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Old 02-25-2000, 09:29 AM
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Join Date: May 1999
Location: Queens, New York, USA
Posts: 38
I have a 83 300SD Turbo Diesel. The ownerís manual recommends using diesel 1. The problem is that we do not have D1 here in NYC but we do have D2. Just recently, a gas station in my area started to sell D2 with an octane of 50. Iíve been using D2 with octane of 40. The answer seems obvious and the tendency might be to use the higher octane. The problem is that the gas nozzle on the higher octane diesel does not fit well in the car. My question is why? Is it because I should not be using the high octane diesel?

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Old 02-26-2000, 12:28 AM
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Octane? Are you sure you don't mean Cetane. Octane is a rating that describes ignition retardation. The higher the octane the slower it burns. You want a high cetane rating, cetane ratings are essentialy the inverse of an octane rating. If it indeed states octane on the the pump then you want the LOW octane diesel fuel.
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Old 02-28-2000, 09:21 PM
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Join Date: May 1999
Location: Queens, New York, USA
Posts: 38
Definitely octane. I went back to the gas station to make sure and it says ďPremium DieselĒ with and octane of 50. Iíll follow your recommendation and continue using the lower octane fuel. Thank you very much for all the tips. Because of this forum and people like you, it is a lot easier to own a MB vehicle.

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Old 02-28-2000, 11:15 PM
Robert W. Roe's Avatar
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Join Date: May 1999
Location: Lehigh Valley PA
Posts: 1,330
I've been under the impression that higher cetane means better fuel. I 've heard that the 350SDL required 60 cetane fuel, so I assumed that higher cetane is better.

Strange thing is, in our area we have a Gulf station selling Gulf Dieselect Premium Diesel (no posted cetane number) for $1.729 when the other local stations are selling 40 cetane diesel for $1.799 to $1.999, thankfully down a bit from $2.299. They have the cheapest diesel in the area.
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Old 02-29-2000, 07:37 AM
Posts: n/a
I believe the Octane sign at your service station is a mistake. It should read cetane.

Maybe I just haven't paid attention, but I don't remember seeing a cetane or any other ratings on a diesel pump in the Southern states. I have always understood that the difference between numbers 1 and 2 has to do with temperature ranges. Because of the mild winters here we only have one number. I know that it freezes at 9 degrees F. Yes it got that cold here once and I looked it up in my owners manual. I was almost stranded on a cold, windy night. Found out later about additives which I now keep in the trunk during the wintertime just in case.

Since I've never seen a cetane rating, I've never known that lower was better. I'm not surprised, however, because almost everything else about a diesel engine is totally backwards from a gas engine. I guess Rudolph just danced to a different drummer.

As you see, you learn something new every day if your reading the postings on this website.

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Old 02-29-2000, 10:35 AM
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Join Date: Apr 1999
Location: Apex, NC USA
Posts: 176
Quite true Larry, Premium deisel is a northern thing. I checked mobil shell amoco and others websites and the only distribution sites in the us are near the northern border.

1991 350 SDL
114,000 miles
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Old 03-03-2000, 08:15 AM
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Join Date: May 1999
Location: Queens, New York, USA
Posts: 38
Thanks guys. A diesel mechanic told me that the premium diesel is mainly for the newer diesel engines. He also indicated that my car would actually run more efficient with the 40 octane diesel since the fuel is thicker and burns slower.
As far as the diesel fuel and low temperatue, one soluction is also to add Kerosene to the diesel. We experienced very low temperature this past January (6F) and I did not have any problem.

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Old 02-21-2005, 09:40 PM
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Join Date: Jul 2000
Posts: 638
diesel fuel performance ratings explained

Diesel No 1 and No 2 refer to the boiling range and viscosity of the fuel. No1 is more like kerosene, and No 2 is heavier (more viscous). No1 will remain fluid without wax crystals to a much lower temperature than No 2.

CENTANE number refers to the ignition quality of the fuel, not the temperature range or viscosity. A low cetane number fuel (say 35) has a long ignition delay--the time from when feul injection starts to when it actually lights off. A high cetane number (say 60) has a very short ignition delay.

