91 Octane Concern

[flywithsean]

I had posed this question in one of my threads but I will put it here so everyone can weigh in on it.

91 Octane is the minimum required octane rating for gas in the smart fortwo. Which is not a big deal unless you live in a high altitude area such as Denver, Salt Lake City, Reno, Albuquerque, and so on. In these areas you may not be able to get gas with an octane rating that high. Even here in Vegas where we are only at 2200 feet 91 is the highest octane rating I can find.

Has anyone in a high elevation city heard anything about the octane situation?

[evoluzione]

the reason higher altitudes have lower octane is that it isn't needed. if a car at sea level needs 91 octane, at 5000 feet it needs 89 octane (or whatever is sold). using a higher octane fuel than is required by the engine is not efficient. octane ratings are just a determination of the knock resistance of the fuel - the higher the octane, the slower the burn of the fuel.


regards,

ken

[GreenBastard]

There may be issues with fuel using high levels of ethanol. Ethanol is a very corrosive fuel and may be an issue in the future on most vehicles on the road today. Running high octane in a 1.0l engine may not make a huge power difference and in some cases can make non-performance rated engines run poorly. My Harley-Davidson motorcycle runs best on 87 octane and with 91 or 93 octane will make the engine stall and run rough.

[OC1Dave]

All new cars have "ethanol proof" fuel systems, and ethanol has a higher octane rating than gasoline alone, so not to worry. The disadvantage of ethanol is that it has fewer BTUs than gasoline and will compromise gas mileage to the extent of % i.e.: more ethanol - lower fuel mileage.

[jwight]

Originally Posted by OC1Dave

All new cars have "ethanol proof" fuel systems, and ethanol has a higher octane rating than gasoline alone, so not to worry. The disadvantage of ethanol is that it has fewer BTUs than gasoline and will compromise gas mileage to the extent of % i.e.: more ethanol - lower fuel mileage.

Dave, glad you posted that. The ethanol thing is misleading at best and does not "save gas." Lower mpg, as you point out. I think a new owner posted elsewhere about an upper limit of ethanol mix (10%??) for the smart but I can't locate it right now.

[OC1Dave]

In a racecar, which is where I have had the most experience with methanol (ethanol is nearly the same molecule), it takes about twice as much fuel to get the same driveability, so adding %age to gasoline will require the engine management system to keep adding more fuel to the charge. Most new car systems, with the exception of GM's E85 engines, can only handle so much more flow, probably limited by injector size. Thus, it is probably the case that smart/mitsu has a threshold of 10%, above which will take engine modification and would lead to a precipitous drop in MPG. My 2 cents.

[Fredvon4]

OC1Dave...I run alky also and know the dynamic fairly well

more to the point I got tired of FoMoCo screwing around with their diesel motors so I optioned in a 3v V10 in my current heavy hauler F350

On pump 87 I have a certain mpg and power throttle response feel ... unfortunately I have to travel (very heavy at 22,800+/-LBS GCVW) several time a year to vacation areas that require I only get the so called "clean air" blends... usually 10 or 15% ethanol, sometimes still the bad MTBE stuff (but that is another rant) in every case I loose significant power and get miserable economy from an already fuel thirsty truck...

It will be a very long time before any one convinces me the alcohol blends are a good idea... even in a vehicle designed to burn e-85

Back on topic...

flywithsean.... another thread had a good post on the effects of altitude on internal combustion motors.... basically the higher you go the less oxygen in the air but more importantly the air pressure is reduced from what it is at sea level.... our semi high compression motors have a barometer in them that can compensate for modest altitude changes... there are limits to all these systems... 91 octane should work fine from sea level up to Denver heights of 5000 feet...

You might actually get better economy using a mid grade 89? for higher altitudes.... The theory being less air pressure has a reduction of compression ratio max pressure

But this really is because one of the strategies the motor uses when it detects too much pre-ignition (PING) is to retard the ignition timing and increase the fuel shot a tad richer...

BTW: Many of the modern computer controlled motor sensors and ECUs are able to detect and eliminate the ping before the driver ever knew it happened... the most you will notice, if you do, is a loss of power

I suspect where you are in Vegas you will be fine with the using the 91 stuff.... IIRC you are in a "clean air" blend area so let us know how she reacts to the ethanol blends when that season starts

[rrrucke]

Say, what's the name of the gas website where you can search for the gas prices in a certain area? Thanks, Regina

[jwight]

One of them is www.gasprice.com.

[Karl Roth]

This one covers a good chunk of North America
http://www.GasBuddy.com
Karl