RON, MON, and (R+M)/2

[y_p_w]

I think most of us know the difference more or less, but I've had some interesting "international" discussions where it broke down because of a lack of understanding about using different scales. People in Europe saying stuff about the "low quality petrol" sold in the US or Canada that was sold with (R+M)/2 (AKI - anti-knock index) as the testing requirement.

I know that they more or less scale a certain way, but it's possible that there are fuels where there's less "spread" between those numbers based on the individual composition. I understand that pure iso-octane fuel has no spread. Or that one fuel with exactly the same (R+M)/2 octane rating might advance timing more compared to another one because the rating is a blunt tool.

 

[Cujet]

Simply review "octane rating" on wikipedia for a good breakdown of each fuel (or component) and it's research or motor octane numbers.

https://en.wikipedia.org/wiki/Octane_rating

There are large differences in each. Without adequate testing of each fuel, they can't be compared. Our lower numbers here in the US, do not correspond to lower real world anti knock performance.

 

[Boomer]

Many years ago, there was a big fuel problem in Europe for gasoline. Somehow the fuel they started to see a very large spread between the RON and MON values and this resulted in engine damage. I think the problem was most acute in Germany and in other nations with no speed limits on the main roads. I believe this was in the 60's or 70's.

 

[eljefino]

The "spread" is documented and called "Sensitivity". Less is better.

 

[y_p_w]

Originally Posted by Cujet
Simply review "octane rating" on wikipedia for a good breakdown of each fuel (or component) and it's research or motor octane numbers.

https://en.wikipedia.org/wiki/Octane_rating

There are large differences in each. Without adequate testing of each fuel, they can't be compared. Our lower numbers here in the US, do not correspond to lower real world anti knock performance.

Yeah - I get that each component that goes into fuel has its own RON and MON and they generally blend linearly. But RON, MON, and (R+M)/2 is a blunt tool. Some owner's manuals I have include equivalent recommendations for RON octane rating. However, I've got a copy of a 2002 Civic owners manual, which says "pump octane number of 86 or higher".

 

[y_p_w]

Originally Posted by eljefino
The "spread" is documented and called "Sensitivity". Less is better.

However, the typical charts seem to place 98 RON European fuel at around 93.5-94 (R+M)/2.