a little fun info on ocatne rating

The swami has been hearing a lot of nonsense around the gas pumps these days.  People are tanking up with the "good" stuff because the commercials imply that it's better for their engine. When the oil companies use superlatives like "Super", "Extra"  and "High"...well it must be better, right?  And of course they wouldn't be charging $0.10 - $0.20 more unless they were putting some really good stuff in there, right?  Sorry...NOT!

"High Octane" is not synonymous with "good" or "better", and does not mean that it is better for your engine!  And the chances are pretty good you don’t need high octane fuel in your scooter.

High-octane fuels only become necessary when your engine has a high compression ratio. It’s a very long and complicated story…that the swami will make short.

First important fact that you must accept:

All gasoline, regardless of its’ octane rating, have pretty much the same amount of energy per gallon. What!!! "Sacrilege" you say? Well, actually, some higher-octane fuels have a few LESS percent energy per gallon…so as not to argue over this small point, for the sake of this discussion we will all agree that the automotive gasoline that you buy at the pump, regardless of octane rating, has the same amount of potential energy.

Second important fact that you must accept:

Octane is NOT a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.

How can this be? If all of the above is true, how do we get more power out of high octane gasoline? We do, don’t we?

Well…yes we do. Here’s how:

But first you must understand "heat of compression". There is a 2,000 year old fire starting device that still amazes the swami. A length of bamboo was hollowed out leaving one end capped. A stick, about the same length as the bamboo, was whittled down until it fit snugly into the bamboo cylinder. A bit of dried grass or wood shavings were placed in the bottom of the bamboo cylinder and the snugly fitting stick was violently rammed down the bamboo tube. The heat generated from rapidly compressing the air in the tube was sufficient to ignite the tinder.

The same thing can happen in the cylinder of an engine. The piston, quickly squeezing the fuel/air mixture into a small space, can generate enough heat of compression to ignite the fuel well before the spark plug fires, with unpleasant results. If the fuel prematurely ignites while the piston is on its way up, the burning of the fuel, in conjunction with the rising piston, creates even more pressure, resulting in a violent explosion. This explosion is equivalent to hitting the top of the piston with a very large hammer. If you want to be able to see through the top of your piston, ignore those sounds that are usually called: "pre-ignition", "ping" or "engine knock". Trust me on this one; in his reckless youth, using this method, the swami turned a few pistons into paper weights.

What we really want is a very rapid burn of the fuel, not an explosion. And we want the burning of the fuel to take place while the piston is in a better position to convert this pressure into productive work, like on its way down. Think of this burning as a very fast "push" on the top of the piston. Despite the violent noises you hear from some exhaust systems, it really is a rapid push on the top of the piston making the crankshaft go around, not explosions.

So that we can ignite the fuel at exactly the right time with the spark plug, instead of from the heat of compression, they put stuff into gasoline to keep it from igniting prematurely. The more resistant the fuel is to ignition from the heat of compression, the higher its octane rating.

Are you with me so far?

Higher compression ratios = higher combustion chamber pressures = higher heat… and it is with these higher combustion chamber temperatures that the magic happens.

At higher temperatures the fuel is burned more efficiently.  So, while it’s true that the higher-octane fuel does not posses any more energy than low octane fuel, the increased octane allows the extraction of more of the potential energy that has always been there. Conversely, lower compression ration engines utilize a little less of the fuel energy potential (2-4% reduction) but there is also less heat generated in the combustion process.

So how do you know if you need high-octane fuel? The swami suggests you look in the owners’ manual! Manufacturers really do want you to get the maximum efficiency out of your engine. They do their best to give a good balance between horsepower and engine life. It’s in their best interests to do so.

There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs. The only benefit is increased profits to the oil companies that have cleverly convinced some of the public that their new "Super-Duper, Premium-High-Test, Clean-Burning, Used-By-Famous-Racing-Types-All-Around-The-World, Extra-Detergent-Laden-Keep-Your-Pipes-Clean, Extra-High-Octane" fuel is your engines’ best friend. The swami is telling you the truth, don’t listen to that talking cartoon car.

The swami hears people insisting that they got better mileage, better acceleration, and less dental plaque by switching to a high-octane fuel. The swami reminds these people that in every pharmacy is a special miracle pill that is often prescribed by doctors, it works wonders because people believe that it works wonders; it’s called a "placebo". The swami warns: never confuse faith with physics!

If you are getting pinging or knocking with what should be the correct octane for your engine, start by checking the ignition timing, also check that the spark plug is the correct heat range. For 2-strokes, check for excessive carbon build-up on the top of the piston, the carbon takes up space and increases the compression ratio.

If all is well and correct, and you still are getting knocking, then try the next higher octane. You won’t go faster, you won’t go farther, but you will prevent an unsightly hole in your piston.

This subject is a whole lot more complicated than the swami wants to bother with. If you are curious to know more, put some of these words into your search engine and enjoy the education:

Antiknock Index

Octane

Stoichiometric Combustion

Thermal Efficiency

Flame Front

Highest Useful Compression Ratio

Compression Ratio

Placebo