The key to high performance and at the same time engine efficiency is ignition advance. (we talk gasoline, not diesel engines. ignition advance is not possible in diesels)

Also required is high compression, and the combination of the two places stress on the fuel and higher octane is required to prevent preignition.

The only method we have these days to increase octane is ethanol, as mtbe and tetraethyl lead are now illegal. This is the reason you can't legitimately find high octane no alcohol fuel.

Yes, we all know high octane fuel doesn't make more power. It allows engines designed to make high power to run, or a least run as designed.

For many years, gasoline has been super expensive in Europe and the response there was smaller displacement, high compression engines ... because autobahn. Some countries penalize higher displacement.

In USA, where gas has been cheap and there were no laws, the answer was super large displacement, and quite inefficient engines.

The USA history has created a culture that premium fuel is some sort of luxury and all cars should only ever require low test fuel. I didn't buy a hot rod, I'm an upstanding citizen.

Well, the Germans don't send us low compression motors, and if we think Mercedes we think high performance. It is culture shock to old timers. The notion expensive to buy, but cheap to run is fantasy.

RichDMB(OP) never answered my question. Is he running 91 octane in his 273???
I suspect the reason he hasn’t answered, is because it would deflate this thread, confirming its status as a troll thread.
It’s getting ridiculous. There is plenty of information to show that running lower octane than recommended is not a very good idea.
In point of fact, it’s a bad idea.
There is no part of the idea that makes it a good idea.
The only time it might make sense is that you might be so low on fuel, and low octane is the only fuel available.
But that’s just making the best of a bad situation, not making it a good idea.
Why in the world would anyone have a performance engine, that requires high octane, then put regular in it?
Also there is the question of top tier fuel.
I don’t know why, but I am actually becoming perturbed by this thread continuing to appear on the new reply list.
The question has been totally resolved.
There is nothing valid left to talk about regarding the issue of this thread.

inject tractor fuel earlier, you advance the burn - which is ignition point - but we don't tend to think like that for tractor engines

we stopped using that around 1975 in the USA and around 1985 in Europe - You can buy Ethanol free Super Unleaded fuel at 99 RON aka 94 AKI and some vehicles need it
Toluene is fantastic at controlling knock and they've used other additives for 30 plus years to control knock. Its other benefits the big players now see driving Ethanol use, a key one it almost gets OK knock control at a budget cost

he should be that's what it runs best on
If you run 99 RON - 94 AKI in a stock 273 it will go fractionally slower, use fractionally more fuel, and cost shed loads more money, for ZERO gains

Agree - Running lower than recco is very much madness

that's why the car is just able to cope - it has a setting to wind back from std running when it detects garbage fuel - but its a get out of jail option, not a daily driver solution

we are confusing Regular and Premium - I suspect this is not the fault of the customer - but rather incompetent marketing that has become such common practise for 50+ years we don't realise any more

the vehicle manu say Premium, meaning don't use cheap petrol with basic additives - buy good stuff aka Premium fuels with expensive additives from big brand companies
whereas the petrochemical fools have muddied the waters trying to offload junk fuel to the masses, and for rich people they put the price through the roof and out of laziness moved good fuels into a high Octane banding alongside the word Premium

that's is the scary point Arrie posted - As MB2timer mentions later in his post running lower Octane is risky

likely dropped into low power - get me to a better fuel station mode

With its later design and likely far more sophisticated engine management, I can envisage why you can see tiny gains in efficiency using lower than Std Octane, but its not a good idea. You are pushing the envelope of the fuel closer to its knock limits - if you drive gently and it doesn't knock, you'll get a fraction better bang for your buck - But the 278 is an engine that gets knock damage very easily. Especially in a refined car designed to have good noise isolation, humans are appalling at detecting engine knock, it's why damage can occur so easily - and to exacerbate we often get knock when we need the power, so backing off is often not the safe choice or even one we recognise the engine needs.

Direct injection can't control knock - knock, is the fuel giving up and misbehaving as its under extreme duress - adding it later in the cycle will knock just the same, its the conditions that are wrong, not the fuel or ignition timing

Higher Octane than std isn't always stupid - if the engine map and knock control of your vehicle is very poor - then a vehicle claiming not to need higher than X, will be a benefit - and here I would run a 278 on higher than recco Octane and never the other way around

what's changed is the petrochemical companies did a deal with green lobby and stupid governments to pretend Ethanol rot is a good idea - the key benefit is the underhand manu supported early death of your car and higher motoring costs (so tax revenues coming in can get handed out to their rich mates in the farming community, as a bribe to filter more death in the human foodchain)


.

