A guy says he’s stopped using premium gas in his “premium gas required” car because it’s too damn expensive. It’s a joke, right? He’s saving 30 cents now, only to threaten his warranty and pay thousands in repairs later? “Yes” is the easy answer. But the truth about cars can be a funny thing, especially when you add fuel and flames.
With apologies to the chemists, theoretical physicists and tuners out there, here is an octane apercu: octane rating measures knock resistance. It has nothing to do with energy content. Engine knock (or ping) occurs when fuel detonates before the piston is in the right spot. The temperature and pressure in the cylinder cause the fuel-air mixture to detonate prior to the spark.
The effect is like tapping the cylinder head with a ball peen hammer, hence the sound. Add the intended spark, and you can have two flame fronts in the cylinder smashing together in the kind of closed-cage match no one leaves. Seriously, knock would destroy a lot of engines if not for octane.
Octane (a.k.a. 2,2,4 trimethylpentane) is a chunk of carbon and hydrogen added to gasoline to prevent premature detonation. In a bit of counter-intuitive chemistry making phrases like “high octane thriller” slightly ridiculous, it actually slows the combustion process. Octane’s fame as a performance enhancement comes from allowing higher compression, which results in more punch. The higher compression comes from the factory, not the pump.
The octane rating itself comes from good old fashioned testing. And I do not use that phrase lightly. To rate octane, testers use special single cylinder motors developed in the 1930s. In the United States, octane is reported as an average of research octane (RON) and motor octane (MON). That’s what that yellow "octane by R + M / 2" pump sticker means. In Europe, it’s RON only, so the numbers run higher. RON tests the fuel under load, but both numbers help engine makers match their designs to available fuel.
The rule, then, is use whatever octane rating the manufacturer recommends. A force-fed SAAB asking for 93 octane fuel really needs it. Fuel with less won’t be able to put up with the pressure of the turbo and temperature inside the cylinder. It will start sounding like the opening of Silly Love Songs, and no one wants to hear that.
On the other side, a Honda Accord is set up to use 87 octane. The environment inside Accord cylinders is not going push that fuel beyond its limits, so it can’t take advantage of the extra resilience of the more expensive brew. It’s added cost with no added advantage. Who wants to waste an extra cent on their fuel?
All rules are meant to be broken, though, which is why dropping gas grade to save money– or picking it up to increase performance– isn’t necessarily a joke. There are times when you may want to stray from the owner’s manual and, as is usually the case, only you can help you know when.
With some cars, there is no choice. Carbon build-up inside an engine might mean you must use higher octane fuel. In older cars, you are the knock sensor and have to respond accordingly.
Newer cars have their own knock sensors, like little U-boat commanders listening for pings. Upon hearing the noise, they retard the timing, firing off sparks when they can still do some good. That means reduced power and efficiency. So, while you’re saving at the pump by avoiding higher octane fuel, you’ll end up paying at the throttle.
Exactly how much difference any of this makes varies for every car. You can (and this is not recommended the author or anyone remotely related to TTAC) take a new Passat, run regular gas and not hear any blacksmithing from under the hood. You will also fail to get that squeal you wanted as the light turned green.
Perhaps more importantly at the moment, your mileage may be off. Whether the drop is enough to warrant the extra cost of higher octane fuel is… uncertain. The only way to really know for sure whether or not premium fuel is worth the money: test out a couple of tanks of gas.
If a test is allowed. Some manufactures "recommend" premium fuel, others "require" it. For the most part, a car manufacture weighs mileage, horsepower and fuel grade against its perception of the ideal buyer. They want to put out the best numbers they can. Conversely, no company wants to tell you to fill up with 93. It’s an added, and inconspicuous, cost of ownership, one that owners are forced to confront often.
Bottom line: if a manufacturer asks you to spend more ON their car, not FOR it, you may want to laugh it off.