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Discussion Starter #1
G'day everyone,

I haven't noticed any postings on what grade octane petrol you are using in your 350's. Also, what is the recommended grade?

In Australia, I think we have Unleaded (92) Premium Unleaded (95) and Ultimate (98) available.

Agent 1. [8D]
 

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06 SLK 55 Firemist
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Fuels are formulated differently all over the world. Even in the US deferent regions (and metropolitan areas) have deferent formulas. As a general rule for US SLK's 91 is the preferred octane rating. Also despite popular (wrong) belief using a higher rated octane will not enhance performance, but will actually degrade performance and add pollutants from unburned fuel.
 

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Julien - 2/13/2005 5:06 PM

Fuels are formulated differently all over the world. Even in the US deferent regions (and metropolitan areas) have deferent formulas. As a general rule for US SLK's 91 is the preferred octane rating. Also despite popular (wrong) belief using a higher rated octane will not enhance performance, but will actually degrade performance and add pollutants from unburned fuel.
Remember that octane is measured differently in Australia and the US. US 91 is about 96 or 97 (I think) in Australia.
 

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I've seen 93 octane in the Chicago area and a gas station not too far from my house has "racing gasoline" rated at 100 octane for $3.25/gallon.
 

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Hi Ross,

Have been using BP Ultimate (98) since delivery of the 350 two weeks ago.

Trialed Shell Optimax & Caltex Premium with my MINI Cooper S and there was a noticeable improvement with BP Ultimate...smoother, more responsive. I hear the Mobil 9000 is pretty good too but yet to try that.

Cheers...alex
 

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Julien - 2/15/2005 10:06 AM

...Also despite popular (wrong) belief using a higher rated octane will not enhance performance, but will actually degrade performance...
Thank you, thank you. [;)] Finally, I know there is at least one other person who understands this.

The octane number is simply a quantitative measure of the fuel's ability to prevent 'knock' or pre-ignition, which is typically an issue at higher atmospheric pressures such as at the coast. It is done by adding additives that actually reduces the speed at which the fuel ignites. (Very important for some high-performance engines at high ambient pressure, to prevent damage.)

That is the reason why Aus fuels are typically a higher octane - most cars run close to the coast (becuase 80% of Aussies live on the coast, or close to it.)

cheers
roadsta
 

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When I collected my 350 from Bremen I asked the technician about fuel. He said 95 rating is fine and the so-called ultimate is a waste of money and does nothing beneficial.
Rusper
 

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roadsta - 2/14/2005 5:40 AM

Julien - 2/15/2005 10:06 AM

...Also despite popular (wrong) belief using a higher rated octane will not enhance performance, but will actually degrade performance...
Thank you, thank you. [;)] Finally, I know there is at least one other person who understands this.

The octane number is simply a quantitative measure of the fuel's ability to prevent 'knock' or pre-ignition, which is typically an issue at higher atmospheric pressures such as at the coast. It is done by adding additives that actually reduces the speed at which the fuel ignites. (Very important for some high-performance engines at high ambient pressure, to prevent damage.)

That is the reason why Aus fuels are typically a higher octane - most cars run close to the coast (becuase 80% of Aussies live on the coast, or close to it.)

cheers
roadsta
All true, but remember that the SLK350 recommends 91 octane as a minimum, not as standard. The car has 10.7 to 1 compression. That's a lot. If you run 87 octane your car will be riding the knock sensor and the car will be so retarded it will run poorly. On the other hand, running 93 to 95 will not hurt one bit, or hender mileage as the pulse width on the injectors will decrease after the unburnt fuel is seen exiting the exhaust. But as stated earlier, higher octane does not mean higher performance. It's all about the temp of that combustion chamber, which increases with higher compression. 91 minimum is also based on the heat range of the spark plugs that are in the car. If you were to increase or decrease that heat range, your octane requirement will change also to prevent detonation or poor operation.
 

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On collecting my SLK350 at Bremen I was told the same as rusper was told " use 95 " which I did on the first fule stop on the way back, then at the night stop over I read the manual and it says at least 95, so the next day on my second stop for petrol I filled it up with 97 or 98 , maybe it's all in my mind, but the higher octan feels better to me.
Will love to read more comments about this. EDD11Y
 

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Discussion Starter #13
Thanks everyone for your comments.

roadsta, what are you using Premium Unleaded (95), or Ultimate (98)? Also, what does the Oz manual recommend?

Agent 1. [8D]
 

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Agent 1 - 2/15/2005 9:52 AM

Thanks everyone for your comments.

roadsta, what are you using Premium Unleaded (95), or Ultimate (98)? Also, what does the Oz manual recommend?

Agent 1. [8D]
The manual recommends 'at least 95'.

I have chosen to use 98. That way I know, even at full throttle and lower revs, I will never ride the knock sensor.

And I also think (guess) that manufacturers take a little more care with the formulation of their top fuel.

Above all, I will only buy petrol from a place that guarantees NO ethanol in the formulation.

cheers
roadsta
 

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sinbat - 2/16/2005 9:10 AM

Wow, only 91 in CA?? I thought 93 is the standard in the US.
No, I have only seen 91. Could it be because of California's extreme emissions standards?

The LA basin area and some other portions of California have to use RFG (Reformulated Gas). Unfortunately, RFG cuts back on mileage and also does not seem to like the elastomers used in new cars so can cause gas leaks. See the link below.

http://www.thecarguy.com/articles/gasoline.htm
 
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