Sorry if this has been asked before. I searched and could not find the exact answer.

My S65 is rated for minimum 93 octane, but where I live we only have up to 91 octane, both in either 100% fuel or E10. I am interested to know if the 91 octane rating will cause any detrimental effects to the engine such as misfires, etc. and if it will matter if I use 100% fuel or e10 based on being at 91 octane. Thanks to anyone that can help.

 

before we can correctly answer that we need to know what market your car was made for - USA use a different octane rating measurement system to much of the rest of the world and then seek to confuse people even more with 3 different acronyms and 2 measurements MON, AKI and PON for _ _ _ _sake - the two that region is supposed to use then under-reads by 4.5 wotsits vs normal people

the best source of info is your handbook that was written with the correct data for your specific car and is not some idiots on the internet telling lies - most data you find on the internet should be ignored

you should NEVER use less than the minimum recco in the handbook - if you must - it should be only as a get you home emergency, and then you should drive gently for the minimum possible time on that fuel - as in light throttle settings and more revs vs large throttle opening and low revs, and avoid hills - the correct way to think about this is if your life is dependent upon moving the car try the what's available, otherwise park it - you could try mixing an octane booster but on a car of this type I would not be comfortable in getting things wrong - the cost of wrecking a good car is a huge amount of money -

 

Thank you for your answer. I've read your replies in other threads and you seem to know gas well.

My car is US-spec so I derive from your chart there that my 91 PON is equivalent to a 95 RON when my vehicle gas cap asks for a 98 RON, minimum 93. Am I interpreting this correctly and if so, should I be octane boosting my 91 PON?

 

I added more ramblings to my post above - yes I agree with your thinking USA 91 is about European 95

but you should use in you car 93 which is European 98 - this is a premium grade fuel that is a lot more expensive - nearly one US dollar more per funny US gallon - as its made using higher quality detergents and anti-knock additives. And the car will drive faster, smoother and happier with lower fuel consumption on this fuel

anti-knock is the main feature your high tune car is seeking to control from that high octane fuel requirement- the more boost (from the turbo's) the more the risk knock will cause harm - knock, AKA pinking, is the shrill metallic tinkling noise you can hear on a hot engine working hard under tough conditions - it really comes from the change in burn rate inside the engine - from a very fast controlled flame front creating high cylinder pressure and high performance to the catastrophic explosion that melts or burns holes in the pistons.

its quite surprising that the car's engine management can't just provide less performance and protect itself when using sub-optimal fuels - but they (Mercedes) tuned it up and they should better understand why they haven't provided that fall back position thats safe for cheap fuels

 

you need to read the handbook fully - that photo of your filler cap (again below), suggests to me its OK to drop to European RON 93 in emergencies and the car will protect itself (but it will also be much slower to drive) that's the behaviour I would have expected - just like one of my BMW motorbikes where more sensibly the manual says

98 ROZ/RON (Premium Plus unleaded)
95 ROZ/RON (Premium unleaded fuel grade, usable with power - and consumption-related restrictions)

if the handbook makes sense and better explains pretty much what the red green text says clearly above - I suspect you can "happily run your local 91 fuel" but the car will be slower and less fun (and use more fuel) than when you can fill it with equivalent of European 98 Octane. On my BM bike I never knew it has this requirement and for a few years 97 Octane was the higher grade you could get and it never made any difference at all. But then we started to get 99 octane and when I put this in, the bike was transformed - its was much happier - even on tickover it made a different noise, and it went much quicker - I still think a 20% performance uplift through the entire rev range and it ran more smoothly - like it had a different engine

the label you have in the filler flap refers to checking the manual it you drop to minimum recco fuel grade - I suggest it'll reaffirm to drive gently and expect it to be slow and horrible to drive - as effectively - unlike my bike you have an even bigger range of fuels it can tolerate - this comes from Knock sensors that detect the pinking noise and very quickly change the ignition timing and boost levels on the fly in a continuous cycle - so basically your car should get another colourful text label

93 ROZ/RON (Budget unleaded fuel grade, usable in emergencies with significant power - consumption-related loses and driving restrictions - always refer to owners manual)

that said try to buy petrol without ethanol it has evil detrimental properties outside of engine "knock"

 

Got it. Thank you for the detailed explanation. Where we are, 91 is always the octane level for "Premium" fuel at almost all gas stations, but I've found some a distance from me that sell 93. I will start going there. It sounds like I'm still safe at 91 per my gas cap, but I'd like to start using optimal fuel for my engine. To my current fuel tank that's filled with pure 100% 91, would an octane booster help my situation to finish out this tank of fuel?

