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  #1  
Old 03-12-2004, 11:33 AM
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Question octane

Just got an E320 (2000) and the owners manual requests 91 octane premium fuel. How important is this? doesn't the ignition self-adjust for lower octane? would appreciate some advice...!!!!!!
Mebedez

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  #2  
Old 03-12-2004, 11:47 AM
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sure if you want to be retarted

the computer will retard the timing so the engine will have less knocking or pinging.

Lower than recommended octane may have the engine run hotter, you may get poorer mileage and for sure less performance. My manual says only use a lower octane if Premium is not available then fill up with premium ASAP. I figure the German engineers know better than I.
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  #3  
Old 03-12-2004, 01:32 PM
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Forget about trying to save a few cents using lower octane gas. Use at least the minimum octane recommended in your Owner's Manual.
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  #4  
Old 03-12-2004, 01:42 PM
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When you have a car with high compression or forced induction (Turbo) etc. you run the chance of detination. That is when the piston compresses the Air/Fuel mixture it may heat up enough to detonate before the spark plug ignites it. This causes large amounts of heat and can melt you pistons. Higher Octane Gas is not as volital, so it prevents detination(pinging, pre-ignition, knocking). Most all autos have knock sensors and the 300E will retard the timing untill it stops knocking if it can.. If you experience this let off the gas and add high octane ASAP. I have been running 90 Octane during the winter with no problem. The colder the intake the less chance of detination. I havn't tried 87 Octane nor 90 during the summer or a hot day..

It has been recommended to me to try 90 and if it knocks/pings to up the Octane. This can be risky business if you don't pay attention to the sound of your engine. The person who owned the car before me said he had used 87 Octane often??

In my Turbo Talon auto I use only 92 or higher. If i turned the boost up beyond 18 PSI I would have to use 105 Octane.

I feel safe with 90 in the MB...

P.S. The lower the altitude your at will also increase the need for high octane.
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Old 03-12-2004, 05:08 PM
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hey stubman - you sound like you have a handle on this...

i think higher octane burns faster...I have read that too.

often i read it burns slower.

what do you think ??
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what i did myself.
CPS
rear shocks
MAS x 3.
SparkPlugs (twice)
Fuel pump & Filter
Window switch
replaced both failed horns with Honda horns.
Cleaned ERG Tube
Oil changes
Oil consumption Fix
Brake Pads
A/C Recharge
Anti-Freeze change X 2.
Front Shocks. Monroe. $90
Mercedes Warrenty Work:
Harmonic balancer,power steering clamp. Cat converters.
Drive Shaft Bearing.s
Replaced Outer Lower Tie Rods and front lower ball joints at 156,000 miles
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  #6  
Old 03-12-2004, 05:51 PM
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Question octane

Thanks for all the opinions guys/(gals) (PC).
Octane does SLOW burning of the fuel and reducese COMPRESSION COMBUSTION, but I would like to hear from owners who actually run their E320"s (2000) on 87 or 90 Octane!!!!
Thanks, Mebedez
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  #7  
Old 03-12-2004, 05:57 PM
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what did you say ?

"octane is slower burning" ? what octane. Lower ? Higher ? Lower octane right ?

you say "octane reduces compression combustion ?" so you need less compression to combust///?

higher or lower reduces compression compustion..lower ..right ?

sorry i am not specific to your particular car.

i sure would like to understand this octane thing. I read so many conflicting things.
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Steve Danville
what i did myself.
CPS
rear shocks
MAS x 3.
SparkPlugs (twice)
Fuel pump & Filter
Window switch
replaced both failed horns with Honda horns.
Cleaned ERG Tube
Oil changes
Oil consumption Fix
Brake Pads
A/C Recharge
Anti-Freeze change X 2.
Front Shocks. Monroe. $90
Mercedes Warrenty Work:
Harmonic balancer,power steering clamp. Cat converters.
Drive Shaft Bearing.s
Replaced Outer Lower Tie Rods and front lower ball joints at 156,000 miles
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  #8  
Old 03-12-2004, 06:24 PM
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Question octane

Steve: Octane is an additive which SLOWS combustion. Gasoline WILL burn more quickly and even explode, with higher compression, so a higher compression engine will require MORE Octane to maintain a "smooth flow" of the flame front. "Ping" or detonation is this explosion which we want to minimize for longer engine life!! Gasoline will combust just from compression and that is what we are trying to prevent. I hope I have explained this a bit better!!
Mebedez

Last edited by mebedez; 03-12-2004 at 06:30 PM.
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  #9  
Old 03-12-2004, 06:30 PM
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It does seem to be backwards. The higher the Octane the less Volatile. The lower the Octane the easier it is to ignite it. One natuarly thinks the higher the number the more power one would get. That is not what it is all about....................

