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Unleaded Premium Fuel Is A Scam: Fact Or Fiction?

Posted in auto industry, Car Buying, Car Care, Engines, Featured, Fuel, General, Tips by Kurt Ernst | April 7th, 2010 | 12 Responses |

Is saving $6.00 per tank worth the risk of a blown motor?

A friend of mine (let’s call him “Stan”) is one of the smartest guys I know. He just authored a book on business process and works for one of the big financial powerhouses as some sort of a training guru. He recently hit me up for a recommendation on a domestic sedan, so I countered with the usual list of questions. One of them was “are you willing to use premium unleaded fuel”, to which Stan replied “unleaded premium is a scam”.

When I stopped screaming at my email, I realized that if Stan thought this was the case, then so do a lot of other people. Manufacturer’s don’t do a good job of explaining this, and dealers are reluctant to point out that your new car will cost you more money to operate than you thought. So what’s the real deal? Is unleaded premium a requirement, a luxury or a scam? If you need it, why do you need it?

Uh oh: pistons aren't supposed to look like this one.

First, let’s start with a little science. A fuel’s octane rating indicates how combustible it really is; contrary to popular belief, low octane fuels (standard unleaded) are more combustible than high octane fuels; specifically, they have a lower flashpoint. High octane fuels have a higher flashpoint because they’re intended for use in high compression engines. Oh, crap, I’ve lost you, haven’t I?

Let’s back up one step and think about how a four stroke motor works. When the piston starts up the cylinder in the compression stage, both the pressure and the temperature inside that cylinder rise. In a diesel motor, this compression alone is enough to create the ignition; in a gasoline motor, a spark is usually required. Why do I say usually? Because using the wrong, low octane fuel can cause the air / fuel mixture to ignite before it’s supposed to. This is called detonation, and bad things happen to motors with uncontrolled detonation. Things that require motors to be replaced or rebuilt.

Connecting rods aren't supposed to look like this, either.

Here’s the good news: modern technology has a way to address this. A device called a “knock sensor”, usually positioned on the side of the engine block, listens for the noises associated with detonation. When it senses them, it signals the motor to retard the timing, which allows the combustion to occur when it’s supposed to. The down side is that when this happens, you motor is making less power than the manufacturer intended it to. It’s literally trying to keep itself from blowing up.

Now some bad news: knock sensors can’t always correct ignition timing enough to compensate for the wrong octane fuel. The editor of an automotive website recently found this out the hard way, when the brand-new Cadillac SRX Turbo he was driving blew it’s motor climbing a mountain pass outside of Ventura, California. The editor admitted to mistakenly filling the press fleet vehicle with regular unleaded instead of the required premium; the sequence of events that followed (retarded spark, turbo boost, cylinder overpressure, cracked engine block) left him stranded by the side of the road. Per Cadillac engineers, use of the wrong fuel was a contributing factor and not the primary cause; still, the incident may not have happened had the editor used the grade of fuel specified by the manufacturer.

Confused? Don’t be, because here’s the breakdown: if your owner’s manual says unleaded premium only, use it every time you tank up. Sure, you’ll pay a few dollars more each time you fill up, but I’ll bet that it’ll still be less expensive over the life of the car than buying a new motor. If you want to see what fuel is required BEFORE you buy a car, just check inside the gas cap lid. If it says, “premium fuel only”, that’s what you need to use.

If your manual says “premium fuel recommended” you can probably get by using the mid-grade stuff for normal driving. If you’re towing a trailer or driving in extreme conditions (up mountain passes, across the desert in summer, etc.), then buck up for a tank of premium. Don’t use the cheap stuff, no matter how tempting it may be.

So what if your manual just calls for the use of “unleaded fuel”? Can you get better performance by running premium unleaded? Is the good stuff formulated to run cleaner, remove harmful deposits, whiten your teeth and leave your laundry smelling spring fresh? In a word, no; running premium fuel in cars that don’t require it will do nothing other than drain your wallet more quickly.

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