Premium Gas v. Regular Gas

Overman

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Overman
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Feb 2, 2006
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What is the difference between premium gas and regular gas?

Better gas mileage, just running cleaner, or just more expensive?

Inquiring minds would love to know...
 
Prevention of detonation in higher compression engines ... especially those that are turbocharged. (Like mine.) Otherwise, I believe it has the same BTU content as regular.
 
Overman said:
What is the difference between premium gas and regular gas?

Better gas mileage, just running cleaner, or just more expensive?

Inquiring minds would love to know...
Click to expand...

Well "premium" and "regular" usually refer to the octane rating of the fuel. This is a measure of the ignition energy required to start the combustion reaction. Higher octane fuels require more energy to ignite. In higher compression engines the air/fuel charge heats up more during the compression stroke than in lower compression engines. If the fuel lights off at too low a temperature it will ignite too early and cause "detonation". This will cause the engine to run roughly and inefficiently and can cause damage to the engine. In the past lead was used as a detonation inhibitor. Contrary to what many people think the octane rating has nothing to do with the energy released during combustion. 93 octane is not "more powerful" than 87 octane. 93 octane fuel will release the same amount of energy as 87 octane fuel.

BTW the octane rating is an artificial scale based on octane and heptane. The seven carbon heptane hydrocarbon is easier to ignite and the eight carbon octane hydrocarbon is harder to ignite. 89 octane fuel behaves as though it it 89% octane and 11% heptane.
 
Yes, higher octane fuel is actually harder to ignite, and is suitable only for high-compression engines though you can use it if you like throwing money away.

In high-altitude areas the standard gasoline is 85 octane, because of the lower oxygen content in the air.
 
Overman said:
What is the difference between premium gas and regular gas?

Better gas mileage, just running cleaner, or just more expensive?

Inquiring minds would love to know...
Click to expand...

Your vehicle owner's manual will generally tell you what octane rating you should use in your car. Exceeding that doesn't get you anything.
 
I believe it should be noted that if one's car is designed to run on higher octane gasoline and you fill up with regular, you will most likely see a drop in horsepower. This is not because the fuel has less energy, but because newer cars with detonation sensors will retard the spark of ignition to prevent detonation -- and retarding one's ignition timing results in less power.
 
My owners manual says I should get higher octane fuel, the salesman said it only mattered in the summer, and in driving it, it does not knock with regular.

Should I really be useing pluss?

My car has a turbo charger.
 
ponderingturtle said:
My owners manual says I should get higher octane fuel, the salesman said it only mattered in the summer, and in driving it, it does not knock with regular.

Should I really be useing pluss?

My car has a turbo charger.
Click to expand...

My opinion -- Yes. It's little insurance against what could be very expensive down-the-road headaches. Plus, you might not be detonating because of your car's fuel monitoring system. Going with higher octane in that case may actually get you more MPG and offset the cost.
 
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Your car likely has a knock sensor as mentioned earlier, and will be fine with regular except for the loss of a few horses.

Now if your owner's manual is specific and warns you not to use regular, then you'd better follow what the manual says.
 
ponderingturtle said:
My owners manual says I should get higher octane fuel, the salesman said it only mattered in the summer, and in driving it, it does not knock with regular.

Should I really be useing pluss?

My car has a turbo charger.
Click to expand...

I have nothing to add except to say that the salesman may not be the most reliable source, especially if gas prices were high when you bought the car.
 
tsg said:
I have nothing to add except to say that the salesman may not be the most reliable source ...
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You don't say !!!

:jaw-dropp


(I actually had a salesman at a Mazda showroom describe the RX-7 as having 4 cylinders.)
 
Where I am, there's a slightly higher octane fuel available for the same price as the regular stuff. But it's an ethanol blend (containing about 10% ethanol). The higher octane makes it a little more efficient, but ethanol has a lower energy density than plain gasoline, and so there's basically no real benefit.
 