Since the base fuel injection timing is fixed (much like ignition timing) the injection pump is set to achieve best performance on a given quality (cetane number) fuel--performance being measured as smoothness, power, smoke emission, etc. The injection timing advances with speed and load to maintin the 'best performance.' Al ot of test stand work goes into determining the advance curve for a given engine. Lots of mechanical things influence the performance.

Higher centane fuel will, for a given engine, tend to run smoother (less diesel knock) with lower smoke. Power may be increased (particularly if the low cetane alternative is lower than what the engine was timed for).

It will not caue any performance deterioration to use high cetane fuel--and it may be beneficial depending on the engine, its condition, and driving habits. Using a fuel with too low a cetane number will cause hard starting, noisy smokey warm-ups, low performance, and lots of 'diesel knock'. Engien damage/destruction will happen if the cetane is too low and the knocking literally breaks things apart.
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Old 02-21-2005, 11:09 PM
Larry Delor's Avatar
What, Me Worry?
Join Date: Jun 1999
Location: Sarasota, Fl.
Posts: 3,114
FWIW, the Hess station down the road has a Cetane Rating sticker that reads: 42
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein

09 Jetta TDI
1985 300D
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Old 02-22-2005, 10:26 AM
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Posts: 1,158
Originally Posted by Cisco
Thanks guys. A diesel mechanic told me that the premium diesel is mainly for the newer diesel engines. He also indicated that my car would actually run more efficient with the 40 octane diesel since the fuel is thicker and burns slower.
As far as the diesel fuel and low temperatue, one soluction is also to add Kerosene to the diesel. We experienced very low temperature this past January (6F) and I did not have any problem.

Could you supply me with the location and phone # of the station that has 50 "octane" diesel?
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Old 02-22-2005, 04:38 PM
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Join Date: Jul 2000
Location: Exeter, NH
Posts: 139
I would rather cut and past than type so here goes. May be a little deep, but lots of good info.

Here is some info i got from

To explain how octane and cetane DO NOT work together, Iíll have to review more crude oil and fuel fundamentals.

The light distillates that gasolines are made from have a natural high-octane index. The middle distillates that diesel fuels come from have a high cetane index. The octane and cetane indexes are INVERSE scales. A fuel that has a high octane number has a low cetane number, and a high cetane fuel has a low octane number. Anything with a high octane rating will retard diesel fuelís ability to ignite. Thatís why each fuel has developed along with different types of engine designs and fuel delivery systems. Gasoline mixed in diesel fuel will inhibit combustion in a diesel engine and diesel fuel mixed in gasoline will ignite too soon in a gasoline engine.

A lot of old-time mechanics added some gasoline to diesel to supposedly clean the carbon deposits out of the cylinders. I have never read anything that said it worked. Gasoline will make the fuel burn hotter, and hotter burning fuels burn cleaner. Thatís probably where the theory got started. In the older diesel engines that belched lots of black smoke even when properly tuned, the result of adding gasoline was probably more white smoke instead of black. This might lead one to believe the engine was running cleaner. Maybe so, probably not. Hereís what happens.

Gasoline will raise the combustion temperature. This might or might not reduce carbon deposits in the cylinder. This also might or might not overheat the injector nozzle enough to cause coking on the nozzle. Thatís a clogged injector tip in laymanís terms. The fuel being injected is the only thing that cools the nozzle. Diesel fuel has a lower combustion temperature than gasoline. The fuel injectors depend on the fuel burning at the correct rate and temperature for a long life. If the combustion temperature is raised long enough, the gums and varnishes in gasoline will start to cook right in the fuel injector and turn into carbon. These microscopic carbon particles will abrade the nozzle. High combustion temperatures alone will shorten fuel injector life, gasoline makes the problem worse.

