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  #1  
Old 08-28-2003, 04:10 PM
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Premium? or Regular is fine?

Hi All,

With our gas prices in CA always soaring, do I really need to use Premium (92 Octane) in my '87 W124? I was thinking of using Regular or Mid-Grade fuel....

Also, I know our cars are not high revving ones so they are not high compression right?

Any thoughts?

Thanks,
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'87 W124 260E (DD)
98K orig. mi. @7/15
CLK 7-Spoke Forged Wheels
Neuspeed springs/Bilstein Sport
4/3 bump (F/R)

'97 993 Carrera
106K orig. mi.
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  #2  
Old 08-28-2003, 05:16 PM
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First, what octane is recommended for your vehicle?

This subject comes up every time gas prices are on the rise. I direct people to read a well written articles from Car and Driver that addresses this issue of octane, and what will (or won't) happen if you use regular instead of premium in your vehicle.

Car and Driver Magazine: Regular or Premium?
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  #3  
Old 08-28-2003, 05:21 PM
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Location: Ontario, Canada
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Re: Premium? or Regular is fine?

Quote:
Originally posted by sjsfiji
Hi All,


Also, I know our cars are not high revving ones so they are not high compression right?



Thanks,
Just FYI , revs have nothing to do with compression ratio.
Your engines octane requirements are largely determined by the compression ratio.
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  #4  
Old 08-28-2003, 05:32 PM
Bud
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The owner's manual for my '91 300E says to use at least 91 octane fuel and if you can't get it you can use SOME lower octane UNTIL you can get the right fuel.

If you use lower octane, you are told to not exceed 3,000 RPM and to take it easy until the correct fuel is obtained.

The cost of fuel is insignificant in the total cost of car ownership.

BTW, I live in the Phoenix area where gas is nearly as costly as it is in California and (recently) where getting premium was almost impossible. Nevertheless, I never considered using the wrong fuel.
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  #5  
Old 08-28-2003, 05:53 PM
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Some cars are equipped with anti-knock sensors and will retard the timing to compensate for inadequate octane.

I'm not sure I'd want my engine running in a degraded mode.

Fuel ain't cheap, but in the long run, good fuel can make a difference. My owners manual calls for 91. In my area there's 87/89/93. I use 93 - wouldn't think of feeding the car anything less, irregardless of what was said in any magazine article.
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  #6  
Old 08-28-2003, 06:28 PM
Bud
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The M103 engine cannot adjust for lower octane fuel. As for those cars that are supposedly capable of using regular gasolines, they are borderline in many cases. When the temps get well over 100F in Arizona, you can hear a lot of American cars pinging like crazy. If I owned a car supposedly capable of using 87 octane, I'd be using at least 89.
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  #7  
Old 08-28-2003, 06:35 PM
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Lightbulb

Gas prices in Vancouver, BC is at 91.9cents CAD/liter for 87 octane nowadays...and has been that price for almost a week now...I usually fill up with 94 octane, but have dropped down to 92 octane during my recent fillup to save a few coins...but I would never think of moving down to 87 octane...

If money is that tight, there is nothing wrong with taking the bus...or walking even...good for your health, and easy on the wallet...
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  #8  
Old 08-28-2003, 07:10 PM
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i'd take the bus only if it has a three pointed star on its
front grill!!!
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  #9  
Old 08-28-2003, 07:30 PM
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pheeewww... thanks....

Hi to all and thanks for the fast reply. I have always used Premium in all my cars...

Anyhow, can someone explain compression ratio and what determines it; how fuel is related... many thanks...
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'87 W124 260E (DD)
98K orig. mi. @7/15
CLK 7-Spoke Forged Wheels
Neuspeed springs/Bilstein Sport
4/3 bump (F/R)

'97 993 Carrera
106K orig. mi.
Always driven like it's stolen
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  #10  
Old 08-28-2003, 07:54 PM
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Location: Between Oakland and Vallejo, CA
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Quote:
Originally posted by joel
i'd take the bus only if it has a three pointed star on its
front grill!!!
i take the bus on occasion, one local agency has Van Hool buses from Belgium. It was 2.37 for premium at a Chevron station i passed on the way to school.
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  #11  
Old 08-28-2003, 08:37 PM
Bud
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A very simplified explanation of compression ratio is the amount the air/fuel mixture in the cylinder is compressed. In the case of a 10:1 compression ratio. The air/fuel mixture is compressed to 1/10th of it's original volume.

The more the air/fuel mixture is compressed, the easier it is to detonate. Higher octane fuels avoid pre-ignition and potential engine damage.

Generally, higher compression ratios result in a more efficient engine both in economy and performance. The only penalty is the cost of the higher octane fuel.

BTW, many brands of gasoline have a higher level of additives in their premium fuels. This is true with Chevron and Amoco among others.
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  #12  
Old 08-28-2003, 09:36 PM
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Mercedes specifies premium fuel for the 103 engine, but the compression ratios are not that high. My 2.6 is speced at 9.2:1 and modern cars of this CR usually only require regular grade fuel.

Contrary to what you said, the 103 is a rather high revving engine with a torque peak in the range 4400-4600 and a power peak in the range of 5600-5800 with little rolloff to the rev limiter. You can feel this SOTP as the engine pulls most strongly from 5000-up and is noticeably soggy down low.

Detonation is primarily a low speed phenomenon. It is most likely to occur with high throttle openings and low revs. I've successfully used regular in my 2.6, but have to modify my driving habits a bit - primarily shifing at higher revs and avoid high loading under 2000 revs (I have a stick shift.) An automatic with a torque converter will have less tendency to load the engine at low revs.

There is a good chance that your engine might operate successfully on regular or at least mid grade. To determine whether it will, let the tank go to near empty, then pump in about three gallons of mid-grade. Drive the car and listen for detonation. Having the driver's window down will help as the sound will bounce off parked cars and city buildings. If it operates satisfactorily, try 3 gallons of regular when it hits empty again. If it detonates add 3 gallons of premium and then try mid-grade again. Keep testing until you have determined what it takes to avoid significant detonation.

Detonation characteristics are affected by ambient temperature and pressure, humidity, your driving style and driving environment (hilly or flat terrain). Because of of all the interacting variables, the only way one can know whether lower grade fuel will work is to test.

Modern cars with knock sensors (103s don't have them) will operate on regular grade fuel regardless of compression ratio. Lower grade fuel will result in less spark timing, which will reduce torque, particularly down low, but most drivers can't tell the difference.

Duke
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  #13  
Old 08-29-2003, 06:28 AM
Manya
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wtf? here in australia our lowest buyable RON petrol is 92RON and over your side that's considered premium?
And here I was complaining that 98RON petrol is the only highest we can buy
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  #14  
Old 08-29-2003, 09:32 AM
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Quote:
Originally posted by tkamiya
Don't W124s in European countries come with a dial to adjust the ignition timing to compensate for varying octane ratings available in some of the countries? If so, perhaps it could safely be re-implemented in US and use regular gas...
Perhaps, but you forget that the average US car owner is a moron when it comes to motoring. It would be more problem than it's worth.
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  #15  
Old 08-29-2003, 09:37 AM
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Quote:
Originally posted by Manya
...RON petrol is 92RON ....98RON......
I believe RON means Research Octane Number. Our gasoline is rated with the formula, (R+M)/2, which is the average of Research and Method octane numbers. Apples and Oranges.
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