The Juice
What is gasoline? A complex, volatile mixture of hundreds of liquid hydrocarbons, usually containing small amounts of additives and contaminants. Today's gasoline's are "dirty". While they reduce emissions, they leave combustion chamber deposits that can be murder on high-performance, two-cycle engines.
Engine deposits can do more than rob power - they can cause expensive engine damage. To make matters worse, today's fuels are more likely to leave serious engine deposits than ever before. As "pure" base crude oil is becoming harder to find, we're getting lower quality gasoline. So it makes more sense than ever before to protect your engine, first by education, then by applying your new knowledge.
Gasoline is a blend of literally hundreds of many different substances, including literally hundreds of hydrocarbons, which are only known to the blenders (and there's no way they'll tell you everything that's in there). So you're left at the mercy of your local station. And it's different wherever you go. Gasoline is blended in compliance with local laws and according to the season, so what I get in Illinois will be much different than that in Alaska, Florida, or Colorado. As of now, more than 33 states have adopted the mandatory use of oxygenated gas.
As a tuner, owner, or engine builder, you must know how to set up your engine's carburetion to make the most of what you're getting from the pump. The name of the game is to build a top end with the proper squish velocities, spot-on compression ratios, and staying on the rich side of the air/fuel mixture. Such precision work is pretty much restricted to professionals. You probably won't find that type of service at your local discount parts house. Remember, talk is cheap, but mistakes are always hard on your wallet. It's a rather difficult to notice any significant changes in your automobile engine's performance, but in two-stroke motors, like the one in your watercraft, changes may become very expensive.
Gasoline blends have been changing in recent years in response to concerns about pollution and resulting emission regulations. As gasoline has changed the amount of additives such as aromatics and oxygenates have increased. These additives are important for the engines in passenger cars, but they can have detrimental effects in two-strokes, especially in the moisture-infested world of marine applications, where the engines also happen to run much cooler than their four-stroke, big-bore brethren.
In two-cycle marine engines, increased aromatics such a toluene, xylene, and benzene leave more deposits in the combustion- chamber. Oxygenates, such as methanol, ethanol, and methyltertiary butyl ether (MTBE), tend to lean out the fuel/air ratio because they release oxygen as they burn. Oxygenates blended with gasoline can affect many fuel properties, such as volatility, octane, heat of combustion, material compatibility, and deposit formation. A lean mixture burns hotter, again causing more carbon deposit build-up. When enough deposits collect, piston rings begin sticking, performance drops, and engine wear increases dramatically, eventually resulting in total cylinder failure.
We are reaching the state of the art in the proper selection of fuel and oils for our particular engine application, where it will be necessary to select not only the proper lubricants, but gasoline additives as well. In the not so distant future, I predict more engine related problems than ever before. Why? Because manufacturers are producing more power in stock watercraft, which demands higher octane, and that's where the problem begins. In the '80s, we used relatively low-compression engines, and they worked well with 87 octane (or even lower) gas. Today, the stock engines are approaching race performance. Where does this power come from? Bigger cylinder volumes, larger port time areas, longer stroke cranks, and higher compression ratios.
Most manufacturers specify 93 octane premium gas for use in their engines. But when you fill up at the premium pump, are you sure that this is what you're getting? Don't bet your life on it. A national study of samples taken across the country showed a large variation. For example, fuel labeled 93 octane was found to range from a low of 90 octane to a high of 94 octane. What to do?
Only by knowledge and selection of the best possible product and detailed work in our engine building and upkeep can we prevent as many problems as possible. We must factor in all the various aspects of our engines (proper compression ratio, head chamber design, squish thickness, squish dome areas, proper squish velocity, ignition spark control, ride plate, intake grate, impeller, and even the size of our rider), and never use a fuel with a lower octane than necessary. Especially when any (and I mean ANY) modifications have been made to the machine.
When my engine is designed to work on 93 octane gas, I always play it safe by adding one gallon of 100 octane race fuel to five gallons of 93 octane. In an all-out race motor' if 116 octane race gas is available, I will program my compression to work at two or three points lower than the fuel's specification. The most important thing is to find a reliable supplier and stick with him like glue, because this is the best insurance you can have.
