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> i've heard that octane only affects engine knock. thus, you should use the lowest possible octane that doesn't cause the engine to knock. however, i've also heard that the octane can affect the performance of the engine. what is correct?
Subject to a few inevitable "it depends," the truth is generally that octane is only about pre-ignition (which manifests itself as ping or knock). There is virtually no difference in energy content; and in fact, the lower octane has the tiniest smidgen more of that. The higher the number, the greater resistance to detonation for a given set of engine operating parameters. That's it. All other things being equal (storage tanks being clean, equal care in the refining, etc.) there should be no performance difference based on octane ALONE.
There is one exception only, and that is if you have a high-compression, high-performance engine that is equipped with knock sensors and the means to alter ignition and valve timing to compensate for different grades of fuel. A fair number of automobiles do have that capability nowadays. If you run them with the high octane fuel for which they were originally intended, they will employ normal ignition advance and valve timing to deliver the maximum power for which they were designed. If you run them with a lower octane fuel than intended, they will cut back to a less aggressive performance level that does not risk knock.
The process does not work the other way around, however. You can't trick an engine into delivering greater-than-designed performance just by increasing octane. Remember, Triumph motorcycles do not have knock sensors or variable valve timing. Few other bikes do have at present. At some slight risk of oversimplification, most are designed to a certain level of performance first, and then the octane required for safe operation of the engine is confirmed.
> say i use 87 on my striple and there's no engine knock and everything is operating as it should. would i get a boost in performance if i put in, say, 91 octane fuel?
No, for the reasons mentioned above.
Over the years, I have read posts from riders who are convinced they can tell such a difference. I have also read posts from riders who have reported just the opposite experience... worse performance from higher octane fuel. I believe both groups are reporting their observations honestly. But it's not the octane number doing it. I've done enough first-hand comparisons over the years to confirm this, as of course have far-smarter people in the petroleum, automobile, and motorcycle industries over many decades.
It's all down to the refining quality, additive packages, and cleanliness of the fuel coming out of individual storage tanks at individual retailers. There can be wide variation in these factors from brand to brand, and station to station. The octane number tells you nothing about the fuel's overall quality--only its knock resistance.
> would the life of the engine be extended by using a higher octane fuel?
As long as you experience no knock or ping with the rated octane, no. I hope that helps alleviate any concerns.
(Having said that, I do admit to a certain tiny degree of superstition. I am running mine in with 89 octane since internal engine parts are slightly more vulnerable during these early miles. For the same reasons you don't want to lug a new engine, I don't want to subject pistons, rods or bearings to the stress of any accidental knock, either, just in case Triumph were a tad optimistic with their specs. I will drop back to 87 octane later if all appears to be going well.)
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John
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