Gasoline and alcohols do have an anti-gel effect on diesel fuel, but these fuels are too thin and will hurt the lubricity. Alcohols work as a water dispersant in small amounts, but also attract water in large amounts. Diesel fuel is already hydrophilic (attracts water) so why add to the problem. The old timers got away with this because high sulfur diesel fuel had enough lubricity to take some thinning. Todayís low sulfur diesel fuels have adequate lubricity, but I wouldnít put anything in the tank that would thin out the fuel, reduce lubricity, or attract water.

Opposites do not attract in this case. Use any of the diesel fuel additives available to clean out carbon deposits, not gasoline or alcohols.

While weíre on the subject of fuels, letís discuss another common question. What is cetane?

Cetane is to diesel fuel what octane is to gasoline. It is a measure of the fuelís ignition quality and performance. Cetane is actually a hydrocarbon chain, its real name is 1-hexadecane. It is written as C16H34, or a chain of 16 carbon atoms with 34 hydrogen atoms attached. All HC chains are also referred to as paraffins. Cetane is a hydrocarbon molecule that ignites very easily under compression, so it was assigned a rating of 100. All the hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. There is very little actual cetane in diesel fuel.

All the hydrocarbons in diesel fuel have similar ignition characteristics as cetane. Cetane is abbreviated as CN. A very loose way to think about cetane is if the fuel has a CN of 45, then the fuel will ignite 45% as well as 100% cetane. Diesel engines run just fine with a CN between 45 to 50. There is no performance or emission advantage to keep raising the CN past 50. After that point the fuelís performance hits a plateau.

Diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. The minimum CN at the pump is supposed to be 45. The legal minimum cetane rating for #1 and #2 diesel is 40. Most diesel fuel leaves the refinery with a CN of around 42. The CN rating depends on the crude oil the fuel was refined from. It varies so much from tanker to tanker that a consistent CN rating is almost impossible. Distilling diesel is a crude process compared with making gasoline. Gasoline is more of a manufactured product with tighter standards so the octane rating is very consistent. But, the CN rating at the diesel pump can be anywhere from 42-46. Thatís why there is almost never a sticker on a diesel fuel pump for CN.

Premium diesel has additives to improve CN and lubricity, detergents to clean the fuel injectors and minimize carbon deposits, water dispersant, and other additives depending on geographical and seasonal needs. More biocides added in the south in summer, more ant-gel added in the north in winter. Most retailers who sell premium diesel will have little brochures called POPs (Point of Purchase) at the counter explaining whatís in their fuel. Please donít ask the poor clerk behind the counter any technical questions after reading this discussion. All they need to know how to do is sell you beer, milk, cigarettes, lottery tickets, and take your money.

Texaco and Amoco are two big names who sell premium diesel in limited markets. Amoco primarily sells its Premier to specialized industrial and agricultural markets. I cannot get either in my area. Most fuel retailers buy additives or buy treated fuel. In the Northern plains states, Koch is a well-known marketer of premium diesel. I buy it when I travel into Northern Wisconsin.

Because there are no legal standards for premium diesel yet, it is very hard to know if you are buying the good stuff. I have good news. An ASTM task force has drafted standards for premium diesel. When the new specifications are accepted, information will have to be posted on the fuel pump. Retailers will no longer be allowed to label cheap blended diesel as Ďpremium.í They will have separate pumps with clear labels on both informing the customer what is being sold. The marketing and labeling will be the same as with regular and premium gasoline. Retailers selling the real thing use this system now. Enforcement of all fuel standards is done at the state level in the USA.

Diesel fuel is an international commodity for industry. Therefore, you should be picky about where you fill up. Shop for price from a large volume retailer so you have the freshest fuel. Thatís about the best advice I can give.

82 Rabbit diesel (first d), 84 Jetta turbo d (300k when sold), 83 240d (305k when sold), 84 F250 6.9d w/Banks turbo(parts truck), 86 F250 6 cyl.gasser(waiting for 6.9d), 84 300d ( 347k Sold 8/04), Y2k New Bettle TDI (185k miles), 95 740IL (wifes), 87 300TD (206k Sold 7/05), 05 Passat TDI Wagon
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