Foster Zygote said:
Well "premium" and "regular" usually refer to the octane rating of the fuel. This is a measure of the ignition energy required to start the combustion reaction. Higher octane fuels require more energy to ignite. In higher compression engines the air/fuel charge heats up more during the compression stroke than in lower compression engines. If the fuel lights off at too low a temperature it will ignite too early and cause "detonation". This will cause the engine to run roughly and inefficiently and can cause damage to the engine.
Click to expand...
Correct, but the point of high octane is you can run a higher compression ratio engine. The higher the compression, the greater the power for a given cylinder size. Of course, if you don't have a high-compression engine, then there's not much point in burning high octane fuel.

Foster Zygote said:
In the past lead was used as a detonation inhibitor.
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No. In older engine designs, the valve guides in the cylinder heads required lubrication. Tetraethyl lead was added to the gasoline to provide this lubrication. Originally, there was a separate, more expensive grade of gas, known colloquially as "ethyl," that people would run a tank of every eight or ten tanks to provide this lubrication; then they eliminated this, putting it in all grades, in lesser amounts, because people would forget and damage their cylinder heads. But lead is known to be a poison, and also interferes with the action of the catalyst beads in a catalytic converter, so they had to make cars that could burn unleaded gasoline; to do this, they used bronze valve guides, because bronze is self-lubricating on steel valve stems. Today, you can't get heads that don't have bronze valve guides and require leaded gas; most even of the cars that originally required leaded gasoline have had their heads replaced with ones that can burn unleaded.

Foster Zygote said:
Contrary to what many people think the octane rating has nothing to do with the energy released during combustion. 93 octane is not "more powerful" than 87 octane. 93 octane fuel will release the same amount of energy as 87 octane fuel.
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Correct. The point is you can burn it in a higher-compression engine, and get more power. It doesn't make more power without that extra compression, and the compression ratio is a design-time choice in an engine.

Foster Zygote said:
BTW the octane rating is an artificial scale based on octane and heptane. The seven carbon heptane hydrocarbon is easier to ignite and the eight carbon octane hydrocarbon is harder to ignite. 89 octane fuel behaves as though it it 89% octane and 11% heptane.
Click to expand...
Correct, although it's not just heptane and octane, it's specific isomers of them: iso-octane and neo-heptane.
 
ponderingturtle said:
My owners manual says I should get higher octane fuel, the salesman said it only mattered in the summer, and in driving it, it does not knock with regular.

Should I really be useing pluss?

My car has a turbo charger.
Click to expand...

Not sure what the dealer was getting at, as, in the winter, the cold air has a denser charge, and if anything would cause a slightly more lean condition in the engine, increasing risk of predetonation.
 
Crazycowbob said:
Not sure what the dealer was getting at, as, in the winter, the cold air has a denser charge, and if anything would cause a slightly more lean condition in the engine, increasing risk of predetonation.
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I think it was winter vs summer fuel formulations.
 
Crazycowbob said:
Not sure what the dealer was getting at, as, in the winter, the cold air has a denser charge, and if anything would cause a slightly more lean condition in the engine, increasing risk of predetonation.
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A mass flow sensor would (should) compensate keeping the air/fuel mixture at optimum. I think what the dealer might be getting at is with a turbocharged engine, the air gets heated through compression which helps fuel detonate more easily ... in winter, a colder starting temperature will result in a cooler charge of air.
 
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Many silly notions persist about "premium" gas. Lots of folks my age recall their fathers telling them to run a tank of premium through once a month or so to "clean things out".
Lots still believe that premium is somehow "better", more powerful, or whatever.

They also think that "jet fuel" (in most formulations little more than kerosene) is some super-powerful item that will double one's horsepower, if only a few gallons could be obtained.
 
Intercoolers are added to turbocharged engines to cool the air before it's fed into the engine. The total compression ratio in the combustion chamber is what's important, not whether it's achieved with a turbocharger or supercharger, or naturally by the displacement of the engine.

It's advantageous to take cool high pressure air from over the hood (or off the windshield) to feed even into a naturally aspirated engine.
 
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