Q: Am I wasting my money buying premium fuel when my manual says all I need is 86 octane, and the bike seems to run fine on it? I always thought I’d get better performance with high-octane gas, but after doing some research, I’m not sure that’s true. My understanding is that the octane boost is in the fuel to prevent preignition (pinging).

Also, a friend told me that by installing an aftermarket exhaust and rejetting the carbs, my fuel mileage should go up because the engine is running more efficiently. I say you can’t get more horsepower without forcing more fuel through the engine, so your mileage should go down. Which of us is right?
John Clayton
Via e-mail

A: Well, John, that’s an easy one. Any increase in octane rating greater than what’s required to avoid detonation in a given engine under a given load or circumstance is unnecessary. Octane or, more properly, isooctane, is a molecule that slows the burn rate of gasoline. The more octane present, the slower the burn. A too-rapid burn leads to detonation, which creates temperatures and pressures that will damage your engine. So, use the lowest rating that lets you ride your motorcycle your way without any perceptible detonation or knock. You might hear someone say his bike runs better when he uses premium fuel, even though it doesn’t knock when he uses regular. This might be because not all detonation is audible. Light or incipient detonation might be sensed, but not heard, by an intuitive operator. If you feel this, use the better fuel. If you don’t, don’t worry about it and use the less expensive juice.

As for your second question, you’re sort of right. It’s possible for an engine to get better mileage after some basic performance modifications. An engine may run more efficiently with a better-flowing exhaust and proper jetting. EPA exhaust and noise emissions regulations force manufacturers to make some compromises that preclude using the available fuel for the best power output for a given throttle opening. Their goal is to produce the least noise and harmful emissions while still yielding an acceptable level of power.

Where you’re not quite correct is your comment about “forcing more fuel through the engine.” The range of acceptable air to fuel is really quite narrow, ranging from about 12:1 to 14.5:1 (by weight). You may have heard the word “stoichiometric.” When used in the context of an air/fuel reaction, this describes a complete, perfect combustion process where all available air is combined with all available fuel. This ratio is 13.8 pounds of air to 1 pound of fuel (13.8:1). Get it down to 12:1 (rich) and the engine will barely run — it will be “sloppy rich.” Get it up to 14:1, and the mixture will be so lean, it’ll also barely run. The controlling factor is really the amount of air you can process, which is pretty much fixed. You can’t “force” more fuel into the engine without quickly getting an overly rich mixture. Thus, you have a fairly narrow band of mixture to work with, about 5-7 percent on either side for a given engine combination. Most engines make their maximum power at about 12.8 to 13.2:1, the best economy at 13.8 to 14:1, and the least emissions at a point somewhat leaner than that. For your bike, or any bike (especially a modified one), there’s probably a spot that will yield better mileage and a spot that will yield slightly better power, each at the expense of the other. Therein lies the compromise that all stock production motorcycles are burdened with — and that’s where the fun of tuning begins!
Doug Meyer

A: If your owner’s manual says that low-octane fuel is what the engine is designed to run on, that’s what you should run. In some cases, higher-octane fuel burns slower and could actually produce less power. In regard to your other question, it’s basic physics. More power requires more fuel, period. Having said that, if you do have a more efficient engine and you moderate your throttle hand, you’ll get better gas mileage.
Nigel Patrick
Patrick Racing

A: The elevated levels of octane are there to prevent preignition, or detonation. High-compression motors like the ones found on modern sportbikes require higher octane levels to prevent combustion before the spark plug sparks. Lower-compression motors, like those found on most stock cruisers, require a lower octane. A common myth is that the higher the octane, the more power your bike is going to produce. This couldn’t be further from the truth. Higher-octane fuel burns slower than lower-octane fuel, in order to resist detonation.

To answer your question, yes, you’re wasting your money by putting anything higher than 86 octane in your bike. I’d recommend running what the manual suggests, and that’s good news with the price of fuel today. As for getting better fuel economy with rejetting and an aftermarket system, you’ll more than likely lose fuel economy — that is, unless your bike was running so horribly rich that it’s now running better.
Chris Reo
Motorcycle Mechanics Institute

RB