I think this is the concept behind deisl engines. It ignites under compression hence no spark plugs.

The engine will retard the timing if the engine knocks/pre-ignition. The problem with that running your car with the timing retarded is you are running your car in a non-optimal mode. You will loose power, gas mileage etc. The money you potentialy save on the lower octane gas would be lost.

Also the ECU can only retard the timing so much and that is where you run the risk of piston melt down.. I have no idea what those limits are.

I have a 91 300E(SOHC 16V) and the manual says 91 Octane or higher. In my area my gas choices are 87, 89, 90 or 92 Octane. I use 90 Octane to be on the safe side and havn't had any pinging, knocking etc..

I have a Haynes manual (European Version?) and it actual shows an item next to the ECU that allows you to adjust the timing so one could use lower Octane fuel.. This item (sorry I don't have the name of it) doesn't exist in my car. I asked the MB dealer/Mech and they commented that it must only be in the Euro Models because they never seen such an Item..
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  #10  
Old 03-12-2004, 07:31 PM
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Also remember that the older the engine the more build-up in the combustion chamber and the higher the resulting compression.

The 190e with 220k miles doesn't like regular, will barely tolerate 89 octane if it is high quality and generally just prefers 91, which is the highest we can get around N. California.

Haasman
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  #11  
Old 03-12-2004, 07:53 PM
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Lots of misinformation, myths, hear-say and error in what has been posted in some earlier comments. eg:
"Higher Octane Gas is not as volital" : gasoline volatility has nothing to do with high or low octane. Gasoline volatility (measured as RVP (Reed Vapor Pressure)) is not higher or lower on high or low octane. Gasoline volatility is adjusted by the refiners according to where/when the gasoline will be used (based on temperature)
"i think higher octane burns faster" Nope. All types of gasoline burn at the same rate, given the same ignition source.
"Octane does SLOW burning " Nope. All gasoline, regardless of the octane spec. will burn at the same speed given the same ignition source.
"Octane is an additive " Nope. Octane is not an additive. There is no such thing as "octane". It is not a product, a liquid or a magical potion. Octane (say 90 octane) is an index, a calculation. The octane calculation is made by comparing a given gasoline to a mix of isooctane and le n-heptane (both an hydrocarbon element) in a lab. engine. Two calculations are made using 2 different techniques. One is called "Research octane number", the other is called "Motor octane number". The average of the 2 gives the gasoline octane index. If a gasoline ignites with the same characteristics as a mix of say 88% isooctane and 12% n-heptane, it will be called a 88 octane number gasoline.
"you have a car with high compression higher compression, engine will require MORE Octane to maintain a "smooth flow" of the flame front. Partly true, partly not true.
The compression ratio is only one element dictating which grade of gasoline (low-mid-high octane) should be used. You can have a relatively low compression engine (say 9.5 to 1) requiring a high octane index gasoline while a much higher compression ratio (10.5 to 1) engine could be very happy running on low octane gasoline index. The spark plug location, the flame path and combustion chamber profile are important elements in defining what octane rating is required.
All gasolines (low-mid-high octane index) have basically the same BTU content. (22,500BTU/pound). As a result, all gasoline grades will give the same power and mpg if burned in similar conditions.
Bottom line: If no detonation is present with a 87 octane index gasoline, running a higher octane index gasoline is a total waste of money.
Sorry for the long post.
jackD
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  #12  
Old 03-12-2004, 09:23 PM
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Just to briefly reiterate. HIGHER OCTANE FUEL DOES NOT BURN SLOWER!!! The octane rating of a fuel is a measure of its detonation resistance. Detonation is the sudden reaction of the unburned portion of the partially burned fuel-air mixture rather than being consumed by the normal flame front propagation. Somehow this gets obsfucated as "premium gas burns slower". NOT TRUE!

Detonation creates high pressure shock waves in the combustion chamber, which vibrates the engine structure and this is the "knock" or "ping" we hear. These shock waves increase heat transfer to the combustion chamber boundary by up to an order of magnitude, so sustained detonation can rapidly overheat pistons and valves, which can lead to structural failure of these components.

Octane is primarily determined by the blend of hydrocarbon species in the fuel. Prior to the banning of tetraethyl lead (TEL), it was used in small concentrations to boost the octane rating of a gasoline blend, thereby increasing detonation resistance, but today, refiners have to juggle octane rating and Reid vapor pressure purely by the blend of components in the fuel. Aromatics - ring shaped hydrocarbons - such as benzene used to be used liberally to increase octane rating, but they are considered to be carcinogenic, so their quantities are now limited, and Reid vapor pressure is regulated to reduce evaporative emissions.

