Premium or regular gasoline?

On Wed, 28 Jun 2006 17:33:15 -0700, James O'Riley somewhere that as long as you keep the revs and power level down you can

what year car? the newer ones have knock sensors that allow you to use lower grades of fuel, but your performance and mpg will take a hit

It's a 2002 E-320. I ran about half a tank and made sure I stayed below about 3500 RPM but occasionally by foot got heavy we went varrroommmm. In town I got 18 mpg and 28 on the road. If that's taking a hit I wonder what it would be like with high test in the tank. ;-)

Thanks for the reply...

Premium fuel ignites at higher temperatures and so avoids preignition.

Try mid grade if your engine's compression ratio is less than 10:1.

Try mid grade when the ambient temperature is low.

Use premium in very hot weather and if the motor's compression ratio is

Modern engines have knock sensors that retard the engine's ignition to avoid knocking or pinging - that causes some performance loss and lower mileage so the savings of cheaper fuel are not as great as you may believe.

IMHO high revs don't matter but lugging the motor will cause it to knock

- that's about the worst one can do.

Thanks friend, that's very complete and easy to understand. I've saved a copy so I can refer to it again.

While I agree with the theory behind the effects of different octane fuel, personally, I'd RTFM and use the minimum octane the manufacturer recommends, with or without knock sensors.

On the other hand, the percentage difference in cost between low and high grade has significantly diminished with the general price increases so you do not really pay that much more.

For example when gas was one dollar and premium was one dollar twenty cents the premium was 20 percent. Now that it is 3 dollars and premium is three dollars and twenty cents the percentage increase is only about seven percent.

So where I live, premium is not that much more and so it is not that bad a purchase.

When you said half tank... was the tank empty before you did that? If not, your reserve premium gas raises the octane level.

Most of the time running on regular lowers the fuel mileage quite dramatically. I had a BMW that runs on premium only... when I used 89, I lost like 4 MPG.

Just shop for cheaper premium gas... in today's world, it makes no difference what brand you use... they all have high detergent level and made by same brand name company... Best cheap gas is the one that moves fast... meaning fresh fuel.

I have to agree. I saw a dramatic increas in milage when I started using Premium (it is 93 octane here). My dad had the car for a year (300E) and ran it on regular. He complained it was sluggish and sucked gas. (actually, it was my stepmom who complained). I found out she was putting in regular gas, I changed it to premium, and viola!! Better mileage, better power.

I realized that after the last fillup. I save $2.50 on 10 gallons of mid-grade fuel. That was really foolish! ;-)

I also realize that it's not all that expensive either compared to the cost in Calfornia in the '50s or even now. And engines are much more efficient. In 1959 my Ford got 16 mpg on the highway, this 'old' 2002 MB gets 28!

I just found that Subaru is bragging about gas mileage being 28 and costs $35,000, again this 'old' 2002 cost me $25,000 2 months ago and gets the same mileage, and a MUCH better overall warrantee including road assistance virtually FOREVER! :-D

T.G. I almost always agree with your advice but knocking, or pre-ignition, occurs because the air/fuel mixture does not burn quick enough. A low-tech but brief explanation of what happens is this. Premium fuel has a lower flash point, thus burns more rapidly. When a lower octane fuel is introduced into a cylinder under certain conditions the air/fuel mixture burns slower and can pre-ignite.

As the piston moves upward compressing the mixture the spark plug ignites the mixture right at the plug itself, this actually occurring before the piston reaches TDC. As the flame front begins moving spherically from the point of ignition the expanding gases begin driving the piston down in the cylinder. However, another process is also taking place, namely the compression of the remaining air/fuel mixture which has yet to ignite and help continue the downward force on the piston.

If this remaining air/fuel mixture is not of adequate octane the mixture simply explodes from compression alone, exactly like that which occurs in a diesel engine. This explosion creates tremendous pressures and the resounding knock associated with the phenomena.

Two things normally can be done to reduce this effect. The first is to retard the timing so that the ignition occurs later in the cycle, thus the piston has moved further down in the cylinder as the flame front expands, reducing or completely eliminating the pre-ignition event. The second is, of course, to increase the ability of the flame front to move more rapidly by increasing the octane rating of the fuel. Premium fuel or the addition of octane booster will solve this problem.

A couple of additional points. First, pre-ignition creates a lot of unwanted pressure and temperature in the cylinder. This can result in the softer piston material wearing away rapidly. If you ever tear down an engine that has undergone pre-ignition for a long period of time you will always note that the damage portion of the top of the piston is furthest from the spark plug gap. Second, the heat buildup in the cylinder because of pre-ignition sometimes causes deposits on the top of the piston to glow red hot and continue to provide ignition after the key has been turned off. This is often called dieseling.

This is a somewhat lengthy explanation. Hope it was more educational than boring. Again T.G., not to dismiss what you say but perhaps expand on it.

Too much like common sense:

- RTFM

- follow manufacturer's advice.

Common sense does not get you far in usenet...

DAS