87 octane versus 93 octane
 

tell me, in the days of microprocessor-controlled engine timing....

you are a car rental company. you rent vehicles that use regular. you rent vehicles that use premium.

how do you fuel your fleet? regular across the board and let the engine control module adjust the timing accordingly. or do you fuel selectively based on the vehicle manufacturers recommendations?

i ask this because hertz rents the majority of its vehicles that are intended for 87 octane, but rents some in its premium collection that are supposed to be fueled with 93 octane. but i have concluded that all are fueled with 87 octane gasoline.

am i right? am i wrong?

The last time I was at a rental car agency I saw two grades being used.

Haasman

Thats a good question, and octane was one of the subjects in a new Q/A column in this weeks Wall Street Journal.

Mercedes benz is very specific about how the car should be used on their premium cars if only medium grade gas is present, no more than 2/3 throttle, no full throttle, no more than 3K rpm, refill with premium as soon as possible.

Rental cars get rid of their cars at what fixed mileage? and if any probems were to develop from using the wrong gas the car would probably be long gone from their fleet.

Could the wrong fuel grade cause an issue where potential litigation forces rental fleets to stock two fuel grades?

most manufacturers set their cars up so that if you put the wrong gas in the tank, it doesnt explode, because if it does its a warranty claim.

its either short sighted, or really brave to sell a car in the usa that wont run if you put the wrong fuel into it.

i live in the mazda world, and we miss out on a lot of cool cars because we (the usa) complain too much.

that is exactly right. they have to because they know some people will put 87 octane for cars that specify 91 (on the flip side, there are people that waste money by putting high octane in cars that don't recommend or need it). On older pre-OBDII cars you could get more performance out of car by chipping computer. That was generally doable because chip would set fuel/ignition/timing curves so car would make more power but at the price that if you don't use 91 octane engine may blow because of knocking. If you search you will find info re: how US 300Es have a resistor that retards timing just in case 87 octane is used, whereas the Euro ones instead of a resistor have an adjustable thing that at the regular setting has no retard (more power than with retard) but requires high octane, but it can be adjusted to retard timing if you are somewhere where you can't get high octane.

Most modern engine control systems have enough timing authority to keep "premium fuel" engines out of detonation if 87 PON fuel is used, and manufacturers are getting away from their dire warnings of "engine damage" if less than the recommended octane is used.

New Corvette owner's manuals say that 91 PON is recommended, but NOT required. This is for the 6.0L 400 HP base engine, which has a CR of 10.9:1.

The new LS7 7.0L 500 HP Z06 engine's CR is 11.0:1, and the same applies.

What you loose is some power, especially at the low end, and fuel economy might be worse, especially in stop and go driving, but probably won't be affected at freeway cruise speed.

Duke

Does anybody here believe, that an average rental car customer would actually fuel a vehicle with premium ( expensive ) gasoline? ;)

Fuel economy of Reg vrs. Prem
 

By accident, my wife gave us a full tank of 87 octane about 3 weeks ago on a roundtrip road-trip from Palm Beach to Montreal. The result was a heated discussion of the "right thing to do" and a few MPG less. NOTHING else. No pinging, no knocking, no anything...

The fuel economy on this 4000 mile roadtrip with premium was 24mpg @ 86mph average. The fuel economy with the one tank of regular was 20mpg... We calculated that the cost for premium is worth it as it gave a better "cost-per-mile" rate.

So, I plan to stick with 93 octane at (an average of) 20 cents per gallon more than 87 octane.

Nearly all electronically controlled engines these days have a knock sensor, which allows the ECU to retard the ingition timing and/or enrich the mixture to prevent knock. The engine will run fine, but lack power and get worse milage on 87 octane instead of 93.

However, on older Benz (and some other engines) that lack a knock sensor (on the Benz, KE-jet never has a knock sensor), running lower than required octane fuel will result in engine damage, usually from detonation at hig rpm/load that is difficult or impossible to hear.

Detonation and pre-ignition can do nice things like burn valves, flame cut piston rings, burn out cylinder wall, and burn holes in pistons. All are expensive!

With new cars, all you will get is a performance/milage hit, and reduced catalyst life.

I assume that most manufactuers will not honor the warrenty for rental cars if the rental agency routinely runs them on the wrong grade of fuel.

Peter

If car companies are so anxious to promote their cars as not expensive to run and maintain, then if running on a lower octane is so harmless why would they not write in their literature that low octanes is useable?

Mercedes has fill for life transmissions, would a shortened life of a catalytic converter be the only reason premium gas is recommended by Mercedes?

Maybe to pass emissions Mercedes needs to run premium, carbon deposites, detonation not manageable by timing management.

Lots of questions.

My '87 260E has run wonderfully for years on 87 octane regular. Better mileage, no pinging etc. excellent starting. Using higher octane gas in a 9:1 compression ration engine is a complete waste of money.

Higher CR, sure.

I agree with Norm. You can use 87 PON in a M103, but you might have to modify your driving habits a bit. For example, with my 5-sp. I need to ease back into the throttle on a short shift to avoid a second of transient detonation, and since I only drive my Merc in the winter it never sees high summer temps that increase the tendency to detonate. An auto trans will likely have less tendency to detonate since torque converter slippage will keep from loading the engine up at low revs like you can do with a manual.

I know of no reason why a lower octane fuel will shorten the life of the catalyst, absent significant detonation, which will probably hole a piston or valve before it harms the catalyst.

As a general rule, lower octane fuels have one to three percent more energy than premium grades, so if anything, fuel economy should be better, but tough to measure since the difference is so small.

Duke

MPG 87 vs 92
 

Parva: One tank of 87 does not convince me that
93 gives better MPG. Most likely driving conditions
or how much the tank was filled before and after
checking MPG gave the difference.

I have been tempted to use 87 durning the winter
since cars don't ping as much in the cold.

Duke,
Funny you should say that.
A friend of mine ( who is an absolute stickler on details, to the point of being anal at times ), tracked this very scenario for a long time.
Results,........lower octane fuel ( at least in his trial vehicle ), consistantly produced better mpg ( 1 - 3 % ), than premium fuel.
His test lasted about 12 months, and every detail was entered into his computer to the 100 th. ;)

87 vrs 93
 

You're right... one tank really doesn't mean anything terribly definative. Suppose I'll try it again but prolly not in the real near future.

A couple of MB techs also told me that it's OK to use one tank of 87 to every three tanks of 93; this is fine for this car... also said that it varies from engine model to model. On a '94 W124 w/ 90k on it, the 1 to 3 rule shouldn't be an issue.

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.

Gas.. Gas.. Where's the beano?
 

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. ;)

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! :rolleyes:

I'm sure Arco has the best gas.. that's what they say
 

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 ). :D
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 ). ;)

Do 87 300Es have a knock sensor?

Premium, 91, ~$2.20 USD PG
 

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. :confused: 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,

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. ;)

and it smells good too.
 

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

Gasoline Brands
 

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...

I'll take a gas martini... shaken.. with additives
 

That makes sense, given the additional infrastructure it would take for all refiners to custom distill/formulate different mixtures. I'm not sure if I would blanket that to include all manufacturers, but it may very well be that way.

Thanks for the insight!