Premium Fuel

why is there a need for premium fuel? It seems that this will negate most of the fuel savings

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Permalink Reply by Rick Masters on January 27, 2008 at 5:56pm: It is a common misconception to believe that using premium fuel in an engine designed for it entails a loss of value compared to one using regular unleaded. Reason A, realize that this is not an American car and regular unleaded has a higher octane rating in Europe and possibly in Japan where the Mitsubishi engine is manufactured. This is because fuel costs so much more over there that manufacturers do everything they can to wring every last bit of power out of the fuel. Over in the States, nobody cared that much, until recently, about fuel efficiency. The Mitsubishi's 657cc base engine design had a compression ratio of 10.8:1. Then this engine was redesigned for the Smart Fortwo: bored and stroked to meet the demands for power of the NA market. Reason B, because the compression ratio was boosted way up to 11.4:1 for the 999 cc engine, it requires premium for two reasons. One, for maximum efficiency the progress of the flash through the air/fuel mixture in the combustion chamber has to be slowed down to provide optimum expansion during the power stroke. Remeber, because they increased the stroke, the piston has farther to travel and you want to have the gas expanding at such a rate as to deliver steady power throughout the entire length of the downstroke. This is what the magic of higher octane does. It makes the fuel burn slower to deliver more power and better mileage. Surprise. Most folks think it is just the opposite. But if you were to run unleaded regular, the air/fuel mix would flash faster, delivering too much power at the top of the down stroke and not enough toward the bottom. See? Using lower octane WASTES fuel. Second, to protect the engine from predetonation, you need this slower-burning fuel to avoid damaging the piston head. As an old California hotrodder, I would not even consider running low octane (below 91) in an engine with a compression ratio above 11:1. The most important thing you can carry away with you from all this is that the engine runs a lot better with premium. So much better, in fact, that you will actually end up paying MORE in fuel costs trying to run it on regular. So the whole premium concern is really a non-issue. Look, it's a Mercedes. Put premium in it with a smile on your face.; ▶ Reply to This

Permalink Reply by neal on January 29, 2008 at 10:45am: Thanks for the comprehensive data on fuel for the smart. I guess the dealership is trying to sell cars when they said that they were told to run regular unleaded. Fuel grade has been a minor consideration and concern, as my big Ford engine runs on regular unleaded. Smart engine specs, printed in their publication, said the Compression ratio is 10.0 : 1 and fuel type is Premium unleaded. I checked the gas filler connection said 91 octane. The Mitsubishi data on the 3B2 engine doesn't list the octane rating, but does list the type of fuel as Regular unleaded gasoline. I have to go alone with you and put Premium fuel in it. I plan to start with premium and then try Medium grade for awhile and check the milage and performance. What do you think? Thanks for your expertise. Neal; ▶ Reply to This

Permalink Reply by Rick Masters on January 29, 2008 at 2:58pm: Smart engine specs, printed in their publication, said the Compression ratio is 10.0 : 1 and fuel type is Premium unleaded.
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You are right. I must have gotten hold of the European specs for this engine. At 10:1 the car is significantly detuned and will last longer if subjected to misuse. Now I'm interested to know the difference between the versions. It could just be the piston. Also, 10:1 opens up the potential for mild turbocharging for high mileage applications. Might still be a bit risky for performance applications at that compression, though.

I've made about a dozen high mileage attempts between southern California and New Mexico in a turbocharged 5-cyl Volvo. The EPA rating is 24 mpg and I have achieved 34 average round trip on Hy 40. I could not do that with unleaded regular. I had to use premium and it turned out to be cheaper to use premium, overall. Another very important consideration was to run the tank down at the point where you filled up with ethanol-free unleaded premium in Arizona. There is about a 30% mileage loss with ethanol which will play havoc with any attempts to measure mileage performance by Smart Car owners. Ethanol does boost the octane a little bit.; ▶ Reply to This

Permalink Reply by Daniel Coté on January 29, 2008 at 6:28pm: I'd like to add a data point here. I've always equated high compression ratios with high octane requirements. I don't know how they do it but my 650cc Suzuki Burgman has an 11.2:1 compression ratio and the factory recommended fuel is Regular Unleaded. That's what I run in it and it runs fine.; ▶ Reply to This

