I am looking for a premium SUV (Audi, Mercedes, Lexus, Acura, Land Rover, Infiniti, etc.) that does not require the use of premium fuel (91 octane). Why do these premium vehicles all require higher octane fuel, as the fuel price is 13 or 14 cents per litre higher than for 87 octane fuel? – Charles
Luxury SUVs have powerful high-performance engines, and those engines need gasoline that won't cause knocking.
"The octane number is a measure of the auto ignition properties of the fuel. A higher octane number means that the fuel is less susceptible to knocking," says University of Alberta engineering professor Bob Koch. "It's not a marketing gimmick. If it's recommended by the manufacturer, it's needed for your engine."
High performance engines have higher torque and horsepower because they're designed to allow for much higher compression of the fuel and air inside the engine's cylinders.
That high compression ratio is only possible with a fuel with a higher octane rating. Put regular fuel in a high-compression engine and it will burn too quickly; you'll get engine knock. That sound – sort of like popcorn popping, but a lot faster – is the gasoline exploding in the cylinder.
In North America, regular grade fuel ranges from 85 to 87, midgrade is from 88 to 90, 91 to 94 is premium, and over 94 is super premium. Most premium SUV manufacturers recommend premium, but many say their engines can run on midgrade in a pinch.
"You can use regular gas in emergency situations, but it will have a negative impact on fuel economy and performance," says Tim Franklin, senior manager with product planning at Nissan Canada.
BMW recommends premium but allows 89 in all of its vehicles except the M-cars (which require 94). However, they say that using midgrade gasoline can lead to knocking when starting your vehicle when it's hot outside. In addition, BMW doesn't recommend using regular fuel.
"Generally speaking, the use of poor-quality fuels may also result in driveability, starting and stalling issues, especially under certain environmental conditions such as high ambient temperature and high altitude," says BMW spokesman Rob Dexter.
If you do use regular gasoline in a vehicle designed for premium, the engine's knock-sensor will detect uncontrolled burning in the chambers and adjust engine timing.
"Manufacturers do this so you will not destroy your engine if you put in a lower than recommended octane number fuel," Koch says. But even though the engine will compensate for the cheaper gasoline, it comes at a cost – you'll lose power and you'll get worse mileage, Koch says.
In addition, he says, increased engine compression actually improves mileage – so, you're using pricier gas, but you're using less of it.
"Premium fuel allows the auto maker to get a bit more power from the engine, which contributes to horsepower bragging rights," says Automobile Protection Association president George Iny. "There is a perception that buyers of these vehicles may pay a little attention to the advertised fuel consumption ratings when they shop the vehicle, but not at all to the grade of fuel."
I checked with the manufacturers and premium is recommended in all luxury SUVs, with a few exceptions – the Cadillac Escalade Hybrid 6.0 L takes regular, and the Range Rover Evoque will only take premium.
Recommended generally means that fuel rated as low as regular (87) gas can be used with the warning that reduced acceleration and knocking could result – but check with the dealer and the owner's manual to be sure.
Iny says the difference in performance will probably be imperceptible to most drivers during normal driving.
But the consensus is that premium should be used if the manufacturer recommends it.
And note that premium fuel doesn't give you any significant benefits in vehicles that recommend regular fuel, says Consumer Reports. If your car recommends regular, use regular.
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