Here's an old rule of thumb for octane, "The optimum fuel for any given engine is the lowest octane that won't detonate under the conditions you may encounter."

This rule may not work well for engines with knock sensors. They typically can retard ignition timing or reduce turbo boost to "tune out" detonation from lower octane fuel. This can result in a significant loss of power or mileage. This will be dependant on the individual engine & it's programming.

I recall a Car & Driver article from the dim past where they tested this. They had a turbo Saab with a knock sensor and tried several tanks of both regular & premium. The test drivers had a difficult time feeling a seat of the pants difference, but 1/4 mile times were almost a second slower with regular gas. Note that the power loss isn't from the lower octane, but from the changes in the engine programming to avoid detonation.

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Norm in NJ
Next oil change at 230,000miles

nglitz's rule is definitely true.

And I'll offer what I know to help explain why.

Their are a couple of properties of gasoline I don't see mentioned in this forum thus far.

1) The reasons why you would use the lowest octane rating your vehicle was designed for (besides price)

and...

2) The difference in quality between formulations of low octane and high octane gases


The higher the octane rating is on a specific fuel, the more resistant it is to detonation. Everyone knows that right? It's because the hydrocarbon chains in the gasoline are longer, and are harder to break.

What does this mean for mileage? Simply that the higher octane gas you use, the worse your gas mileage will be. Why? Longer chains of hydrocarbons require more energy to break during combustion (providing resistance to pre-ignition by heat), and less energy is released by them during their combustion (as properly initiated by the spark plug).

Now, that's not to say that you should put 87 in a car that wants 91 because you think you'll get better mileage.

By retarding the timing, the ECU will pretty much negate your mileage increase. You'll be into the pedal more, because of the power difference, and use more gas as a result. And you'll negate the perfomance edge designed into your timing maps.

So if you have a Toyota Tercel, by all means, use 87. It's cheaper and your mileage will be better.

But as we all know.. the higher the compression, the higher the tendency for pre-ignition (pinging/detonation). So, can a Mercedes deal with low octane gas?

Any modern one can, without much short term harm. So why use 91/93?

For reason number 2.

There is a difference between grades of gas that is not accounted for by its octane rating. Generally speaking the higher the grade of gas, the more and/or higher quality detergents are added to it. And this means, less deposits on valves, heads, pistons, etc. Chances are good that you will more than pay the difference should your car require a repair brought on by poor quaity fuel (a duel head redux on a 600SL is 72 hours of labor time by the book, no parts!). Carbon deposits in the combustion chamber can have sharp edges that become very hot, when they are hot enough they'll ignite the mixture in the chamber before the spark plug can. Your engine isn't a diesel (at least for the purpose of this arguement), it isn't supposed to run like one!

Don't take my word for it, ask a chemist, and take a sample of all three octane ratings and have a chemical analysis done on them (if you're anal, like I am). Should run you less than $200. Another good thing about analysis is that if you frequently fuel at the same station, the analysis will tell you how dirty the gas is.

Yep, dirt! And you thought you were just buying gas. Your local station makes their money by selling gas, not by making sure that their storage tank is operating room clean. If you find your usual station very dirty, move on! Dirty fuel clogs filters, destroys pumps, hampers performance, and brings on repair costs you don't have to have.

What does this all mean to you? It means, use the gas the manufacturer recommends. It's what you might call cheap insurance.

So why didn't I just say that?

Oh yeah, and if I had a rental fleet, I'd only use higher octane in the really expensive cars (mb, porsche, bmw, jag). They don't rent many of those.

Rental car places ditch their cars at 20K miles typically. That's the best balance of use and depreciation for a lot of cars.

Sorry about the length, just thought I'd throw in my 2 cents.

Justin

Justin

That's a pretty accurate description, at least for the average person, of what it's all about.

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2007 C 230 Sport.

You need to check this out
www.toptiergas.com

It is a new gasoline standard set by 4 major automakers. The new standard calls for more additive in the gasoline than the EPA standard calls for.

Car Nut

Some years ago ( 15 - 20 ), I had quite some lengthy conversations with a group of Ford Powertrain engineers.
When questioned about their biggest challenges facing them, as far as emissions, driveability, component reliability, etc., the all agreed it would be nice if SOMEBODY would provide a decent gasoline.
Upon further probing as to who had the worst quality fuel, wouldn't you know ...........our largest national oilcompany!

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2007 C 230 Sport.

So Manny... Whose distillation do you pump?

Well, being in Canada, I normally use Esso.
Guess that's Exxon to you people ( on the " other " side ).
The two brands I avoid like the plague is Shell ( sooooo many horror stories ), and Petro Can ( hint, this used to be our government-owned oil company ).

