December 7, 2012 - The intent of this blog is to end a long-standing myth about premium gas. If you thought premium gas, was just better gas—don’t worry, you are not alone. Just the word premium alone implies that one gas is in some way superior to the other gases at the pump! Next to each type of gas there is a sticker that gives you the octane rating. Most of the time, regular gas is rated at 87, regular plus at 89, and premium at 91. If you’re like me, you at one point just started assuming that more octanes meant more power, or more fuel efficiency. I had it in my head that more octanes meant less “other stuff” that could potentially corrode or clog my engine. I no longer feel bad for thinking that way because I was only thinking exactly what gas stations (and oil companies) wanted me to think. It is misleading on purpose. Gas stations (and oil companies) want you to spend more money on gas, even when it’s not necessary for your vehicle.
Despite what mainstream marketing has led you to believe, high octane fuel is not put through some kind of special refining process that keeps your engine cleaner. In fact, it is required by law that all types of gas, from regular to premium, contain some kind of detergent to help fight corrosion.
The primary, and the only important, distinction between premium gas and regular gas is that premium gas is less combustible than lower octane fuel.
That’s kind of counterintuitive isn’t it? You would assume, as I did, that premium gas would be more combustible, meaning that there would be a larger explosion of power inside the cylinders of your engine. Alas, this is not how an engine works.
All of this has to do with engine knock—a phenomenon in which the air-fuel mixture inside the cylinder of your engine ignites on its own from the compression of the piston, and NOT from the spark plug. We all know the cliché: Timing is everything. Well, this holds true for your car’s engine as well. It is important that the air-fuel mixture in each cylinder of your engine is ignited at the correct moment—the correct moment being, when the spark plug ignites it. If the mixture ignites on its own before it’s supposed to, it causes engine rattling and vibration that will not only reduce the performance of your engine, but it can also damage it mechanically. Simply put, engine knock is your engine not functioning the way it is supposed to and that’s bad.
So what does all of this have to do with premium gas?
Premium gas, because it is less combustible, is less likely to cause engine knock. Careful here, I am not saying that you should put premium gas in your car because it is better for preventing engine knock. Standard cars are all designed to handle fuel with an octane rating of 87. That means that your engine will not knock if you use regular gas because your engine is designed to handle fuel with a higher combustibility.
The key here is that some luxury vehicles and high performance engines require a fuel that does not combust as easily.
Without getting too technical, because I couldn’t even if I tried, high performance engines have a higher compression ratio. Compression ratio is the volume of your engine’s cylinders when the piston is at its lowest to the volume when the piston is at its highest. A higher compression ratio allows the engine to extract more energy from the air-fuel mixture, but it also increases the likeliness of engine knock. In an engine where the compression ratio is higher, the air-fuel mixture is more likely to combust on its own without the spark—engine knock. Thus, premium fuel with a lower combustibility is needed for some high performance engines to prevent it from knocking in cylinders with a higher compression ratio.
Modern cars all come with something called a knock sensor. This is a sensor that picks up on engine vibration and sends a signal to the ECM. If your engine is rattling or vibrating due to knocking, the ECM will adjust the rate that the spark plugs are firing to reduce or prevent engine knock from occurring. That makes knock a very rare occurrence in modern cars. However, adjusting the timing of your spark plugs can reduce engine performance and fuel efficiency just slightly. Your average driver probably wouldn’t even know the difference.
Here’s what it comes down to:
If your owner's manual doesn’t recommend that you use premium fuel, you shouldn't be using premium fuel.
You are not doing your engine any favors by using premium fuel over regular—you won’t get better fuel efficiency, it won’t prevent engine knock, and it won’t keep your engine cleaner. Your car is designed to use fuel with an 87 octane rating. Give your engine the fuel it is designed to use. If your car “recommends” that you use premium fuel, then you have a judgment call on your hands. From everything you’ve read here, you have to assess if the benefit you get from premium fuel is worth the extra 10, 20, or 30 cents. The fact that your gas cap says premium fuel is “recommended” means that it is probably a marketing tactic by car companies to make you feel like your car is in some way better than cars that use regular fuel. Or they might just be working with oil companies to get you to spend more on gas than you should. It’s probably both. After all, oil companies were the ones that initially got you buying into the myth (literally) that premium gas was better than regular. Either way, the only time that premium fuel is absolutely necessary is when your owner’s manual explicitly states that your engine requires premium fuel.