 

Since we're talking fuel, let's assume here forward that I move to 93. If my car has been slow on 91 then I'm anxious to see what 93 looks like. I think I know the answer, but if I have a choice between 93 with no ethanol and E10, which do you recommend? I was unable to find any discussion of this answer in my manual as to which one is preferable.

 

double check post #5 - you are replying faster then I'm writing an eloquent accurate response - post #5 now has the new blue text bit re. min spec that I think your car can handle

we also need to be clear which unit system we are on - 93 PON - yes the car will transform to a better, far faster, nicer car

E10 is 10% ethanol filth - post 5 updated to say try not to buy any fuel with ethanol - its a myth its better for the environment - a lot of research suggests its worse - but its cheap and the ethanol gives great anti knock properties meaning they can peddle budget fuels with high knock resistance and increase profits.

some other posts today with nasty elements highlights

modern petrol is a disaster zone - and now you should never leave old petrol in the vehicle long term... ethanol absorbs water assisting in corrosion of the pump and injectors etc... but remarkably, the nastier thing is it can grow bugs in there, and these excrete acetic acid that leads to erosion of metals, which then assists in the corrosion issues from the water - it also separates which can lead to injector pipework blocking with a jelly type mess It also MUST burn 6% more fuel to create the same energy release as std petrol - but the crooks adjusted it so it tends to use 20% more fuel to go as far as real petrol can

 

I saw that other post so thank you for sharing your thoughts. I also appreciate you updating your posts to address the questions; I think this is going to help any other searchers who are serious about what they put into their vehicles.

I think I'll add an octane booster into my car to get me to 93 PON (from 91) on this current tank of 91 PON, then use 93 PON 100% fuel (no ethanol) exclusively. If that makes my car even more fun to drive, it'll be well worth it.

Thank you for taking the time to help me!

 

with octane booster short term - u can try it to reassure yourself of the benefits u aren't currently getting at the moment

USA user manual should explain in words that makes sense to the unit system in place over there - but the sticker on the filler flap is clearly saying European 93 RON and it just about copes (horribly), will cope reasonably on 95, and should fly on the correct 98

 

In USA premium pump octane varies according to altitude. 91 is it where I am at mile high.

 

there is less air pressure the higher up you go - so on a NA engine (normally aspirated - AKA ones where atmospheric pressure fills the cylinders with air), you end up with a low compression engine, and thus it needs lower octane fuel to get the optimal burn of a boring state of tune. Come down the hill and you must fill it up with proper fuel.

but if the car has a turbo or a super-charger then these devices just work harder and produce the missing air pressure - so these do not become low compression situations - so they need to retain the correct higher octane fuel (unless its engine management is sophisticated enough to work it all out and just run with reduced output)

Thus the situation described in this thread is perfectly normal we get lower Octane up the hill all around the world - South Africa is the same - they use Euro measurements for Octane with low octane 93 up the hill in Pretoria, higher octane 95 or 97 down the hill in Cape Town.... where you suddenly feel the car goes much better. However if you drive hard in Pretoria with their nasty fuels and oils, its common to blow up the turbo from over exertion.


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Thanks for the chart, is it possible to include AKI in the comparison as well? Thanks!

 

AKI (anti knock index) = PON (pump octane number) - as you measure Knock resistance in the USA using a slightly better MON test routine (rather than the ridiculous European RON test) and are quite comfortable to then choose to call it by another unit system at the pump - why not just call the stuff at the petrol station RON as well and we can all sleep happy and stop squabbling around the world ?

 

So following up on this, I can only find ONE gas station around me (within about a 30 mile radius) that sells 93 octane but it's 10% ethanol. So now which do I choose as the winner, E10 93 octane or no-ethanol 91 octane?

They sell E85 around here quite a bit but I don't know what a tune for that on my car would run me and if it's a good idea based on everything you've said about ethanol.

 

anything without ethanol

ethanol is a budget octane booster - nothing else about it is any use - sure I wrote this, they all did the dirty on us as this rubbish became common place - 10% ethanol by the maths has to mean the mpg drops 6% (the same quantity of ethanol has a lot less energy) but world wide most are see 20% decline in mpg - and to make sure you don't feel bad about it they seem to have made the other stuff worse too

 

I was afraid that'd be the answer, so I'm back at 91 octane non-ethanol fuel. Is my solution then to just use an octane booster in every tank of gas (I don't even know what's in that stuff) or to try to mix this with 100 octane race fuel to try to get a balance? It stinks after all this work and research that my solution right now is full circle back at 91 octane.

