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When it comes to gasoline grade choices at the pumps, the terms 'regular,' 'mid-grade' and 'premium' refer to the octane rating.Jonathan Hayward/The Canadian Press

You want your car to last, so you treat it right. That means regular oil changes, weekly washes, seasonal rustproofing and, of course, nothing but premium gasoline with every fill-up. Only, in some instances, that premium fuel you insist on might not only be a waste of money, but it might also be damaging your engine over the long run.

When it comes to gasoline grade choices at the pumps, the terms “regular,” “mid-grade” and “premium” are misnomers; there is usually no difference in quality. What they really refer to is the octane rating: 87, 89 and 91 (or sometimes 93 or 94).

The octane rating measures how much compression a fuel can withstand before igniting: the higher the octane rating, the less likely the fuel is going to preignite (sometimes referred to as “pinging” or “knocking”) before the spark plug fires. If knocking should occur, the danger is that peak cylinder pressure occurs before the piston reaches top dead centre, so it’s trying to shove the piston backward in its rotation. That could give you a very big repair bill and a generally bad day.

Turbocharged and supercharged engines, and also some other engines on higher-end models, usually need at least 91 octane (premium) gasoline, because the pressures and temperatures inside the cylinders are higher than in normally aspirated engines. But as Chris Muir, an automotive professor at Centennial College points out, using higher-octane gasoline in your mainstream vehicle isn’t helping it at all.

“If you’re running 91 or 94 [octane] in a car that requires 87, there is no advantage to having a fuel that requires a hotter cylinder temperature to ignite,” he says. The fuel doesn’t have enough time to burn fully, Muir says, which leaves behind unburnt hydrocarbons, and long term, “there’s a possibility of misfiring and fouling the [spark] plugs. It’s not as prevalent as it used to be, but it’s still a possibility.

“If you go into the 2016 Volkswagen Golf FSI owner’s manual, for example, it tells you that using premium fuel in that car has no advantages and, in fact, could damage the car.”

These days, all engines have sensors that monitor for knocking every few ignition cycles. If a knock is detected, the engine will automatically retard engine timing, which will save your engine at the expense of horsepower. But some automakers offer engines that can run on any grade of fuel safely, albeit with a variation in performance.

Mazda, for example, advertises that its Skyactiv-G 2.5 T turbo engine, found in the Mazda3 and CX-30, can run on any octane; it will get 227 horsepower and 310 lb-ft of torque on 87 octane, but if you want to squeeze more performance out of it, 93 octane will bump those numbers to 250 horsepower and 320 lb-ft of torque. So how does that work?

The Skyactiv engine was designed from the ground up to run on 87 octane, according to Daniel Grenier, national manager of technical services at Mazda Canada. But along with an advanced computer engine management system, variable valve timing and specially designed pistons and rings, it was also designed to manage heat, which is critical in avoiding engine knock.

“The cooler it runs, the better it is; the more performance and durability you can have,” he says. The Skyactiv engine was constructed to prevent heat from one cylinder being transferred to the next through an intricate exhaust manifold, he explains.

“We also have an exhaust gas recirculation system that runs with a cooler; you don’t see that very often. So, we’re cooling the exhaust gas before sending it back to the engine.”

Even though it’s safe to use lower octane fuel in the Turbo, Grenier says there are instances where high-octane fuel should be used. “There’s no detriment at all with running 87 octane, but the only thing we recommend is that when you’re towing or doing high-performance driving, running it harder than usual, then you should use premium fuel, for all the power and making sure the engine stays cool. Because the octane itself isn’t what gives better performance; higher octane only prevents knock.”

Regardless of what vehicle you drive, Muir has some final advice when it comes to filling up.

“Follow the manual. Put in what the manufacturers designed the engine for, and until you start doing internal engine work or heavy tuning, you do not need to go for a high-octane gas. Go for the cheapest gas you can get that meets the car’s minimum standards.”

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