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  #1  
Old 04-25-2003, 09:29 AM
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Premium fuel only.. Why???? Is the compression ratio that high??

What is the minimum octane requirement for the 300SEL?? Does it suggest premium fuel in hopes that you will get the best detergents??? Just wondering what the reason is...

Jay
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  #2  
Old 04-25-2003, 10:29 AM
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I don't think it is about detergents so much as it is about octane, which relates to how well a fuel resists detonation. The higher the octane, the less likely the fuel will detonate. Octane requirements for a particular motor depend on ignition timing, compression ratio, and I don't know what else. If you put regular gas in a modern car designed for high-test, performance will suffer because the lower octane will cause detonation, the knock sensors will pick up the detonation way before your ear does, and the computer will retard ignition timing. In older cars, using regular gas in a high-test car will shorten engine life because there are no knock sensors and no computer to retard the timing to prevent detonation.

I'm no expert, but that's my story and I'm sticking to it.
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  #3  
Old 04-25-2003, 11:04 AM
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Higher-octane fuels have a higher "flash point"...that is, it requires higher temps to ignite.

Internal combustion 4-stroke engines are designed to ignite the fuel at the precise moment when the piston reaches the "power" stroke. Lower octane fuels would tend to ignite prematurely when the cylinder heats up in the compression stroke...this is called "pre-ignition", or detonation as dculkin described. This would sound like "pinging" or "knocking". Left unchecked, the end result will be pistons with holes blown through the top!

The emission control system in your car is also designed to work with specific parameters. If the lower-octane fuel forces timing adjustments that would degrade performance, the emissions control system would suffer as well. Too much retard on the timing, and the catalytic converter will be overworked, dealing with the less-than-optimum mixture at the exhaust manifold. Translation: overheated cat!

As dculkin also mentioned, newer vehicles have computer management systems that monitor this situation and adjust timing to avoid damage. But for older models, it's cheaper to just fill up with premium fuel than to risk a bottom-end rebuild...
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  #4  
Old 04-25-2003, 11:27 AM
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Not exactly.

Pre ignition is not ignition ocurring before spark, it is basically the fuel burning too fast. If the timing and octane are adequate, the fuel will burn slowly providing even pressure as the piston goes down.

If octane is too low or ignition is too advanced, pre ignition occurs at the time of spark. Basically this means that there is a very sudden pressure "spike" as the burning occurs. This is why you hear it. If I could easily draw a curve it would have pressure on the vertical axis and time on the horizontal. With preignition at the time of spark there would be a very high spike that showed tons of pressure at one instant. The curve for proper ignition with adequate octane fuel would have a lower maximum pressure than the other curve, but would spread out over time.

Proper ignition provides burning over a relatively long period of time. Since the piston is going down increasing volume of the cylinder over time, the combustion maintains similar pressure during the entire descent of the piston.

Any of the MB engines sent to the US in the '89 time period have relatively high compression ratios requiring high octane fuel. You can use low octane fuel and the knock sensor will detect knock and the ignition module will retard timing enough to eliminate the knock. With the timing retarded you are decreasing the efficiency of the engine thus requiring more fuel to be used. For this reason, using a fuel with less octane than premium is usually false economy. The fuel is less expensive, but it will get you fewer miles down the road.

Hope this helps,
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  #5  
Old 04-25-2003, 11:41 AM
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I guess the REAL question is???

What is the octane requirement???

From what I understand, PREMIUM, means a lot of different things, in different parts of north america..

I told my wife to just use a mid grade from one of the major companies, and that would certainly be sufficient.. Did I tell her wrong???
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  #6  
Old 04-25-2003, 12:14 PM
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Here's some good information...

There was a lot of discussion on this last summer. Just search using the word "octane". The most comprehensive answer I remember was in a link provided by 95E320cab:

http://www.caranddriver.com/xp/Cara...keywords=octane
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  #7  
Old 04-25-2003, 12:46 PM
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Re: Here's some good information...

In normal combustion the flame propagates across the combustion chamber in a predicable manner. Detonation is the sudden reaction of the last portion of the unburned fuel air mixture before it is consumed by normal flame front propagation that causes a very sudden rise in pressure and temperature. This creates shock waves in the combustion chamber, and these shock waves are the "knocking" noise we here as they vibrate the engine structure. The real problem with detonation is that it dramatically increases the rate of heat transfer to the combustion chamber surfaces, so sustained detonation can damage a piston or valve.

Preigntion is the initiation of combustion at a point or time other than the spark plug when it fires. It's essentially the same as advancing the timing, so preigntion can lead to detonation. Detonation can also lead to preigntion, because it can create a hot spot that acts as a preigntion source that leads to more severe detontion.

Detonation and preigntion are NOT the same thing, but one can lead to the other in a positive feedback loop. Also, normal flame propagation speeds are the same for all automotive gasoline. "Premium burns faster" or however this goes is a MYTH!!!

There are many variables to detonation, but increases in the following will increase the propensity to detonate on a given octane fuel - compression ratio, throttle opening, ignition timing, inlet air temperature. Compression ratio is the primary parameter in determining recommended fuel grade and you should consult your owner's manual to see what is recommended for your specific year/model/engine.

Detonation usually is most prevalent at low revs. Since turbulence increases the rate of flame propagation, the end gas is exposed to high temperature and pressure for more clock time at low revs and is more prone to detonation. Modern engines with knock sensors that are designed for premium fuel can be run successfully on regular because the engine controller has two igntion maps - one for premium and one for regular. If the knock sensor detects repeated detonation, it will switch to the less aggressive timing map. You might notice a loss of low end torque, but less aggressive drivers usually notice no change.

