QuestionI drive an old Honda CT90 because it is so affordable and better on fuel than the latest Hondas. The thing that I have been troubled with for a while is what fuel will have less emissions? I have tried 87, 89 and 91 octane and they all run very well in the bike. 91 octane seems to give me a little more power, if you can believe that. I have heard from others that 87 gives a knock but I haven't heard it myself. They all use 91. What do you think? I don't care about spending the extra money. I just want to know if it makes a difference when it comes out the pipe.
AnswerI don't know if different octane fuels emit more or less emissions. I don't think they do--I think as a rule they're all the same fuel, what's different is the rate at which they burn. So let's start with that--there's no difference between 87 and 89 and 91. (Ignoring the fact that many fuels are now cut with ethanol, which is supposed to reduce emissions...more on that later.)
The lower the octane rating, the faster the burn. This is not important in and of itself, but it is important to your engine. Some engines (generally, low compression) like lower octane, faster burning fuel. Other engines, generally high compression, prefer slower burning fuels in the 91-93 octane range.
The preference comes from the design of the engine and the way it burns fuel--lets use the powerband as an example.
When a car, truck, or motorcycle reaches its "powerband," it's this magical place in the rpm range where more power is suddenly created using the same engine and same gasoline. What causes this is the fact that the rate of fuel combustion and way the engine uses it are in perfect synchronicity--the engine is running at optimum performance. More power = more Point A to Point B motion using the same amount, or less, of fuel. So in theory a fuel that keeps you in the powerband longer will use less fuel, and therefore, create fewer emissions. Of course you'd need to work at using the powerband for the hard work, and this assumes that other, non-powerband rpm ranges weren't using the fuel extra poorly....
So if there's a fuel that you feel gives your bike more power, that'd generally be the one I'd assume uses less fuel for the same amount of miles, and makes fewer emissions. The bike is using the fuel more efficiently.
Here's my big boondoggle. In Minnesota where I live, they cut gasoline 15% with ethanol, which is supposed to burn cleaner. The problem is, when I use oxygenated fuel, my mileage goes down, yep you guessed it, 15%. So I have to use 15% more fuel to go the same distance...which would lead me to believe that I'm not creating any fewer emissions after all. But that's a controversial subject and lots of people have lots of different opinions.
Generally, I seek out 91 octane non-oxygenated fuel (non-ehtanol) for my bike, because it seems to make the most power and get the best mileage. If your bike likes 91 the best, in theory, that's the best for emissions, too.
Pat
