I bought my 08 S550 in April 2022 and bought premium gas. When it spiked to $4.50/ gallon I tried regular which was $1 per gallon less, saving $25 per fillup. I never noticed any decline in engine performance or gas mileage. Nor did I ever hear anything close to a knocking sound coming from the engine. I can get 25 mpg on highway driving around 70-75 mph. These cars have computers that adjust the air fuel mixture based on multiple variables.

I know people go on and on about how using regular on a car that should get premium can "damage" certain components. If this is true, tell me exactly what can be damaged. I've been using regular gas for about two years now and the car runs fantastic.

 

the 273 is designed to run normal octane - it will go slower and do worse mpg if you put high octane fuels in it - however you must remember to buy premium brands (so additives like stabilisers, detergents, anti fungal stuff are made by adults not the local kebab shop) and ensure its not full of the death additive ethanol

ethanol trash fuel is worse for the planet (that's the whole idea....) it uses from start to burnt, more energy than it produces making the wheels go round - but it does make them more money and it does damage your vehicle
you'll use more and go slower at higher cost - whilst damaging your car and the planet

 

knock - pre-ignition - pinking, is all pretty much the same thing - its an explosive ignition of the fuel earlier than is desirable - effectively the fuel having a moment and is trying to reverse the engine rotation, just as the driver wants more forward motion - in a conventional piston engine we want petrol that burns with a very rapid expansion of a controlled flamefront, building pressure progressively - not a massive shockwave explosively damaging the piston crown, shattering the piston rings or eroding components in the combustion gas environment, and or causing localised overheating melting areas of the piston and smearing down the rings till its game over in a slower death

the 221 has knock sensors that retards (delays) the ignition to reduce power and lower combustion chamber temps allowing the fuel to get over itself - its a virtuous circle if it doesn't clam down, power drops out till it does - its happening all the time making changes in milliseconds - however its not a miracle worker - terrible fuel, terrible driving, severe overheating, the wrong oils etc. will eventually lead to some engine damage

the 278 engine with CGI was built with this disease intentionally - and they now pretend to help the mismanagement with a later spec engine oil that doesn't contribute to the engine wear quite so much - designed to fail - in action - if you won't lease the flipping car we will take so much money out of your pocket you will have to walk or get on message !!!

by 2015 the idea has been lease for 4 years and then just scrap it !


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What do you call “normal octane”?

The M273 in my E550 asks for min 91 Oct.

 

What BOTUS defined detonation as, is absolutely true. However, he didn’t explain normal gasoline combustion. Gasoline is designed to combust in a ICE, very controlled, over a much longer time(comparative to detonation combustion, ie diesel). It is supposed to be smooth, with power being delivered with a steady increase throughout the combustion stroke, and a steady burn down at the end of the stroke . Higher octane fuels decrease any tendency for the combustion stroke to detonate, ie explode all at once. Detonation is zero to full expansion over a very short time, with no burn down at the end, and no build up at the start. That’s why detonation is so destructive to gasoline engines, they are not designed for it, they are not strengthened for it, they are not built for it, thus they will be destroyed by it, much earlier than their natural wear and tear would last.

 

std stuff - I deliberately wrote it like that as Europe and much of the world use a different std to measure octane

>1500m 93 RON
sea level 95 std RON
higher performance engines 98 RON high octane

RON = research octane number, a rather basic useless test for anti-knock, but for some madness we stick with it
I forget what USA and Canada use - is it Mon motor octane number - an average of a usable car engine test and another - - these days seem to have changes its name (same units new name to AKI anti-knock index)

often you just take 4 units off RON to get equiv USA AKI ratings

 

I use premium for one, it smells good, two, it's factory recommended and third, I hope high Tier gas is what they say it's suppose it to be.
There are two discussions I stay away from, premium vs regular and Mobil 1 vs other oils LOL