Though my '88 190E 2.6 was designed for premium and has no knock sensor, I can run regular, but have to change my driving habits - basically just a lower rate or throttle tip-in following a shift to the next higher gear (It has a manual transmission), but I can essentially "drive around" the detonation envelope.

Duke
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  #8  
Old 04-25-2003, 01:22 PM
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So how does dieseling (not in a diesel) relate to all that, Mr. smarty pants
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  #9  
Old 04-25-2003, 01:28 PM
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All I really want to know is????

What is the minimum octane rating for an 89 300sel????

I don't have an owners manual..
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  #10  
Old 04-25-2003, 02:07 PM
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"Dieseling" is usually a preigntion problem. Emission controlled engines run less timing at idle than pre-emission engines, which increases EGT and combustion chamber boundary surface temperatures. This helps oxidize HC and CO in the exhaust system with or without a converter, but can lead to preigntion. Dieseling is often accompanied by obvious detonation. This is because the engine is turning very slowly, so combustion is slow and the end gas is exposed to high temperature and pressure for a relatively long time. Thus preignition is followed by detonation, even though the throttle is closed! Dieseling was primarily a problem on carbureted cars from the seventies and some OEMs installed "idle stop solenoids" which closed the throttle an additional amount when the igntion was cut off to starve the engine of air.

Modern cars, including the KE system on eighties vintage Mercedes cut fuel flow when the ignition is shut off, so dieseling should not be an issue.

Most US version Mercedes engines (I think an exception are the early 190E 2.3s) are tuned for premium fuel, which would be a minimum of 91 PON. The 300SEL engine is a variation of the 103 engine in my '88 190E, and it might be able to run on regular as I reported in my prior post.

Suggest the following experiment. Allow the fuel to approach empty, then add about three gallons of regular. If you hear detonation, attempt to "drive around" it as I did. If there is no detonation add three more gallons of regular, and continue to experiment.

An automatic transmission equipped car will have less tendency to detonate because the torque converter limits low rev loading. On the other hand, the W126 body is much heavier, so it will take more throttle to accelerate at the same rate than a 201.

If the engine detonates on regular and you can't find a way to drive around it, try mid-grade and see if that will work.

The 103 engines have modest compression ratios (9.2:1 on my 2.6), which is rather low to require premium. Four-valve engines without knock sensors and up to 9.5:1 CR usually only require regular. The "problem" lies in the basic combustion chamber design of the 102-103 engines. It can be classified as hemispherical, but does have a small quench area. "Hemis" allow generous valve size, but they aren't very resistant to detonation compared to a wedge or pentroof chamber. The four-valve pentroof chamber with small quench zones adjacent to the both the inlet and exhaust valves is very compact with a centrally located spark plug that results in shorter flame propagation distance. This reduces the time for combustion completion and shorter combusion time reduces the tendency to detonate. The relatively quick combustion time of a pent-roof chamber also requires less ignition advance and has better thermal efficiency because less time is available to transfer heat to the cooling jacket.

Current four valve engines run CRs up to about 11:1.

Duke

Last edited by Duke2.6; 04-25-2003 at 02:12 PM.
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  #11  
Old 04-25-2003, 02:10 PM
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How about the reverse, any adverse effects on the engine if you throw a tank of premium in every once in a while in an engine that only normally requires "regular" (87 octane)? Better performance? Shortened engine life? No difference?
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  #12  
Old 04-25-2003, 02:11 PM
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Exclamation Re: All I really want to know is????

Quote:
Originally posted by jay3000
What is the minimum octane rating for an 89 300sel????

I don't have an owners manual..
91 octane or above...
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  #13  
Old 04-25-2003, 02:23 PM
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Buying octane above that needed to prevent detonation or preignition is wasted money.

Have a great day,
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  #14  
Old 04-25-2003, 02:24 PM
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Quote:
Originally posted by iwannabenz
How about the reverse, any adverse effects on the engine if you throw a tank of premium in every once in a while in an engine that only normally requires "regular" (87 octane)? Better performance? Shortened engine life? No difference?
Generally, no! The difference between regular and premium grade fuel is octane. All commercial gasolines have deposit control and other additives, though some premiums might have a higher than minimum dose.

In California the blending of fuel, including additives in regulated and the only functional difference is octane.

Sometimes an engine tuned for regular might feel a bit smoother on premium especially when accelerating at half to 3/4 throttle from low revs. This is usually because on regular, the engine is detonating marginally - not enough to actually hear. Modern engines with knock sensors are set up to operate on the ragged edge of detonation, because this operating condition yields the best torque and fuel efficiency.

Duke
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  #15  
Old 04-26-2003, 12:14 AM
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Unhappy premium or regular?

I drive a 1994 C180 Euro Benz usually on weekends only and it seems to be running better on the regular then the premium. Books recommend the premium, but the motor at idle with "D" on, feels like it is very tensed and vibrates a lot more then when on regular. We use unleaded here. I have changed to regular and it feels a lot more relaxed now and less vibrations although the car doesn't have the "go" it had on premium before. Is this the right thing to be doing or will I destroy the motor using regular only? Can I mix with half of regular and half of premium or should I fill with full premium followed by full regular and so on? Will the experts please comment on this???
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