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.