Gasoline Question

[ralphie]

I know your supposed to use the mimum octane rating that your owner's manual states for your car and exceeding this octant (for example using 93 when all you need is 87) will not increase performance. Does the same hold true for motorcycle? Meaning using 93 when all the manual calls for is 87 wont get you any gains, right?

[josowsky]

My (limited) understanding on this issue is that on cars the octane issue is present because the car 'computer' (or ECU, engine control unit) takes into account the temperature of the outgoing (expended) gases. Because the computer is configured that a certain temperature range is 'ideal' it will de-tune the fuel / air ratio after a few minutes of driving with a higher octane ratio then usual (to go back to the lower temperature.
To my knowledge this didn't become a factor until cars in the early 90's given that prior cars either a) didn't have a computer, or b) the computer wasn't as advanced. I don't think a lot of bikes are as complex as cars today, but maybe some of the new ones are.

[flynrider]

The minimum octane (anti-knock index) for your bike should be fine. Octane is a measurement of a gasoline's resistance to detonation in an engine. This is primarily determined by the compression ratio of the cylinders. A higher compression ratio will require a higher octane to keep the engine from knocking or pinging, which is caused by the fuel/air mixture exploding in the cylinder prematurely. Higher octane gas is less explosive than lower octane. If your bike runs without knocking or pinging on regular gasoline, there is no benefit to be gained from using a higher grade.

My Nighthawk specifies 87 octane (regular unleaded) gas, but in the heat of the summer here in the desert, I occasionally get some detonation when I pour on the throttle. As such, I tend to use the mid grade gasoline in the summer and regular the rest of the year. My truck is the same way.

[ZooTech]

I know your supposed to use the mimum octane rating that your owner's manual states for your car and exceeding this octane (for example using 93 when all you need is 87) will not increase performance.

That's incorrect. More often than not an engine can "tolerate" 87-octane gas, it doesn't necessarily prefer it. My truck pings like a popcorn maker on 87 (the minimum recommended octane) but runs very smooth on premium. The reason is the ECU is able to advance the timing more when running on premium which usually results in more power, especially in the higher RPM's.

Check out Nissan's website for the Maxima and look at the horsepower specs. There's a small "2" next to the rating (265hp) which denotes some fine print lower down. The fine print states: "Use regular unleaded fuel with 87 octane. For maximum power, use premium fuel."

[flynrider]

Higher performance with higher octane gas only applies to later model car engines that are equipped with knock sensors in the head, tied to the ECU. I don't know of any motorcycles that are set up like that (yet).

[ronboskz650sr]

Are you talking about a bike with an ECU? If not, it can get even more complicated, but nothing a couple of trips to the pump won't fix (just ride more). My bike...oldy but goody...runs poorly on premium, and even fouls the plugs, around town. On the highway, it runs fine on premium, but gets worse mileage. Remember, late 70's state of tune, points and condensor, weaker spark in general (not on purpose), lower compression ratio...designed to burn bad, cheap gas. Until I had this bike, I had no idea premium could make a bike run worse...but in my case it does. I say, if it pings, put it in. If it doesn't stick with the recommended octane...especially for old bikes.

[jmillheiser]

Hes right many modern cars will produce more power with premium.

Most modern cars run at the very edge of detonation on regular unleaded. They spec 87 because it makes a tiny reduction in emissions and the engine will still run it. 91 or better octane in most modern engines (especially a high strung one like a honda VTEC motor or nissan VQ35) will give you a few extra HP and smoother running. The same thing should apply to a fuel injected bike that specs 87 especially since bikes have much higher specific outputs than a car.

Also in some cases you can pick up a few mpg buy running high octane. A smarter FI engine will lean out the A/F ratio during cruising if there is no sign of detonation. My 91 olds is a prime example, with regular unleaded it barely manages 20-22mpg and loses a little power as its on the edge of detonation with mid grade it gets 25-27mpg and much smoother running and more power, going to premium however affords no extra benefit on this car.

[ZooTech]

Higher performance with higher octane gas only applies to later model car engines that are equipped with knock sensors in the head, tied to the ECU.

For the most part I agree. But if you took a carb'd bike or car and manually advanced the timing it could make use of higher octane gasoline by igniting it sooner. The benefits, if any, would probably only be felt at the upper end of the powerband, but it would still mimic the electronic advancement done by an ECU-equipped engine.

I'm not suggesting anyone do it, but if your timing was off and you didn't know it you may find higher octane gas works better for ya.

[jmillheiser]

my one non computer controlled car gets nothing but 91 octane however. The engine in my bug has had its compression raised over stock ,a more aggressive cam, and an increase in displacement. The bug needed mid grade even in stock form due to being air cooled.

[ronboskz650sr]

That's a good point. Brings up another tip...If you set points with a credit card and time by ear for your rough timing...make sure you use the same gas until you take the time to do it right. One time I did that out in the sun in a hurry...couldn't see the light, and wanted to ride. Worked fine, until I filled up again...dummy, lesson learned...do it right the first time or accept the possible consequences.