Posted by: engineguy | July 1, 2007

Using Regular Gas — Low Octane — Is Likely Costing You More Than If You Used Premium — High Octane Gas.

If you’re you trying to save money by using a lower octane gas than your owner’s manual calls for, I can almost guarantee that you will actually spend more money on gas (depending on the cost difference of the choices between 87 octane and 93 octane). Why? Because it’s a certainty that you will get terrible gas mileage. Isn’t that ironic? And horsepower? Forget it. But the worst part is that it will eventually harm your engine. Badly. And you won’t know about it until it’s too late.

Surprised? Sure. I’ll bet that you’ve heard that there is no reason to buy premium gas because the regular gas is just as good. That’s wrong! Then there’s the belief that the higher octane premium gas is just another rip-off by the oil companies. Wrong again!

First let’s look at what octane means. It’s not about horsepower or heat energy. All an octane measure of 87 or 89 or 92 or whatever, means is that the number signifies the average of two methods of testing a fuel’s resistance to detonation. They typically range from 87 octane for regular unleaded gas to 93 octane for premium fuel. If you read most of these signs you will see a notation that the octane has been calculated using the formula R+M/2. We could go into another page to explain that those are shorthand for the two tests performed and divided by 2 for an average between them.

Without going into all the scientific stuff, this means that a premium, higher octane fuel has a much higher resistance to detonation than does a regular fuel of 87 octane. You read it right. Resistance to detonation. So what’s this about detonation? The resistance to detonation is the primary quality of a good gasoline. This is desirable so the detonation inside the cylinder head can be ignited evenly; the spark plug ignites the flame front in the cylinder head as a mixture of gas and oxygen. The flame moves in a deliberate manner across the top of the piston in the cylinder head. And that’s how it’s supposed to work.

So the better the fuel’s resistance to spontaneous igniting the more controlled the burning of the gas and the subsequent expansion of the heated gases that pushes the piston down to turn the crankshaft. But when a fuel with a lower detonation is used where it shouldn’t be used (in an engine whose manufacturer says it shouldn’t be used) the whole process goes out the window.

Abnormal combustion is what it’s called. And that’s because the intense heat and pressure in the cylinder subjects the lower detonation fuel (the 87 octane fuel) to an environment that it wasn’t meant for. So pockets of the fuel can spontaneously self-ignite creating secondary flame flashes. When those meet in the out-of-control burning in the cylinder head, the engine gets a shock wave. This shock wave is the knocking sound we have all heard in an engine at some time or another, usually when accelerating or under a heavy load hill climbing. Sometimes it sounds like marbles rolling around in the engine. Other times it actually sounds like the piston rods are loose and rattling. Whatever it sounds like, it’s not good for your engine . . . really not good!

So now comes the part where technology almost outwits us again. In older cars – those that didn’t have computer controlled engine functions, we all knew that the knocking sound meant that either we were using el cheapo gas or the spark timing was wrong. That was easy enough to fix. Just buy better gas or have your mechanic re-set the timing. Some guys could do that themselves in their own garage or under a shade tree. Just use a timing light and synchronize it with the main belt running by turning the distributor to adjust the spark up or down. Old, old cars used to have a little lever on the steering column by which one could adjust the spark after the engine started. Kind of like adjusting a choke until it sounds right.

But now we have these ingenious high-tech engines that are monitored by an on-board computer and they are constantly making adjustments to correct or improve what the computer perceives as a problem. Here’s where we fool ourselves. Or rather where the engine fools us and we believe it. These little critters are equipped with knock sensors.

When they detect the typical bad detonation from using low octane fuel, the little devils automatically retard the spark timing or take other precautions to protect the engine. So, in reality, the sensors are de-tuning the engine to accept the fuel’s octane. The designers put this in to protect the engine from damage, but only for the short run. But with the spark timing compromised, you may not have a clue that it’s happening. But the engine’s performance is just shot. This is why it looses horsepower so badly, as I pointed out earlier.

In addition to robbing the engine of power, the electronic computerized de-tuning has put the engine into a mode that, by the nature of the low resistance, uncontrolled detonation, wastes the fuel’s energy to power the vehicle. So, if you think you’ve outsmarted the oil company, think again. You’re among their best customers because your vehicle’s miles per gallon are in the toilet. Said another way, you use more of their product than you need to.

The techno-wizardry of this is amazing. But the down-side is that the electronic computerized controls – the techno-wizardry — can mask the inferior detonation so effectively that you may never be aware that it’s happening.
So you are merrily driving along thinking how clever you were to not fall for that premium gas trick at the service station. Meanwhile your car is running sluggishly and you are using a great deal more gas than you should be using. But you sure saved a lot of money on that last fill-up, didn’t you?


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