All of the above consipires to raise the price of producing gasoline that meets the octane and myriad regulatory requirements, which is the big reason why gasoline prices are heading up across the country, and California's unique "RFG II" blend is the most expensive to produce.

Duke
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  #13  
Old 03-12-2004, 09:38 PM
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Ah, but higher octane fuel does ignite in buld less readily -- it also has slower flame probagation rate, so in an engine that does not require it, it CAN reduce power output and economy from poor combustion. Higher compression (the main reason for needing higher octane) generates higher heat, so the higher octane, less volatile and slower to ignite fuel is more volitalized, more easily ignited, etc.

RVP is an "average" for the fuel, not the volatility of the entire fuel bulk, and volatility and burn characteristics are related in complex ways. Gasoline isn't a simple mixture by any means, and most of the compounds that burn slowly (ie resist detonation more) are unsaturated, with lower vapor pressure and lower volatility than fast burning compounds (that resist detonation less). The highly volatile compounds will always vaporize, so the spark will ignite the fuel, but the bulk of the fuel will ignite less readily (ie have less tendency to detonate).

The whole point is to have highly combustible fuel that ignites only with the timed spark that will also burn competely in a controlled manner in a short time (before the pressure and temperature drop too far). Mixtures that ignite very easily will ignite under compression heat and the influence of "hot spots" (spark plug tips, hot deposits on exhaust valves, etc) before or simultaneously with the timed spark. TWO flame fronts in a cylinder at the same time isn't a good thing, and premature ignition of the mixture (one form of detonation) isn't good either.

Part of the price we pay for having the power output MB engines produce is a requirement for high octane fuels. No way around it, as you risk serious engine damage from running low octane fuels -- if it knocks or rattles, use higher octane fuel. Otherwise, it's gonna be valve job time soon.

Incidentally, octane requirement can go UP as engines age. Deposits from various sources build up on the pistion crown and on the exhaust valves, and this can both raise the compression some and provide red-hot spots. Spark knock can then increase the deposits from poor combustion. Using higher octane fuel controls the knock and can reduce the deposits -- I've had several 150,000 mile plus engines do this.

Peter
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Last edited by psfred; 03-12-2004 at 09:54 PM.
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  #14  
Old 03-12-2004, 11:47 PM
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Quote:
Originally posted by psfred
Ah, but higher octane fuel does ignite in buld less readily -- it also has slower flame probagation rate, so in an engine that does not require it, it CAN reduce power output and economy from poor combustion. Higher compression (the main reason for needing higher octane) generates higher heat, so the higher octane, less volatile and slower to ignite fuel is more volitalized, more easily ignited, etc.
This is so full of myth and misinformation it's tough to know where to begin.

Flame propagation rate is primarily a function of turbulence, which is a function of combustion chamber design. That's why different design engines have differences in best power ignition timing. For engines of the same bore, lower optimum best power timing usually translates into better thermal efficiecy, which means more power for less fuel. Difference in fuel blends have little effect on flame propagation rate for normal combustion. Once an engine gets into detonation, which is "abnormal combustion" the rules change.

Higher compression engines don't generate higher heat. They convert more of the fuel's energy into usable work at the crankshaft. Therefore, as a rule, they have lower EGT, which transfers less heat to the cooling jacket as the exhaust flows out the port. The energy of commercially available gasolines is similar, but I've seen reports that show some regular blends actually have a little more energy than some premiums.

Fuel "volatility" is its willingness to evaporate as measured by Reid vapor pressure. In a modern fuel injected engine the fuel is invariably evaporated and mixed homogeneously with the air when the spark occurs, initiation combustion.

Duke
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  #15  
Old 03-13-2004, 01:21 PM
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Duke:

Higher compression ratio equals more heat at the top of the compression stroke -- above 12:1 you are in the compression ignition range of 400 F or so, and in fact some early deisels run at 12:1 -- usually require some sort of pre-heat to get started, but once running will continue. Terribly ineffecient, but that's a different story.

Higer octane fuels would have to resist self-ignition of the mixture under compression heat, else they would pre-ignite.

High compression ratio engines convert the heat into work much better, so that less is wasted as hot exhaust gas and heat dumped into the cooling jacket, but the mixture and burn temps are higher --- again, this produces more useful work, but only if the fuel/air mix burns correctly at the proper time -- lighting off the mixture on the compression stroke makes for VERY hot combustion chambers!

Peter

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