Octane rating

[zak]

I have been using 91 octane regularly for the last 20 years in my 250 SL.
Recently I filled up at a Sunoco gas station that had 93 octane gas so I tried it.
Tank too about 16 gallons.
After that the engine felt a bit more responsive and eager  and revved a bit free-er.
Am I just hallucinating or does the higher octane fuel make a difference ?

jz 

[Bonnyboy]

I think it does. 

I am more concerned about ethanol in the fuel so when I fill up in the USA, I buy the only ethanol free fuel available locally which is 89 octane.  After going through a tank of that I find a definite difference in performance and knocking with the lower octane fuel.  Once I fill up with the Canadian 94 octane ethanol free fuel, my car becomes happy again.   

[mdsalemi]

Last time I used "regular" fuel, 87 octane by USA standards, my Pagoda sure let me know how cheap I was, and voiced its displeasure. Since that time, I've always used premium which is generally 91. More recently the I've found some mid-grade, 89 octane and I didn't get too many complaints. So yes, Zak, I think you were not dreaming.

"Rec gas" is easy to find here in the Carolinas, what with hundreds of NASCAR drivers and thousands that THINK they are not to mention so many boats around the lake here. That's a/k/a ethanol free. It is quite a bit more costly. It was harder to find when I lived in Michigan (something peculiar about driving 15 miles to get fuel...) thus never used it.

For all the complaining that people do about ethanol in the gas (corn politics aside) I used E10 for 23 years and over 20,000 miles, with lots of storage in that time, like 6 months each year. In all that time, only ONE fuel system problem, which I cannot positively attribute to E10, but say it was: it was one $20 hose from the fuel feed pump to the steel fuel line at the back of the car.

Not going to ethanol free anytime soon. Maybe I'm living on borrowed time, but the car runs very well. Don't mess with what works.

[JamesL]

For clarity
Octane ratings differ across the pond (different scale  - like you have a different gallon)
Here we generally have 95/98RON at most pumps
In the United States gas stations describe the types of gasoline based on the Pump Octane Number (PON). That number is the average between RON and MON.

If a fuel is 98 RON then it will be 93 PON -> 93 PUMP
If a fuel is 95 RON and 87 MON then it will be 91 PON -> 91 PUMP

Mine runs very happilyon 95RON E10 - although I sometimes treat it to 98

[MikeSimon]

One thing about "octane ratings". Higher octane does not mean better performance. Higher octane fuel has a higher number of ignition inhibitor additives in order to prevent uncontrolled combustion, i.e.: "pinging". If your engine has a compression ratio which does not require a certain octane rating, going higher than that, means you will end up with unburned fuel residue. This can cause "wash down" on your cylinder walls and consequently wear on the piston rings. There is a widespread misconception that higher octane means higher performance. It is just not the case.

[mdsalemi]

...There is a widespread misconception that higher octane means higher performance. It is just not the case.

well, sort of. I think.

Modern cars are different are they not? The "rated horsepower" of many performance cars (such as the Ford Taurus SHO we had, or a modern SL) is usually quoted with "premium" fuel (with the octane rated denoted around the gas cap or such. These modern cars have complete computer control on everything, so they won't knock or ping on 87 octane but they won't perform the same. Ignition advance and retard is computer controlled. This may be "throw it on the dyno and test it" kind of differences.

I never "needed" the performance on the SHO so she drank regular gas, with 10% ethanol and behaved just fine.

[zak]

I cannot find any non ethanol gas in northern NJ. So I use the e10 and my baby blue doesn't complain..
JamesL, I had to laugh at your ron/pon/mon because I don't know what it means.

jz

[Ed Riefstahl]

Greetings,

We've driven our 1966 230SL from coast to coast and Main to Florida in the past 22 years. We've had to use every and any fuel available, buy always try to use 93 if I can get it, even though it usually has 10% Eth. However my 280S with carbs. will hardly run on ethanol crap.

