Federal Regulations require autos sold in the U.S. 'be capable' of running on gasoline with a rating '87 ' with up to 10% ethanol and has a specified amount of detergent that will keep the engine clean enough to meet the emissions limits for 50,000 miles---if maintained and operated within manufacturers operating and maintenance requirements stated in the Owners Manual. The EPA fuel economy and emissions testing is conducted with a special 'certification fuel' that is much better in all respects than '87.' That Certification fuel has much more detergent and is much higher in 'octane' than '87.'

These modern engines now have 'knock sensors' that detect harmful 'detonation' (which is perceived as 'knocking' or 'rattling'). Detonation is spontaneous combustion of the as yet unburned portion of the fuel/air mixture. This 'detonation' reduces power produced, fuel economy, and causes excess heating of the detonating piston and its parts. Detonation can be very mild--called 'trace knock' or very harsh. Trace knock is generally the maximum allowed by the 'knock sensor' and ECU ignition timing controls. VERY LOW anti-knock property (low octane) can 'knock' so badly the ignition timing reduction cannot keep the engine from knocking badly. HEAVY knocking will damage engines, often breaking the top piston rings and pistons, leading to total engine destruction. Driving carefully under light loading will reduce knocking tendency often avoiding any knocking. However, even brief heavy detonation can damage the pistons and piston rings, ruining the engine.

A more serious 'knocking' , called 'Pre-ignition' can occur when 'detonation' is heavy and prolonged. 'Pre-ignition' is when the fuel/air mixture ignites BEFORE the ignition spark ignites it.. Pre-ignition causes much higher 'explosive' pressure in the cylinder and can destroy pistons (and the engine) in a very short time. Pre-ignition will typically happen under higher loads after a few seconds of 'Detonation', or if deposits in the combustion chamber begin to 'glow.' 'Pre-ignition' can occur at cruise speeds and may not be audible over other noise, with the operator unaware until there is significant power loss or engine damage results. The modern 'knock sensors' may not detect 'Pre-ignition' because the explosive vibrations are of different frequency than 'Detonation' caused vibrations. Retarding ignition timing may not stop 'Pre-ignition' once it has begun. Only immediate load reduction is effective.

To summarize, one should always use a fuel having high enough anti-knock property ('octane') to avoid both 'Detonation' and 'Pre-ignition' from occurring during their daily driving to avoid engine damage.

Driving 10,000 miles a year averaging 20 miles per gallon uses 500 gallons of fuel. At a price savings of even $0.20 per gallon, the potential savings is only $100 a year. IF a cost difference of $100 a year is meaningful to a Mercedes Owner, that Owner might better by served by a much lower cost vehicle.

 

Been doing some more reading about this over the last year or so.

According to SDS, the octane readings that you can set for the car are based on RON (Research Octane Number). There's also MON (Motor Octane Number), which is typically about 10-12 points lower than RON. In Europe, they use strictly RON. Here in the United States, we average RON and MON (the formula is {RON + MON} / 2).

What this means is that our "87 octane" regular gasoline actually has something like a 92 or 93 RON. Remember that the max setting in SDS is 93 (and that's RON). That's why burning regular gas works so well in the M112/M113 engines; apparently even with the 10:1 compression ratio, our engines were designed to run well on what we would consider pretty "bad" gasoline (think 84 octane {RON + MON} / 2). At higher altitudes, they do sell 85 octane gasoline, so this info is also applicable to, say, folks in Denver, CO metro area or the Mexico City metro area. But at anything even approaching sea level, 87 octane (US spec) has a high enough RON to be just fine.

Now, in a forced-induction engine like my S600 TT, or an S55 AMG, I would use 93-octane ({RON + MON} / 2) gasoline due to the greater air pressure provided by the super/turbo-charging.

 

Good to know! I put regular in my car last fill up and it made no difference whatsoever in how the car runs. I think I'll keep doing that, considering it's an extra $0.40 per gallon to use premium.

 

But cheap gasoline may not clean as well as premium or TOP TIER Gasoline. Then you should do as the user manual says or as the car workshop or car shop says. okay knocking is the engine should be avoided

 

At higher revs, octane is well in MON. the test performed well between 600-900 RPM .the octane in RON is well up to 600 RPM . If these where he has logged like this, it is enough that the standard for the test was developed a long time ago when cars maybe had long connecting rods and maybe had a maximum speed of 3000 rpm

 

You should also go by the fuel consumption per mile. and in the long term also if the fuel has a good cleaning effect on the engine. if the engine sounds rougher, it could be a sign that the fuel lubricates worse or maybe that the octane content is just at the limit for the engine