87 vs 91 octane fuel

[Walking Tall]

Can anyone give me a reason why I should run 91 octane when my owners manual calls for 87? I've been told by a reputable local tuner with 30 years of experience working on Triumphs that 91 will allow my bike to run cooler, while 87 would produce more h.p.

 

[idk]

I phoned Triumph USA about this a few years ago and the answer is that there is no reason to, unless it is pinging. TuneECU can take care of any pinging so the answer is still no. However, if you have performance mods then the answer may change.

 

[sonny]

Our bike are low compression. Well those that have stock pistons anyways. You can put 91 octane in and run the bike i guess but why? Runs fine on 87 or 89 octane. I do not believe the bike will run cooler using 91 octane or that 87 will produce more horsepower. I do believe running real gas and not corn syrup makes a big difference. I run 89 octane after adding the street cams. ( 3 into 1 Paul's exhaust and ramair) Seems to run smoother. Tried 91 for a gas tank or two and figured i will save my money for better things.

 

[Joesmoe]

No reason.

The engine as delivered will provide best performance, as defined by mileage, power, while meeting CARB requirements, on 87 (avg Research and Motor) octane rating.

 

[Joesmoe]

And let's remember our basic organic chemistry.

Back in World War II, in the quest for ever greater performance, basic physics tells us that performance is limited by the absolute temperature difference between the hot and cold reservoirs. So the push was on to have better metallurgy that could withstand *higher* temperature and pressures, and concomitantly, fuel was required that could withstand the higher temperature and pressure before intentional ignition.

Chemists delivered on these needs with longer chained hydrocarbons, branched chained hydrocarbons, and the infamous tetraethyl lead discovered to delay the onset of detonation.

Octane ratings reached 140, permitting 36 cylinder motors with two stage turbocharging to exceed 3,000 hp normal rating, and upwards of 4,000 hp in emergency rating. These fuels have all but been phased out, and virtually all aviation fuel today (for reciprocating engines) is 100 LL (low-lead).

Higher octane fuel is more expensive, because those extra carbon atoms means there are fewer gallons of higher octane fuel from a barrel of oil, than lower octane fuel.

To this day, a pilot of a piston engined airplane will adjust the fuel/air mixture on the fly based on the exhaust gas temperature gage - at the lower end of the range, the mixture is richer and can produce more power. At the upper end of the range, the mixture is leaner, to greatly extend the range.

So just the opposite, for a motor designed to produce greater power by forcing more fuel into a smaller space, it will run hotter - not cooler.

 

[Griff]

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)

 

[Joesmoe]

Thank you @Griff

Yes, details matter.

 

[sonny]

Harley requires 91 octane. Fiat requires 91 octane. The low compression engine of the rocket Which will tear both a new rear 89 octane.

 

[Dr.D]

The 2014 Roadster ran fine on 87 octane but I like to waste money buying 91 and 93. So I put in high compression pistons and big azz cams to match the high octane fuels.

 

[Walking Tall]

I phoned Triumph USA about this a few years ago and the answer is that there is no reason to, unless it is pinging. TuneECU can take care of any pinging so the answer is still no. However, if you have performance mods then the answer may change.

Not doing TuneECU, doesn't seem to be anyone around me that uses it (although to be honest, I didn't look real hard). I'm going with PCV/Dynomax, using a reputable shop with 30 years experience tuning Triumphs.