In the US, there are really only two official ways to classify gasoline: octane (which may be conflated with words like "regular" or "premium" but that's all it means), and whether the fuel is part of the Top Tier consortium.
https://www.toptiergas.com/gasoline-brands/

It is also important to note that if you have a Top Tier fuel, even the "regular/87" grade meets these standards. The higher end product with higher octane often exceeds these standards. But there IS NO standard for what premium actually means other than higher octane, which is the only consistency across fuels.

While you will find OEMs who back this, I have NEVER seen an OEM actually cite "Top Tier" in an owner's manual as a fuel requirement. And I don't think they are implying it when they use a word like "premium." In the US, "premium" generally means 91+ AKI (91 is the best you can get in CA, and up to 94 elsewhere in the country - 93 is very common on the east coast and is what my cars get).

So I suppose the quality scale would go something like this:
Lowest: non-Top Tier 87 octane (marketed as "regular")
Low: Top Tier 87 octane (also marketed as "regular")
Better?: non-Top Tier 93 octane? (marketed as premium, supreme, or similar)
Best: Top Tier 93 (or at least 91+) (also marketed as premium, supreme or similar)

Then ethanol free goes in there somewhere but may not meet octane requirements.....

At least the fuel pump will (usually) say Top Tier right on it but it has nothing to do with the naming convention of the different fuel options.

Just in case you don't believe me, right from the Top Tier website:

California is all lawnmowers, prius' and fake liberals, so not much surprise they don't understand anything
What do they sell around Detroit - they ought to know in that area ?

I posted something on the other big thread on fuels we had - the USA fuel supply was set up to suit some mad military supply chain needs and I think this has greyed what owners of car's think to this day

in the good old days when they wanted you killing people - we deliberately put lead in to UK / USA fuels to make you violent

the UK had 5 Star 99 Ron Octane (94 AKI) - this was withdrawn when Vietnam ended and some cars had to have the ignition wound back - with horrible drops of 20% in performance and economy
so we dropped to 4 Star 97 Ron Octane (92.5 AKI)
when Russia capitulated we moved to 95 Ron Unleaded (91 AKI) pretending it was for the little children

the gains on efficiency from better materials (aluminium alloys), good combustion chamber and piston design, better cooling systems and engine management got us around most of the knock issues, and we got more or less similar power with lower costly Octane increasing additives for the fuel companies...

Ever greedy the petrochemical giants alongside German's love for attacking Autobahns, meant they invented a need for Super Unleaded fuels at $1 a gallon more
in the UK where the power wasn't needed they pulled a fast one and created 97 Ron Octane Super Unleaded (92.5 AKI) which did the square root of nothing - as the Germans engine management systems only switch to full power map when they had 98 RON (94AKI) and above Octane
as they hadn't yet created the farming and processing capacity for the switch to Ethanol death fuel - we first went 5% and then 3 years back switch to 10% death rot in 95 Ron rubbish - the maths proves this is drops energy available by 6% - but in some slight of hand its closer to a 15% power drop in the crap we buy today

Then with a few million gallons of expensive real additives around 2018 they pushed the price up again and magically made Ethanol free 99 Ron Octane (94 AKI) Super Unleaded - and all the old (2010 era) German performance cars and bike woke up for the first time ever and flew

I can buy containers of VP ethanol free 94 AKI for the low, low price of $22.80/gal.

I run regular 87 octane and I have never got knocking from the engine. Supposedly knocking would be the sign of pre-detonation (aka damage) to the engine. In the absence of that, I ask what is being damage in my 273 engine, I say nothing. Why spend an extra $20 per tankful for premium. These cars have computers and are designed well to accommodate the conditions they are under. I reiterate that my acceleration is powerful and smooth and my miles per gallon is low to mid 20's.

87 is a terrible choice unless you live and mostly drive above 5900 ft

if you live around sea level like most people on the planet - you should be on 91 AKI (read the fuel cap) and running 87 in these condition you will have damaged the engine - it might be slight (it may well be a lot worse), humans can't hear the damage of slight knock - that's why when technology allowed we fitted electronic sensors to do the job

if you run rubbish (87 AKI) at sea level the engine management drops into an emergency state of tune - to try and reduce the damage

Elevation of Tewksbury, MA, USA

Location: United States > Massachusetts > Middlesex County >

Longitude: -71.234224

Latitude: 42.6106478

Elevation: 37m / 121feet

Barometric Pressure: 101KPa

In Oregon, I fill up with Chevron non ethanol 92 octane for $5.50/gal. Car pulls like a freight train.