Permalink Reply by mr bad example on February 5, 2008 at 11:47pm: sorta like spud guns, barrel lengths and fuel burn rates, you want it still burning/expanding as the tater leaves the barrel, otherwise the spud slows as it approaches the end of the barrel. with spud guns you experiment with barrel lengths and fuels. if you search the internet, there is some wildly detailed combustion/expansion rates of fuels on spud guns! (seems like a lot of brilliant minds have a lot of free time!!!!) i used that data to design a few spud guns (my first gun blows away 95% of others on the internet with power/velocity, you need a trailered or stationary spud gun to surpass my 5ft little gun) http://www.youtube.com/watch?v=OqBfxPMP_9E :); ▶ Reply to This

Permalink Reply by kerry on December 14, 2008 at 10:02am: Hi, I know this is a question from January, but I wanted to say that I have two friends that one has a mercedes slk, he always has used regular and has had no problems with performance or milage. I recently spoke with someone that has a SMART. She drives about 150 miles per day and showed me her rather obsesssive fuel charts. She is getting high 40s mostly highway. I asked her if she uses premium and she stated no, not even the medium grade, only the cheapest and she does not sufffer from any engine knocking, lower milage, etc?? I have had mercedes cars in the past and they all demanded premium. I will continue using premium in my SMart but it appears some people do not adhere to the recommendations without any short term consequences.

Kerry; ▶ Reply to This

Permalink Reply by 451 Kahn on December 14, 2008 at 3:54pm: The ECU has a knock sensor which monitors the engine. You can add any fuel you want but the best performance you engine will get is from the 91oct. The ECU just slows down the engine as poor fuel octane is used. It is your car put what ever you want. The car will still run and the ECU will work for you. This is why they call it a Smart car.; ▶ Reply to This

Permalink Reply by Gilbert on December 14, 2008 at 5:03pm: That's right 451 Kahn... knock sensor does the works same as my C230 MBZ requires 91 octane I put 87 unleaded it works without knocking but,,,,,, the car performs slower.

A) same in mini cooper S 2003
B) Not the same with my 1962 cooper classic its ping not good

i've never tried puting 87 octane in my Passion trying to learn in this tread...; ▶ Reply to This

Permalink Reply by Ian Pellant on December 14, 2008 at 7:44pm: This is an old and well worn thread.

1962 was pre unleaded fuels... by a decade or so. They generally don't like low octane unleaded. They certainly didn't have any ECU or electronic igntion system... we learnt how to tune by loosening the distributor clamp and advancing / retarding a bit.

smart advise that a mix with a lower octane fuel than recommended will work... ECU adds more retard to the ignition; you get less power and fuel consumption may go up. Even with Australian fuel prices (usually twice or more than US prices), it doesn't save anything dropping to the standard RON 91 octane.

On the other hand.... there is a trend here (in Australia) to drop the RON 95 (premium unleaded) pumps in favour of the higher grade RON 98 fuels; leaving the regular unleaded RON 91 as the only other choice. With my pulse turbo, I can't notice any real advantage with the 98 fuels (Shell V-Power) over the 95. smart recommend RON 95. The local Shell stations remodelled their pumps towards 98 fuels in October, getting to a 95 pump is not easy anymore. Maybe smart will offer us a few BRABUS turbos which need RON 98...?

I dunno how true it is, but I read on Wikipedia that there are only two grades of fuel delivered to the service stations: the lowest and the highest octanes... the other grades are created by mixing at the pump... no real guarantee that those intermediate grades are accurate from one pump to another?; ▶ Reply to This

Permalink Reply by DAN on December 14, 2008 at 7:35pm: What will happen if you use 93oct. Premium here in NY comes as 93oct. followed be 87 and 83.; ▶ Reply to This

Permalink Reply by 451 Kahn on December 14, 2008 at 9:01pm: 93oct and the car runs faster. I am waiting one day to add 100oct (Racing fuel) to my car @ $7.99 a gallon and see how fast will it go.; ▶ Reply to This

Permalink Reply by DAN on December 15, 2008 at 3:06am: Dear 451 Kahn

As always you are a pioneer when it to trying new items on Smart car. Well done!!!. I am sure that with remap and 100otc the will go faster that any other smart in US.

Thanks,

Dan; ▶ Reply to This