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  #1  
Old 08-13-2004, 04:24 PM
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Octane?

So, what will be the result if I run "regular"- 87-89 octane in my 1989 300te? How does less octane effect engine life and performance? How do I know if it is harming the engine?

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Old 08-13-2004, 04:56 PM
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Octane ratings on the pumps today reflect the AKI (Anti Knock Index) which is the average of the old RON and newer MON indexes. To get to the point, your best bet is to use the fuel recommended by the engine make, so long as that fuel does not cause premature detonation, sometimes called "ping" or "knock." That easily discernable sound (turn the stereo off, please) while accelerating is the result of fuel being compressed and combusting before it reaches the top of the stroke and before it reaches TDC. The early combustion can result in damage, although not immediately, to a street car engine.

You can use the lowest octane that doesnt result in ping or knock. Temperature and humidity can cause an engine to require higher octane as well. Hot days with low humidity, like in the Southwest, can cause some cars to need the next higher level of fuel. Except for a measley 1-2 HP, there is no power gain for using higher octane fuel in our fuel injected cars. You get more horsepower through more air and fuel, not octane in particular.
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Old 08-13-2004, 09:37 PM
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As per general conensus on a recent thread (look up in search function) it was generally agreed to spring for the extra 2 bucks or so at fill up and stay with premium, that is how the engines were designed to run on.... 2 bucks.... maybe 3....
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Old 08-13-2004, 11:07 PM
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You can try it, but listen for detonation, which is most likely at low revs/high load. I run regular unleaded in my car with no signficant detonation, but I only drive it during the winter months, so the cooler inlet air temperatures help reduce detonation tendency. When I drove it year round it would sometimes detonate slightly on hot summer days in traffic with the AC, even though I used premium.

I've had to modify my driving habits a bit by not short shifting and loading the engine quickly. With an auto. trans, the torque converter reduces the amount you can load the engine at low revs relative to a manual trans.

Duke

Last edited by Duke2.6; 08-14-2004 at 01:13 AM.
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Old 08-13-2004, 11:15 PM
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Well, as I've said before, we have run regular 87 octane in our 300Es for years with no adverse effect.

Positive effect on our pocket book, though.

We've tried it both ways in both cars and can tell no difference. But, YMMV.

And maybe I'm just a cynic, but when you fill up with what you think is premium, how do you know it's really any different from what comes out of the regular 87 pump?
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Old 08-14-2004, 04:42 AM
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Using Unleaded 95 Octanes

Hi gays. I am using unleaded 95 octanes on my 300E and 190 E, 2.6, year 1988, because it is more clean and has better quality control.
Our outside temperature is 80 Farengheit degrees around the year and relative humidity is 60% or more.

Then, I have to calibrated for a new air- gas ratio?

The % readed between point 2 and 3 in the X11 diagnostic plug has to be the same 50%.?

How to get more power and efficiency?


Thank.

Mario Farias
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Old 08-14-2004, 05:16 AM
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Um, yeah. Here's the deal. If the car was engineered to run on 87 octane, it won't do you any good to run 93+. There is no more power to be had. No more cleanliness to be had. To take advantage of higher octane, an engine must have a higher compression ratio, or must be supercharged/turbo charged.
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Old 08-14-2004, 12:29 PM
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Mercedes specifies premium fuel for M103 engines, but with only a 9.2:1 compression ratio, many will run on lower octane than typical pump premium. It comes down to individual cases and there are a lot of variables - ambient temperature range and altitude, individual driving habits, and internal carbon buildup.

The only difference between regular and premium grades is octane rating - nothing else of any consequence, so select the minimum octane fuel that will allow your engine to operate without significant detonation. Occasional transient detonatation will do no harm, but sustained detonation under any driving conditions can be harmful. Detonation rapidly overheats combustion chamber boundary surfaces and can lead to piston and valve failures.

The Bosch KE system automatically compensates for variation in air density, fuel specific gravity, and fuel oxygen content (if any) to maintain the proper air fuel ratio for all driving conditions.

Duke
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Old 08-14-2004, 08:35 PM
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Excesive Octanes?

Please, Excuse me by my ignorance. I have understood that too much

Octanes can increase the Engine temperature. Is it right?

Is it possible that operation temperature change from 80 oC. to a higher

value using 95 Octanes?

In our cars are recomended 91 Octanes.

What does mean 9.2: 1 compression ratio.?


Thanks.


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Old 08-14-2004, 09:21 PM
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Thumbs up

MTI and Q have hit it on the head. Duke 2.6, too. Octane rating is only an anti-knock rating.

Nothing else.

Not power, not cleanliness, not "quality", not goodness, not premiumness, not "more $ is better" ness, not sex appeal. There is no "insurance" to be gained by spending that 2 or 3 dollars more. It is not "better" for your car, unless the lower octane fuel knocks.

If anything, higher octane fuel will burn dirtier than regular. That higher octane is achieved with additives. The additives are used to slow down the burning rate of the fuel/air mixture. Tetraethyl lead used to be the additive of choice but we know where that went. The current additives still slow down the burn rate but leave behing traces of themselves. As buildups of crud on sparkplug electrodes and valve faces. Neither is a good spot for unwanted deposits.

Edited to say that I stumbled across an illustration of excessive fuel additive causing a buildup on sparkplug electrodes. Serendipity.

http://aa.bosch.de/advastaboschaa/Product.jsp?prod_id=75&ccat_id=26&language=en-GB&publication=1&flash=

Click on the "Practical tips" link at the bottom and use the back/continue links to scroll through the different plug pictures.



The optimum fuel for any gas engine is the lowest octane rated fuel that doesn't cause knocking, AKA detonation or pinging.



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Last edited by nglitz; 08-15-2004 at 12:04 AM.
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Old 08-15-2004, 12:06 AM
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There is no difference in "burn rate" between premium and regular. Likewise, one doesn't burn "hotter" or "colder" relative to the other in normal (detonation free) combustion. Both are consumed at about the same rate as long as there is no detonation, which is abnormal combustion.

Detonation is a phenomenon that causes the remaining portion of unburned mixture to spontaneously react and literally "explode" like a stick of dynamite, before being consumed by normal flame front propagation. As the flame front advances, the unburned mixture is subjected to higher temperature and pressure. All gasoline blends have an autoigntion temperature. Compress them enough and they will spontaneously explode ("detonate"). Higher octane simply means that the gasoline has a higher autoignition temperature/pressure or lag time, so it is more resistant to detonation.

The shock waves from this abnormal combustion "ring" the engine structure, which is the "knock" or "ping" we hear. These shock waves also increase heat transfer to the combustion chamber boundaries by up to and order of magnitude, so sustained detonation can rapidly overheat pistons and valves, which can cause them to fail.

Commercial grade gasoline octanes are achieved by varying the blend of hydrocarbon species to attain the advertised rating. Oxygenates such as ethanol and MTBE are octane enhancers, but if oxygnates are blended in, the amount of high octane hydrocarbon species are reduced so the same PON is achieved as non-oxygenated gasoline.

Duke

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