Octane?

[Eliot]

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?

[MTI]

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.

[yosshimura]

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....

[Duke2.6]

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

[brewtoo]

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?

[MARIO FARIAS]

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

[Q]

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.

[Duke2.6]

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