JR

Just to add my 2c worth... I have a 2015 C300 and from the time I got the car I have only been using 98. (a year now)

I decided to try the 95 fuel for the last few weeks and I feel like she prefers the lower octane.

Is it a placebo effect or is it possible for a car to prefer a lower octane fuel?

with the introduction of ethanol death additive - most vehicles are now just set to std tune using 95 RON, 91 AKI at or around sea level

before ethanol some high performance vehicles could retune to std high end grunt if fed 99 RON, 94 AKI but I don't believe ANY normal merc ever used this grade - these same vehicles will fall back to slower map if fed std junk (think K1300 bikes or M5 Cars, and probably Brabus or Black series cars)

Tamish - if you run 98 RON it will be slightly slower - try 97 BP sell it, or a 50 50 mix of 95 and 97 (all RON you'll need to play with adding around +4 for the AKI scale)

Last week my tank was on empty and the station I went to had 93 octane (often 91 is the highest). It was a full $1 higher per gallon than regular. I took the plunge though I was tempted to get a half tank of 93 octane and another half tank of 89 to average 91 (wonder if many people do that). I have driven about 125 miles since and don't notice any difference yet, though I didn't expect to as I no had issues over the last 2 years with 87 octane. Just crossed 125K miles on the odometer as well, got the car in April 2022 with 101K. I plan on continuing using premium fuel after all I've heard on the board.

knock happens most in hot engine, hot ambient and large throttle opening conditions - often just before peak torque turns up and you want to get on with it

This talk about fuel octane has been in the forum in many places and most "experts" seem to ignore one important detail with the engine types.

The normal port injected engine for sure is sensitive for fuel octane as the fuel enters the cylinders already mixed in air and this air/fuel mixture is then exposed to increasing pressure under the compression stroke. With increasing pressure, the gaseous mixture temperature also increases, and premature detonation (knock) can happen if fuel octane rating is too low.

I own and used to use as the daily driver a 2010 E550. This car with the NA engine clearly runs better with high octane fuel. I made a test a couple of times with 87 and 89 octane fuels and the engine performance went down both in fuel mileage and engine power. I measured around 15-20% lower fuel mileage and this percentage was probably present with the power output too, at least it felt so.

But the Direct Injected engine should be different. Fuel does not get pre-mixed with air that enters the cylinders and does not experience the temperature increase with the compressing air. The fuel is also injected in the cylinders just before the detonation with spark plug so knocking should not happen that easily.

My daily driver now is a 2012 S550 with the DI engine, and I ran a trial with 89 octane fuel during a long road trip and the result was better fuel milage by about 10%. This was all highway driving without heavy accelerations and I did not feel or hear any problems at all. I wrote about my experience in another thread, but I don't remember which one it was. It was this year though a few months ago.

I now fill up with 89 oct fuel especially when I am on highway for long distances. For city driving, that is minor part of my driving, I fill up with 91-93 as during city driving heavy foot on accelerator is more likely and under heavy foot the engine could knock.

I have not tried 87 octane fuel with this car yet but plan on doing that soon.

I hope MB (or someone else) would have done some research of the difference in required fuel octane between NA and DI engines as it just cannot be the same due to the so different method in fuel delivery in the cylinders.

the way you explain it sounds great - the reality is its much worse at getting knock and they've had to re develop oils to try and make the engine last a little bit...

https://www.sciencedirect.com/scienc...715?via%3Dihub

It's actually the opposite, forced induction engines require premium fuel due to their higher compression ratios. The lower octane gas provides more explosion, thus hurting the engine long term. Yeah, you get better fuel economy (because more explosion), but it's not worth it. There's a reason that most turbo engines require premium, even some economy brands.

I used to get knocks even on 91oct. because California fuel is so regulated that the quality is horrendous. I also want to mention that the knocking isn't really heard, but is seen in datalogs. Sure, if you use very cheap fuel in a non-turbo engine, you might hear it.

What MB2timer and Botus said.
For me, I have always used the recommended octane fuel developed and recommended by the car manufacturer engineers.
The manufacturers spend millions to design these engines, and I tend to believe they know what they are doing by the testing and data, rather than what someone “thinks” they know better with lawn mower gas.

JR

I was thinking why direct injection engines are often damaged by LSPI , especially after the points Arrie was saying re the early Merc adoption of DI...

