FIRST, A SCIENCE LESSON:
Octane is a hydrocarbon chain with 8 carbons chained together. It can be compressed (in combination with air) much more before it ignites without a spark. Heptane is a 7-carbon chain and ignites under much lower pressure.
If the “octane rating” is 95, for example, that means it’s 95% octane and 5% Heptane. Basically, the lower the octane, the more flammable the fuel is. That means if an engine is meant for 95 and you put 92 in it, it might “knock”. That’s because the fuel is igniting too early. Either because it is igniting under pressure or because it’s burning too fast and starting to push the piston(s) too far before Top Dead Center (TDC).
BASICALLY?
Fewer-cylinder-lower-revving engines need that slower, firmer push on the pistons. That creates good low rpm torque. Multi-cylinder-high-revving engines do better with fuel that burns faster. (Faster=lower octane). This is VERY general, OK. Basically, the way the engine is engineered is the way to go. Changing octanes is slightly asking for trouble, and definitely not an advantage to go either way. 98 is probably more expensive because of the process involved in refining it. Not because it will work better with your engine.
The number of cylinders, intake method(s), compression ratio, average expected RPM on the torque curve, ignition timing, and even bore/stroke ratio can be factors that decide which fuel is best for an engine. If your vehicle says “9X” on it, use that! Probably many engineers spent hours and hours tuning it to be what you want. Messing with that won’t help you.
IF YOU STILL DON’T GET IT, HERE’S A CHEESY PERSONIFICATION:
Think of a nurse in a hospital. She’s on her feet all day. (95 octane shoes). She does fine. One day, she wants to buy shoes that are a little smaller (92 octane shoes). She wears them and her feet hurt very badly (“knocking”). Ouch!!! Knocking HURTS!! Then she buys oversized shoes (98 octane). She can walk around with no problem, but she becomes clumsy, and maybe the sliding around inside might cause blisters after a long day. Stick with what you’re meant for. Buying bigger shoes won’t make you more comfortable.
PLAY WITH IT:
If your bike uses 95 octane, run it down to a nearly empty tank and then take an empty plastic bottle to a filling station (I said EMPTY, dammit, not a water bottle that still has droplets in it) and fill up $20 of 92-octane. Then buy 98-octane in the bottle. Drive around for a while and see if it knocks or behaves differently. If it does, stop and drop the 98 in. It mixes quickly, you’ll notice the difference right away.
If you normally use 92, try 98 and see what happens with $20 worth of 98. If it feels different, just go to the next station (which is probably not far) and then fill it to the top with the recommended fuel and then this “testing” is over.
Not a problem these days. Taiwanese gas is actually of very good quality, according to tests done by BMW. The only issues you might have are those stations with leaky tanks, or those that haven’t cleaned them in many years (which is a lot). If you go fill up when their tank is almost empty you may be unlucky and get a shot of filthy water from the bottom.
