I know this has been discussed before. The topic is all over the internet and the general consensus is that you should use the fuel which is recommended by the manufacturer. However, the MB manual is not very clear on what the best octane is. It say to use 91 min and makes it clear that you should not use anything under 91. So is 91 the recommended octane or is it minimum 91 but higher is better?

Will this engine take advantage of higher octane fuel or is it simply a waste? What is the exact octane that the engine is tuned for? As I said the manual states 91 min. which is not clear. I am not asking for money saving reasons I am asking for pure technical reasons so please only technical answers.

 

You can only put in what is available in your location. If you want to drive further and find other you will have to make the determination if the extra expenditure, mileage is worth it.

 

It says 91 minimum, thus you can use anything 91 or over. I personally do not think you will get any additional performance over 91 as thats what MB suggested and that should get you all the rated performance out of the engine and do no damage. Anything beyond is not harmful but also not beneficial. Thats my take.

 

I don't think the engine will advance timing to use the extra octane. Some of the older M-B had a device that could be overcome to take advantage of 93 octane fuel. It is a pain but when gas reached $4 a gallon I blended a few tanks with 89 and 93. I have never had the engine ping with either method.
In my area, there is no 91 octane available. I have another non M-B vehicle that "prefers" 91 octane but I run 87 in it with no appreciable disadvantages except occasionally I get a "bad batch" at Costco and it will ping noticeably. It will yield about 2 mpg better with premium and if pushed hard, the acceleration is slightly better (seat of pants gage). I think you run the lowest octane you can. Pinging is pretty harmful to the engine. If you hear it regardless of the brand / octane fuel you are using, I would change.
I do not know how many knock sensors are utilized in an M-B engine but they are effective at reatrding ignition timing and preventing engine destruction. As has been said before, $.30 gallon is not a big factor in the operatring cost of these cars.

 

The 91 octane as I said in my previous statement on this thread will give you the rated power of this engine. Using anything more will not give you more performance. there is only one knock sensor in most engines. Guess what?? It has to knock at least once to have the sensor retard the ignition. Even one knock repeated over time can do damage. The savings of using any lower octane than 91 is not worth the potential damage and is really a false economy. The additional cost of using more than the 91 is also insignificant.

 

To the OP: as far as the "technical" answer, engines have compression ratios that determine which RON (or AKI whichever rating you want to use) is necessary for the proper air/fuel mixture to explode on time (not too early or too late.) That's what "timing" is about. It needs to explode at the right time in the right place (TDC depending on engine configuration.) A lower octane than specified for the compression ratio of your motor will explode prematurely (hence the sensors retarding the timing if you put in lesser octane than required.)

If you use a higher octane, it won't explode prematurely (doesn't detonate as readily.) So you'll be safe. But if the compression ratio of the motor doesn't require the higher RON, then it's a complete waste, i.e. it's totally unnecessary.

If the manual says use a minimum of 91 octane, then that means the motor was built to compress at that octane and it won't explode on its own (prematurely.) If you put in 98 octane, nothing changes 'cause it will still only compress at 91. It's a waste of money unless you change the timing to raise compression like when adding more boost to FI motors.

The only advantage to using higher octane is you know that the fuel won't burn prematurely. Any fuel that burns early can burn in several places in the combustion chamber causing excessive high pressure in the cylinders. Boom.

One issue is the octane rating of gasoline is not consistent across the country. One state may require a minimum octane rating of 91 for premium gasoline, while another allows 89 octane to be called premium. Those yellow stickers are supposed to legally tell you the proper AKI. Higher octane doesn't clean the engine any better, either. The EPA requires that all grades have detergent additives (although I like to add some Techron once in a while.)

Even though there are anti-knock sensors, what happens if they fail? Here's NYT article on that actually happening when a GM car's computer failed to retard the timing from using lower octane gas. I guess if you're really paranoid or suspect the octane ratings, then just use the highest octane you can find since it's harder to detonate too early, i.e., it can be compressed more before it ignites.

 

Lower octane fuel can be used as stated in the "owners manual" but you must follow the limitations listed.

During Katrina only 87 octane was available and I can assure you our E500 ran well with good fuel mileage on 87. However you are restricted to throttle limits and rates of change. Following the manual instructions we had no issue using 87 octane.

91 octane it the "normal" min octane I will use provide it is available. When not available I use the next higher octane. If I still need gas and only 87 octane is available I will use 87 and follow the MB Owners Manual.

 

You are correct in most of your statements. However you do state that you can raise compression by advancing the timing. This is incorrect. To increase the compression ratio you must modify the cylinder head and/or replace the pistons. This is pure physics as it is the ratio of the uncompressed volume of the cylinder to the maximum (TDC) top of the piston stroke volume of the cylinder. Perhaps you meant that the timing can be adjusted to compensate for the change in compression ratio.

 

As we all learn from each other on the site. You can also raise compression without changing the base engine,, just add turbo or supercharger and they will also raise comression based on boost and waste gate settings

 

Turbocharging or Supercharging will increase cylinder pressures but will not change the design compression ratio of the engine. These devices force more air into the cylinders allowing more fuel to be burned, thus more power. In many cases paticularly with superchargers, its actually required to reduce the compression ratio before supercharging to prevent detonation and damage. This of course depends upon the boost. Most automotive turbos and superchargers today are really low boost compared to what was used years ago. Superchargers give you a very rapid boost as they are mechanically driven as opposed to turbos which have a lag due to the spool up time driven by exhaust gases.

 

Thanks for that great answer. I was under the impression that the engine has several timing maps and can adjust downwards and upwards up to a certain limit. i.e up to 94 but race fuel would be a waste. i.e my 911 takes 93 but can benefit from 94. Your answer makes sense and I have to apologize (while not admitting that I was wrong) to my wife and let her fill it up with 91.

 

Arent' we having fun.. Lets go build one and race

 

No, you're quite right. The CR can't be changed by timing of course (only by piston design, etc..) I was just implying that timing can be adjusted to optimize the CR by advancing the timing to keep compression in the piston chamber high before detonation occurs. I'm thinking that the majority of stock engines may never see max cylinder pressure because of appropriate fuel issues and less than optimum compression. Timing (ie, tuning) for 100 octane can be chancy because of fuel issues, imho. That's part of why there are tunes that blow motors, I'm guessing.

Maybe I'd rather have a lower-compression engine with a good ignition advance curve, than a higher-compression one with retarded timing to utilize crappy available fuel. I dunno. But I do get nervous with my AMG 6.2 liter with such a high CR. I realize they use internal thermal barrier coatings, etc., to help contain the heat inside the chambers where it does the most good. A long time ago Porsche's response to the elimination of 98-100 octane fuel was to back off the compression ratio. And then when fuel efficiency became such an issue they increased the compression ratio and backed off the timing. Apparently the increased efficiency from higher compression was better.