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Fuel octane number for US 6.9?

Started by Jan S, 31 October 2022, 06:09 PM

Jan S

Engine is a 1977 US 6.9

For years I have used unleaded 95 octane fuel. Is that correct? Should it be 98? Location is Europe, Norway to be more specific.

I checked the owner's manual for the US 450 SEL 4.5 on the forum. It says: "unleaded gasoline: average octane of Research and Motor 87 (RON of 91)". See pic.

Does RON of 91 means closer to 95 in Europe? Is my interpretation correct?

What does the US owner's manual for the 6.9 say? Anyone have that?

Any tips highly appreciated.
1975-mod W116 450 SE with 6.9 engine

daantjie

I'm far from an expert on this but due to the relatively low compression ratio you do not need super high octane like you would for turbo engines for example. Altitude above sea level also plays a role of course.
More importantly avoid any ethanol in fuel.  Not sure if you have this garbage in Europe in your fuel.
Daniel
1977 450 SEL 6.9 - Astralsilber

rumb

#2
This article explains octanes.

https://www.eia.gov/energyexplained/gasoline/octane-in-depth.php

Does Europe use Octane or MOR or RON?


More info on octane and compression:
https://auto.howstuffworks.com/compression-ratio-octane-ratings.htm

Note our engines do not have knock sensors.

That said I have used mid-grade in my 1991 300se for decades on advise of my mechanic and it works perfectly. Our 2 newer cars use Premium only.

The chart above I think is saying octane of 87.  Which they correlate to RON of 91, which is technical incorrect, since octane = (R+M)/2 , but none the less is close enough.

'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS

raueda1

This explains pretty well:
octane

You're fine with 95 octane.  Try slowly accelerating up an incline at low speed.  If octane is too low you'll hear knocking.  If you go with 98 octane you can probably get more power by advancing your timing a few degrees.  That can be fun to play with.  ;-)   Octane requirement is reduced at higher altitudes.  Standard grades in the USA are 87, 89 and 93 octane (RON).  In areas over about 1500 m you tend to see 85, 87 and 91.  Reduced atmospheric pressure at altitude effectively reduces engine compression thereby reducing octane requirement. 

I've been using 91 RON at altitude and the 93 RON at sea level.  Also running my ignition a few degrees advanced with the encouragement of Rumb who is a big fan of this stuff.  Never had any knocking problems. In fact, this motivates me to see how much more timing I can add before knocking.  Has anybody tried to see how far you could go?  Seems to me that the timing specs are very conservative.  Fully agree with Daniel about using ethanol-free gas if you can.  But often I can't find it and honestly I can't tell the difference.  But I'd never store the car for long periods with the ethanol stuff in the tank. 

sidebar: The compression in my car is 8.6:1 (pistons are +1), not 8.8, and it has never knocked.  A while back I did some research and discovered that the compression in these cars is actually all over the place.

Happy motoring and cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Jan S

Thanks to all of you for comments! I need to read and digest all the information and links you provided.
1975-mod W116 450 SE with 6.9 engine

ptashek

US 6.9 would use 91 AKI, which is 95 RON in Europe. Euro 6.9 would need 98 RON, but can run with 95 with ignition retarded 1 degree per octane below 98.

Norway probably has ~5% ethanol in standard petrol as most of Europe these days?
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE

Jan S

Thanks, ptashek!

Trying to summarise ... a few facts and my understanding after reading the information from all of you:
- The octane number in Europe/Norway is RON.
- I've been using 95 RON with 5% ethanol (95 RON ethanol-free fuel is not available).
- 95 RON in Europe is equal to approx 91 AKI in the US also known as "Octane" on the US pumps.
- I'm considering using 98 RON because it's ethanol-free here in Norway.
- The car is normally at sea level (requires higher RON compared to altitudes).

- The engine is designed for 95 RON. However, 98 RON will not harm the engine. Correct?
- And I can advance the timing ... a few degrees .... maybe 3-5 deg or even more when using 98 RON. Correct?


1975-mod W116 450 SE with 6.9 engine

rumb

Using a higher grade causes no harm and doesnt make any difference in performance either.

But since the higher octane makes knocking less possible, then yes you can advance the timing some to get a wee bit more power with no concerns about knock.

As Dave mentioned I'm a fan of increasing timing a few degrees even at altitude (>5000ft where I live)The 6.9 shows noticeable power increase.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS

Jan S

Thanks! Should have changed to 98 ethanol-free fuel years ago.
1975-mod W116 450 SE with 6.9 engine

ptashek

What I learned a few years back in Ireland is that our 95 RON with 5% ethanol, at the pump, is actually closer to 97 RON. So check with your local suppliers.
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE

Jan S

Quote from: ptashek on 04 November 2022, 10:44 AMWhat I learned a few years back in Ireland is that our 95 RON with 5% ethanol, at the pump, is actually closer to 97 RON. So check with your local suppliers.

I want to avoid ethanol. So 98 it will be.
1975-mod W116 450 SE with 6.9 engine