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2007 C 230 Sport.

Do 87 300Es have a knock sensor?

Exxon would be Mobil here, Chevron is considered good around these parts, that's what I use. Chevron bought Texaco, but the gas isn't the same. 76 is Conoco/Phillips, Shell is shell, and Arco is worse.

Justin

No.

Duke

Thank you.

This was true up until the discovery of tetraethyl lead. About WWII era. After that, it's all additives, additives. additives.

True only if the engine has a knock sensor or you manually retard your ignition timing.

Last i heard, Uncle Sam has made this illegal.

Except that the octane improving additives leave their own deposits. Typically whitish, but just as evil.

Evil preignition!!! First step on the road to detonation. But carbon deposits are more likely caused by too rich a mixture or oil burning, not lower octane.

Agreed. Far and away, the biggest contaminant in gas is water. Amazingly enough, the station charges just as much for water as for gas. Yeccho!! One time I got such a load of water, the car sputtered for half an hour of thruway running before all it got pumped out into the cylinders. If that car had a carburetor, it would have filled the float bowl with water & stopped. Thank goodness for fuel injection that recirculates the fuel back to the tank. All that said, I've had two fuel injected cars stopped by plugged fuel filters. The filters had so much dirt in them, I couldn't blow through the filter. Cut them open and they were solid with black crap I'd bought at the pumps.

Great fun,

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Norm in NJ
Next oil change at 230,000miles

fascinating thread. i am pleased that i started it.

after all that has been posted, tell me, at public stations, how certain are you as to the octane rating of the gasoline that you select?

and tell me this, if you can, how often do any regulatory agencies sample and evaluate the grades of gasoline being sold?

how often do they test pumps to validate the accuracy of the gallonage purportedly delivered?

if they do these tests, how often do they perform them? and are they surprise inspections? or do the stations have advance warnings?

there are many reasons for my asking these questions. from what i have experienced and viewed, i have been forced to conclude that there is only one octane rated fuel in those tanks. and it ain't 93 octane.

now, let us assume that one would want to document the octane rating at major oil company stations. how would you recommend that you go about it?

and how would you prove the octane rating without an octane rating engine? is there any other way?

all ears.

albert

I can only answer a portion of your question re. calibration of gaspumps.
We ( in Canada ) have regulations that require testing/calibration of gasoline dispensers every 12 month.
This is policed by the Weights & Measures Dept. of the government.
A sticker is applied ( very visible while you're fueling your car ) to the pump display.
Having been in a related business ( fuel dispensing ), I can tell you that equipment such as fuel dispensers are designed/built to go " in the consumers favour ", as natural wear takes place.

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2007 C 230 Sport.

nglitz,

Here's a bit of info I discovered, that confirms that detergent additive concentrations vary by gasoline grade.

I've seen no mention of a law against it, nor can I fathom a reason as to why there would be one.

http://www.chevron.com/prodserv/fuels/techron/techback.shtml

Also, detergents, though they are additives, are not necessarily octane boosters. Their purpose is to reduce deposits in all fuel system components, as well as in the combustion chamber.

I've had more than a few modern engines apart, and only those very poorly cared for have any serious "white" or "yellow" deposits in the combustion chambers. I have seen this phenomena in those low mileage Japanese engines that receive ZERO maintenance. Deposits are also determined by your driving style. If you baby your engine all the time, deposits will form more quickly.

From all evidence I can find, octane is still determined by the fuel and additives that increase the length and or strength of hydrocarbon chains.

Modern fuel injected engines do not run constantly lean or rich unless they are not in normal operating condition.

Oil burning may seem like a deposit former, but again, excessive oil burning is a state of disrepair. If your engine burns enough oil to quickly form deposits, it needs to come apart.

Justin

I'm a commercial real estate appraiser and I had an assigment recently to appraise a gas station. What I learned from that is very interesting. It turns out that gas is distributed regionally, and in my region (new england) there are only 2-3 major depots where the gas is distributed from. All of the stations get their gas from these depots, every brand station from the no-name cheapos to the top tier brands. In fact, all of the gas is EXACTLY THE SAME. The difference is the additives (detergents, etc) which are mixed and blended when the trucks pull up to fill up. Shell has their formula, Exxon, theirs, etc. The gas is actually "formulated" right then, just as the tanker is filling. Cheaper no name stations will use generic formulas with minimal additives while the top tier stations use their own proprietary formulas, but the GAS is all the SAME, it all comes from the same tanks at the distribution center. There is no brand difference in the GAS, just in the ADDITIVES...