The only times I can remember really noticing a real difference were a few times I had to use 97 and once coming east on I-70 into Denver up that 10,000 ft incline. It still cruised up at 60 mph like a champ passing many overheated vehicles pulled off.

I'd like to add this comment while talking performance though. The single best improvement I've made to my SL was switching to electronic ignition in 2010, including the better/hotter coil that is available with the purchase. It really does run better on the ethanol. I can't remember off hand if I'm using different plugs though. I did put hotter plugs in the 280S, which made a big difference with the non-eth 90 that I have to use in it.

For those who've never considered electronic ignition, it's super easy to convert with no permanent modifications to the car. There is also no way to tell unless you were to remove the distributor cap. My car runs better, it's always perfectly tuned and I've never had to touch the distributor in over 20,000 miles. I do keep the points in the trunk and can put them back in the distributor, if for some reason I had failure with the electronic unit.

Good luck,

Ed Riefstahl
Erie, PA

1966 230SL (Ms Magoo)
1970 280S (Miss Daisy)
1989 300SE (Majestic)
1999 BMW Z3 5 speed
1991 BMW 318I 5 Speed
1997 Toyota Paseo Convertible - Red 5 speed (have you ever seen one?)
1997 Ford Ranger step side (Mater)
2023 Mazda CX 5

[bracurrie]

The owners manuals of 1960's Mercedes with higher compression engines recommends that if you use lower octane fuel that you have to retard the timing a bit to prevent pinging. Because I have had issues with my two way switch that controls the all at once retard system and I then disabled it, I learned how to set my own distributor timing. I run 93 E free whenever possible so I set my timing as advanced as possible without pinging. I got stuck having to buy 87 E10 one day and had to retard the timing as it was pinging under load. The combination of the lower octane E10 and the lower spark advance setting was noticeable. Because of this I am looking forward to installing a 123 Ignition distributor with the app control so I can adjust on the fly for lower octane fuel and still run as much advance as the engine can take without pinging.
My question is this: How does ethanol affect the detonation characteristics of fuel? 

[MikeSimon]

If you are forced to use lower octane gas, you can always use a fuel additive. There are plenty around which eliminate pinging, add lead, solve ethanol issues etc. No problem.
Chemistry 101: The component in fuel that carries energy and produces performance are CmHn molecules. If in a given amount of fuel, a certain volume is taken by ignition retarders to achieve higher octane ratings, there are less CmHn molecules to make power. When I attended University, we did a Dyno test on a varaiable compression AVL test engine and the fuel with the highest specific gravity (engine oil) produced the largest amount of hp

[mdsalemi]

I cannot find any non ethanol gas in northern NJ. So I use the e10 and my baby blue doesn't complain..
JamesL, I had to laugh at your ron/pon/mon because I don't know what it means.

jz

Must be some thing with New Jersey. The following link to pure gas shows eight stations in New Jersey, while we have over 1100 in North Carolina.

https://www.pure-gas.org/

I think you can ignore the ones that are at an airport because I do not believe you can put aviation gas inside anything, but an airplane by federal law. Unlikely they will allow you to pull up your SL to an aviation fuel pump and fill up. But there is at least one marina…

[BobH]

I'm not sure where you can buy ethanol free fuel in the UK, apart from direct from this supplier, only costs between £ 3.80 and £ 4.65 per litre, depending on how green you're feeling that day

I think i may continue destroying the planet until prices drop and it becomes more available, probably have a long wait!

https://coryton.com/lab/news/first-sustainable-fuel-for-classic-vehicles-fuel-launches-for-public-purchase-in-the-uk/

[MikeSimon]

Usually, you find ethanol-free gas close to maritime activities. Boats normally do not use ethanol spiked gas and the filling stations carry ethanol-free accordingly. I have a (Marathon) station 2 miles from me and it is about 20 miles from the closest body of water that has any boating.