Compared to older days, gas prices haven’t exactly been kind over the last few years, and with continued instability around the oil-producing parts of the world, it may not get much better anytime soon. Given how expensive the stuff has already gotten, premium gas in particular seems like a major purchase every time you go to the pump. Do you really need it? Could you just skip it and go for the cheap (well, comparatively) fuel?
As usual, it depends. Every car will list what octane it requires, either on the filler cap or in the owner’s manual. If yours only needs 87 octane, going for premium grades won’t help you any. In fact, even if higher grades are recommended (we’re not talking required here), you’ll probably be fine with the less expensive stuff. Furthermore, you can usually skip the lesser-used mid-grade gasoline – that is, unless you live in a high-altitude state like Colorado and mid-grade is your 87 octane.
But what about the cars that do claim you are required to shell out for higher octanes? Is that really necessary? Unfortunately for your wallet, the answer is yes. The entire point of high-octane gas is that it reduces knocking, which otherwise might seriously damage the engine.
What is engine knocking?
Maybe it’s not appreciated enough, but a car engine is basically an explosion machine. A mixture of air and fuel is pumped into a cylinder, the spark plug creates an ignition, and the resulting boom makes the car go vroom. It’s a very delicate operation, so if something goes wrong and explosions start happening that the engine isn’t designed for, it’s not good news.
Knocking means that explosions are erupting in the cylinder even before the spark plug fires; in other words, forces are being unleashed in a timing that the engine isn’t meant for. This can happen in engines with high compression ratios, where pressures and hyperlocalized temperatures get higher. To counterbalance this problem, higher octane fuels are used, which are less likely to spontaneously combust.
At its worst, knocking can cause serious damage to the engine, which is one of the most important and expensive parts for your car. A lot of knocking over an extended period will almost certainly lead to major problems. So in high-compression engines, it’s really critical to make sure you’re putting in high-octane fuel, which should eliminate the problem altogether.
Compression ratios in brief
What exactly is a compression ratio? This can get technical, but essentially, it’s a way of measuring how far into the cylinder the piston actually travels; the farther in it’s designed to go, the higher the compression ratio. Roughly speaking, a higher compression ratio will get the piston (and, thus, the engine) to do more stuff per amount of fuel used, leading to better performance and fuel efficiency.
That sounds great, but as mentioned before, this makes for a more volatile system. Higher octane fuels are one way of solving for this, but another is to simply engineer for a lower compression ratio, which makes knocking less likely anyway. If you can get good horsepower and fuel efficiency in another way — like, say, a turbocharger — then you’re good to go, and lower octane fuels can be used. That’s why the vast bulk of cars on the road only need 87 octane and won’t see any positives from using anything higher.
To follow that thought through: Higher octane isn’t better for performance, it’s simply more stable. So, putting it into a low compression engine doesn’t make it go any faster.
