87 vs 91 octane fuel

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Griff said:

Just as an aside re Octane - my recent acquisition was a 2018 Indian Roadmaster. Read the manual and it said 91 Octane. So I put 91 Octane in it as I've done for 40+ years and half a dozen bikes (I've never NEEDED to put higher octane fuel in any of my vehicles because they didn't ping or knock, and given it's so much more expensive.....). Now, the issue is that Octane is measured in different ways depending on where you are. After the first fill, the bike ran like a tractor, I got engine misfire codes on the Ride Command system, had no acceleration and was generally just yuk. Did some Googling, got to the closest auto parts store and put in some octane booster. $34NZD and 150kms later, all of a sudden, he woke up and said, 'here I am' and started purring as before. The engine light also went out! So, in the manual it called for 91 (R + M/2 method). Our pumps show the 'RON' on the pump (who knew?) so 91 here is equivalent to your 87 Regular. There's a table on Wikipedia that shows the comprarisons. I needed to use 98 Super which is the equivalent of your 91. All going well now. I quickly ensured that my Touring's fuel was correct after that! (It uses 91 as measured in NZ)

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You experienced just the reverse of most of us. Since Triumph is a "foreign" maker, most of the units are in European standard including the fuel rating in RON instead of the USA standard of (R+M)/2 (also known as AKI or PON). Since your Indian is a USA manufacturer, the units are expressed in USA standards. So you are correct in that the international standard of 91 RON is roughly equivalent to the USA standard regular of 87 PON/AKI.

I like to make gas references in terms of Regular, Mid-Grade and Premium (some call it Super). That is generally understood internationally. In the USA, Regular is typically 87 octane (slightly lower in high altitude western states) as measured using the (R+M)/2 method. Internationally Regular is 91 octane using the RON standard. Similar values exist for the other grades.

Higher octane has an effect on the propagation of the flame front within the cylinder. The higher the octane, the slower the propagation. The goal is to get as complete fuel burn as possible thus liberating the most potential energy in a controlled manner within the cylinder. Too fast of propagation can result in an uncontrolled explosion, aka detonation. Too slow of propagation results in unburnt fuel spilling out the exhaust system. The unburnt fuel obviously does not release its potential energy (ie heat) thus the reason for "running cooler".

In the age of electronic fuel injection, it is important to match the fuel type, fuel quantity, and compression with the airflow in order to achieve complete combustion without detonation. The EFI tune controls that mixture. Changing any factor in that equation will result in less than optimal conditions.

just my 2c.