designate the compatibility of a particular grade of gasoline to how well it will perform in a given engine.

After fuel is added to the gas tank and you’re on the road - gasoline and air are compressed in the engine’s combustion chamber. If the pressure reaches a certain point, the gas will spontaneously combust and cause a “knocking” sound. You don’t want that to happen. This is called an uncontrolled combustion and happens more frequently when lower octane rated fuel is added to a vehicle with a high performing engine.

In this scenario – gas can combust prematurely; the piston will try to go down while it is still being pushed up by the crank shaft and you will hear a pinging or knocking sound. That sound is metal banging into other metal components when it’s not supposed to. As you can imagine, this can lead to expensive damage.

For lower octane rated cars (e.g. 87- rating), adding premium 91-rated fuel with an expectation to get “more power in your engine” isn’t actually the case. The engine will synthesize the 91-rated fuel through your fuel system as effectively as with 87-rated fuel.

Drive a high performance vehicle and that is when octane ratings really matter. These vehicles have engines with a higher compression ratio and require gasoline with a higher octane rating to prevent premature combustion and engine damage.

Higher gasolines grades are less prone to auto-ignition and can withstand a greater rise in temperature during the compression stroke of an internal combustion engine - without auto-igniting. This prevents the “knocking” sound of an uncontrolled combustion that would occur should a lower rated gasoline be used than what is recommended for that vehicle. Higher rated fuels allow for the combustion process to occur at precisely the right time it is needed, and the engine is able to maintain operating integrity during use.

The bottom line is that your vehicle has a recommended octane rating and you should always fuel up with that number, as a minimum. Using a lower octane rating than recommended could cause engine damage.

No matter what kind of gasoline you use, there will be some carbon build up on your valves, pistons, fuel injectors, etc. This is especially prevalent on the intake valves of the GDI (gasoline direct injection) engines that are found in more and more vehicles because the gas is injected directly into the combustion chamber and the detergents in the fuel do not wash over the intake valves. A professional fuel system cleaning is needed to clean and restore the efficiency of your fuel system and to keep your fuel economy up.

Ask your NAPA AutoCare Center or Service Advisor if it’s time for a professional fuel system cleaning.

Posted July 23rd, 2015

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