Octane Question

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Permalink Reply by Daniel Coté on April 15, 2008 at 8:03pm

I don't have personal knowledge ... yet. I filled up with mid-grade yesterday and will do a comparison with past experience now that my smart seems to have settled into the 42-43 MPG range. (We have approximately 2,000 miles on the car)

There's an owner on another board who claims to have used nothing but regular in his car without a problem.

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Permalink Reply by midnight_crossing on April 16, 2008 at 6:43am

only 95? You don't have 91 or 92 Octane? Where do you live?

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Permalink Reply by SATO on April 17, 2008 at 1:13pm

I think Smart wants Us to use something around 91-95 range but the only thing that should be affected by lowering the octane is your fuel economy not much else.. Let me ask someone for an updated reason and I'll post his perspective (an ex-Exxon guy)..

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Permalink Reply by smartster on April 17, 2008 at 5:15pm

91 is the base octane you want to be using in the smart due to its compression and often high RPMs, using normal gas will allow the smart to work but you will not want to push the smart, the burn rate of the fuel is designed for 91+ so anything less then that will cause the smart to not burn/compress/use the gas properly.

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Permalink Reply by SATO on April 17, 2008 at 7:17pm

backfire or ping

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Permalink Reply by canuck451 on April 21, 2008 at 6:01pm

The story on octane isn't as straight forward as you might think because there are a couple different ways of defining octane for gasoline.

The 451 manual (and even the gas tank cover) say you should fill the tank with gas with an octane rating of nothing less than 91 RON (Research Octane Number) and preferably 95 RON. If you live in Europe or Australia, gas octane is measured in RON and, hence, you should fill your tank with 91 or 95 octane.

Another way of defining octane is MON, or Motor Octane Number. This is mostly used in aviation and is measured slightly differently... so the MON of a particular gasoline will come out about 8 to 10 points LOWER than RON... same gas, different measurement technique, different number. So, a gas with a RON of 91 will have a MON of 81 or 83.

In North America, octane at the pump is shown in something called PON, or Pump Octane Number, which is mathematically defined as (RON + MON) / 2 (you can actually see this equation on some pumps if you look for it). Since MON is 8-10 points lower than RON, the PON will be 4-5 points lower than RON. Therefore, a 91 RON gas is the same as a 86-87 PON gas (which is why you won't find a gas pump in Europe pumping out anything less than 91 octane and in US/Canada our pumps go down to 87). Therefore, in North America, you're perfectly safe to put 87 octane in your tank (equivalent to 91/92 RON) or 89 octane (equivalent to 93/94 RON).

There's plenty of resources on the net if you want to double check what I'm saying. Some starting sources:
http://en.wikipedia.org/wiki/Octane_rating#Measurement_methods
http://www.torquecars.com/articles/fuel-octane-ratings.php
Google RON MON or gas octane ratings

And to answer that old myth... no, if you put a higher octane gas in your car than the manufacturer's recommended octane you will NOT NOT NOT get better fuel economy or horsepower. Octane is NOT a measure of a gasoline's 'power'... it's purely a measure of its self-combustion under the stress and heat of an engine cylinder. That said, if you put a LOWER octane gas than your manufacturer recommends, you WILL get worse fuel economy and horsepower because your engine will knock (the gas will pre-detonate in the cylinder), your knock sensor will detect this and change your fuel mixture to stop the knocking... this will lower your performance. The Smart engine is rated at ~71 horsepower... you can't increase this by putting a higher octane gas in your engine.

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Permalink Reply by SATO on April 21, 2008 at 6:23pm

you my friend are starting to stirr the nest of contravercy..
So did Smart know this and compensate for it in their computations? are their requirements to use 91 or higher pre or pos (equivalent to 91/92 RON) or 89 octane (equivalent to 93/94 RON).? Or thid they not take it into consideration, telling us to use 91, a more expensive fuel than 89 in mistake?
I dont think they would make an error that would be so unpopular with the possible clientelle looking at the smarts cost effectiveness in a market that wants to have cars run 100mpg on the cheapest fuel at the pump.

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Permalink Reply by canuck451 on April 22, 2008 at 12:01am

Ok... Go check your manual. I'll wait.

Now check the label on the inside of your fuel filler nozzle. I'll wait.

Did both say "91 RON (minimum)"?

Prove me wrong that PON = (RON + MON) /2. Now prove me wrong that pumps in North America use PON and pumps in Europe (where the car was made) use RON.

I don't care what the dealer said. If they told me to fill it with 95 proof rum but the manual and the car said to fill it with 91 RON I would trust the manual first. To be truthful, my dealer also told me to fill it with 91... but he's a sales guy looking at a number and obviously no understanding of the difference between RON, MON & PON. He's been trained in Europe... but octane isn't octane everywhere around the world.

Ultimately it's up to you... it's your money. I have done my research and have no interest in damaging my wonderful new car. 89 PON is what I'm putting in mine and I have experienced no pinging due to pre-detonation. I'll let you know if I do.

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Permalink Reply by WDSMGuy on April 22, 2008 at 12:30am

Well, now. Living in the land of corn in Iowa, does the idea of getting your 89 or 90 PON octane gas from ethanol change anything? (This from the guy that will pick his up this weekend).

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Permalink Reply by canuck451 on April 22, 2008 at 11:12am

Be careful with using ethanol blended gas and NEVER use pure ethanol gas (ie: E85) in the Smart. Only engines specifically designed for Ethanol gas (many of which are labelled as "Flex-fuel" engines) can run E85 - running it in your Smart will actually damage your engine due to the differences in burn rates, viscosity, etc.

Blended ethanol gases CAN be used but in the U.S. Ethanol blended gas can be up to 10% ethanol, which studies have linked to premature engine wear through corrosion of ferrous parts and disintegration of rubber seals and hoses (http://www.environment.gov.au/atmosphere/fuelquality/publications/2... which is a study of 20% blended ethanol gas... but the effect is similar but not as dramatic for 10%).

Personally, I'd avoid gas with ethanol in it. I prefer my corn on the BBQ or in my tortillas... not in my engine.

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Permalink Reply by SATO on April 22, 2008 at 11:23am

I strongly second your opinion Canuk!

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Permalink Reply by mr bad example on April 22, 2008 at 12:32am

i'll buy that and save a dollar! i've been running 93 octane since new, i'll try BP silver 89 US octane next fill-up. i haven't had my car over 4000 rpms yet (i top out at 75mph when i get the chance) i prefer to keep it at 3000 rpms (60mph). i plan on spending weekends at my dad's cabin a little over 200 miles from home, if i can get away with 45-50mph, i'll do it! it'll take an hour longer, but i should see 50+mpg, i have a good stereo installed, so i'll sit back and enjoy the ride (hoping for cruise control soon)

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