 

you only need the octane in the summer when its warm and dry enough to try and have some fun - not that I'd advise, but you could even use the junk if you're using the car a lot and get rid of it and get a few tanks worth of real fuel in there well before the damp and cold of winter comes back

 

Good discussion on fuel. I run 91 and do my best to avoid ethanol. I’m at 2,500’ or less MSL with low humidity(density). Motor and TT run great and never had a ping. Occasionally I’ll run a tank of 93 as a wiper. I would love to tune this thing one time and take it to a track. You should be fine with 91 unless you romp on it alot.

 

when you play with dead dinosaurs and split it apart, you get various levels of rubbish and some exotic fun

closer to the bottom of the pile of rubbish is tractor death fuel (AKA Diesel - note big clue in the name)

close to the top we get petrol (does the name gasoline, come from the fact it can evaporate at room temperature becoming a gas?)

towards the top we get to the aromatic posh stuff, but it has a tendency to get you cancer and other ailments. Toluene is one of the ones at the top and is awesome for antiknock - and many octane boosters used this - its not good for you, but if you like glue sniffing its right up there


oh dear....

Somewhere if I look up (I did) Methanol is just a more dangerous and corrosive form of alcohol like Ethanol - stats - turns out you need 33% more methanol than petrol to go just as far and 25% more ethanol to go as far as petrol. Ethanol is less chemically toxic than Methanol, and it has slightly more energy... Ethanol contains about 75 percent of the energy of gasoline per gallon, compared to 67 percent for methanol. Both thus achieve fewer miles per gallon than petrol, but in competition no one cares, and if Methanol is more corrosive than Ethanol just use more resources up buying components.

From 1964 to 2005 Indy car ran Methanol. In 2006 the fuel changed to 90 percent ethanol, 10 percent methanol. In 2007, it became 97 percent ethanol, 3 percent gasoline. The gasoline was to make the fuel safer - alcohol burns with an invisible flame, and gasoline does not. In 2012, Indy car went to pump-grade E85…85 percent ethanol, 15 percent gasoline.

nut jobs

Back in the 1980's when Formula One didn't do fuel stops (coz its stupid, expensive, boring and dangerous), unlike indy car of that era, they couldn't run Methanol because the tank would run out in their 2hr or 200 mile races, And because they'd also gone down the turbo era, they'd brought in small capacity engines of 1.5 liters, to try and slow the fragile deathtraps down a bit. But competition is competition, so the teams with no budget caps just moved up the scale of silliness and ran 86% toluene and mega boost pressures. In qualifying they were up to 1300bhp in a 300 kg go-kart (with no traction control, no power steering and a manual gearbox) - so back then we needed real driver's like Ayrton

Toluene at 86% by volume fueled all the turbo engines in F1 during the 1980s, first pioneered by Honda the team. The remaining 14% was a "filler" of n-heptane to drop to the Octane limits F1 used. You can run 100% toluene but it doesn't like to vaporise unless preheated to 70 °C (158 °F). Honda solved routing the fuel lines through a heat exchanger


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Where do you live? High altitude fuel blends are usually two octane lower than the same fuel at low altitudes. Engines don't require the same amount of octane to retard ignition, regardless of the vehicle, at higher altitude.

 

wrong answer for a boosted turbo or supercharged engine - see post 12

 

Oklahoma USA. Only 1000 feet above sea level, which is why I'm confused why we don't have 93 octane here.

 

yes that is odd, should be on normal fuel

for example in South Africa, Johannesburg is 5700 foot above sea level with nearby Pretoria around 4400 foot up - they get the next levels down - remember below to take 4 off for PON ratings u use

Depending on your vehicle you should, in theory, get better mileage and fuel economy from a higher octane fuel (so 95). However, in the highveld you have approximately 20% less atmospheric pressure than at the coast - which makes it harder for fuel to ignite. In areas like Johannesburg or Pretoria, you have the option of filling up with 93. You won’t find 93 in coastal regions (e.g. Durban or Cape Town) because it will cause knocking, so you’ll have to use 95 or 97.

 

Don’t get hung up needing a high octane number. The CR on our motors is 10/10.5:1. Even with TT, these motors can run 91 all day long. 10:1 is mild.

Race motors at 12 -16:1 absolutely need higher octane rating to stabilize fuel from pre ignition. MB motors don’t have that problem. Octane adds no power. It stabilizes fuel to match a known compression ratio to prevent preignition in high CR motors to generate crazy horsepower numbers. That’s the short version.

if you hear pinging, you most likely have a tune issue….especially on a used car.