I think in laymen's terms we can explain the foibles of direct injection with these two points, and why today 12 years down the line we now have MB 229.71 after realising the mess they created with DI

1) in old world science - to burn correctly the entire fill of the combustion chamber was a beautiful mix of 14.7:1 fuel and air by weight - With DI where they go super lean and inject just enough fuel for a quick burn around the plug, we end up with excess air in the rest of the chamber - it's way off the old world normal... and whilst tech now means, better design, better control, etc. its opening pandoras box to localised extreme temps, and way too much unused air, just waiting to get up to mischief if it can find a reason to explode....

2) with fragile fall apart cyl linings in modern engines - we have the opportunity for engine oil to end up where it shouldn't. In the old days the fuel already in the combustion chamber would pull oil off the walls and mix it, ready to use as part of the original burn - but in the disaster that is DI, the oil is loitering well away from the usual burn (on low speeds where most oil rocks up). So when we rip it making the engine work hard then get off the throttle - all the oil turns up on the overrun as we slow for the fake road works and or a google traffic jam. So now we have oil in the cyl but likely our next acceleration phase is meandering in low speed conditions - creating the perfect conditions for oil in the cyl to act as a secondary ignition and fuel source and we destroy the engine with knock

FYI reading one article - you must use Merc spark plugs as the electrode and the thread are set up to mean the blind side of the plug is not in the way of creating the flame front - whereas after-market plugs don't care were it ends up

I would also hazard and educated guess Start Stop creates fun when you need to get a gap in traffic - is the perfect pre cursor to Knock

I usually use 98oct gasoline in my 2007 m273 cl550, am I doing it right? or should I put 95oct?

both a 273 and a 278 should have 95 RON (91 AKI)

lower is dangerous
higher may feel smoother but should be fractionally slower and may cost lots more

if you a 278 and drive briskly and tend to have hot climates and lots of slow driving - 97 RON or 92 AKI is probably a good idea - as would be looking into 229.71 oil

not having ethanol should always be seen as a win - and its likely there is less in higher octane fuels
disabling start stop destroy my engine technology - should be done in ANY vehicle that has it - it to extract money not help save the planet (just like the introduction of ethanol death fuel)

Thanks for your reply. Here in my country, there is no gasoline less than 95 ron, this would be the normal. I always put 98 ron and I notice that it runs well, I don't feel it slow. The manual indicates not to put less than 91 ron, but it doesn't indicate anything about the maximum.

that's half the point of this whole thread - it will run its most efficient when the fuel exactly matches the tune of the engine (which is 95 RON or 91 AKI) - run higher Octane and efficiency drops - but we are talking 3% max drop, and very few will be able to feel anything less than 5% missing - however the higher Octane fuel under extreme conditions will behave better, and you might feel this win (more often than you should). In general the slightly slower, longer burn will often take a few rough edges off the engine's natural NVH (which is the benefit most people notice every fill when they run slightly higher Octane than std).

the other general factor is the more expensive fuel should be better quality, and that tends to win on all points too....

go to extremes of overly high Octane and the far longer burn can cause very high exhaust valve temps and cause damage

RichDMB, able to find 87 commonly available at sea level is very strange and plain wrong - either its the bottom of a big hill and lots drive up it regularly (where about 7000 foot up 87 AKI will help) or the people have no idea what they are doing....

.

Compression ratio has nothing to do with it, amount of compression does.

DI engine forces the fuel in the cylinder at the moment it is needed for the ignition, just like in Diesel engines. Diesels could never operate if fuel was mixed in air before entering the cylinder as it lights up from the heat from compression. Detonation could happen wildly at different moments.

DI engine also keeps the fuel from wild detonations while injecting it at the correct moment. This just has to make difference between port injection and DI.

Again, you’re just plain wrong. If you were right, Mercedes would’ve recommended 87 or 89 oct fuel.

The higher the compression ratio, the better the engine is designed to extract more power out its fuel provided. Using lower octane fuel will cause predetonation and knock, therefore damaging your engine.

Put anything you want in your car, but don’t argue that it’s better to use lower octane fuel in engines designed and tuned not to use it.

You are so lost with this it is unreal. Predetonation occurs if the detonation (ignition) is not controlled. Big part of this control is Direct Injection, if you know what it means. Diesels control it only by injection timing as there is no spark. “Spark” is the heat from air compression that is present before anticipated detonation so fuel is Durect Injected at the correct time for the detonation.

You argue there is no difference in octane requirement between port injected and Direct Injected other vice similar engines and you are wrong. I can argue this even with Mercedes and it could just be something they never bothered even to look at as the higher octane will work.

Everything a car maker says is not the absolute truth, but it takes an engineer to understand this.

Done!