As an engine builder, you will have to resort to testing your fuel sooner or later. It's possible for anyone to perform a limited but very important test which is affordable for even a one-man operation. (Special Note: These suggestions do not apply to each watercraft, but if you want to tinker with your engine you might want to follow them fairly closely.)
To determine the presence of alcohol in fuel, fill a 100cc measuring device (such as a syringe) with 50cc of water and 50cc of fuel. Shake it well, then let it settle. If the amount of water increases, say from 50cc to 60cc, that indicates your fuel is 10% alcohol. The second most important device in my shop is a gasoline specific gravity tester, which is a priceless device for comparing fuel for race engines. It's not an absolute authority for the proper selection of gas, but it will give you a pretty good idea when comparing fuels purchased at different locations. Another tool that I find quite handy is a Gas Check, which is designed to test fuel for automotive applications, but is handy for other uses. This gas analyzer will test for lead, water, or ethanol/methanol in the fuel, as well as testing the High Fuel Vapor Pressure, which to me is a very important factor.
When you use them in combination, these will give you a very valuable piece of information regarding the decision you make about which fuel to go with. Reid Vapor Pressure (RVP) is an important indicator which relates to a daily temperature range, and is adjusted with the season (such as summer or winter blend).
All liquids are disposed to vaporize, to become gas. The more volatile the liquid, the higher the liquid's vapor pressure at a specified temperature and the faster the vaporization. For a given temperature, therefore, the vapor pressure of a liquid is its volatility. This applies only to vapor pressure exerted by a liquid. Pressures exerted by vapor disassociated from the liquid are functions of volume as well as temperature, which covers a wide range of values.
When a petroleum product evaporates, the tendency is for more volatile fractions to be released first, leaving lower volatility elements behind. Reid Vapor Pressure is very significant for gasoline, which contains a portion of high-volatility fractions such as butane, pentane, etc. These fractions exert a major influence on vapor pressure. This is why it's hard to start an engine in cold weather - the cold lowers the pressure, lowering the volatility, making it very difficult to vaporize gasoline. Conversely, if the pressure gets too high, as it can in a too-hot engine compartment in the summer, you can be the victim of vapor lock, which is a partial vacuum that may exist at the suction end of a fuel pump, promoting lean runs and possible seizures in two-stroke motors.
Have you ever noticed that it's easier to start your engine if you've just filled the tank? That's why I try to run my craft close to empty at the end of each session, even if it's just going to be out of the water between weekends. And when I store fuel, I use steel containers and never premix the gas and oil. I use only freshly mixed fuel, one gallon at a time, so the unused fuel stays fresh and clean. This way, I always have a tank freshly filled with just-mixed fuel every time I go to start the engine. If you're a racer who's traveling, it's a good idea to bring your own fuel using containers designed for fuel transport.
Common sense dictates that in order to get big horsepower output we must use high-octane fuel and compress it to the highest possible level to get the biggest possible bang inside the cylinder, all the while managing to keep everything in one piece. That's the BIG challenge in the modern world of ever-changing petroleum products.
In my experience, Amoco 93 octane pump gas and Union 76 100 octane fuel is the best combination to date. I use these products with excellent results by mixing them at various ratios, running in power plants producing 60-120 hp.
It should be evident that you can't expect the same performance from gasoline that's blended at different locations. Once again, in four-stroke motors, you may never experience any difficulty, but two-strokes may suffer serious problems when they're asked to do the impossible, such as digesting the wrong petrol diet. Let's take a look at how sloppy dimensioning or miscalculation affiects-an engine's performance.
An engine's power delivery depends on several factors. First of all, the tuned pipe must be working with the exhaust port time area. The selection of gasoline octane and compression ratio must match. The most critical point relates to the squish thickness and the squish diameter area, as these two are responsible for more seizures than almost anything.
Let's take a hypothetical Yamaha 650cc motor. The reason it's so reliable is that it seems to have been designed so that it would run well if you put kerosene through it. It delivers 50 hp at 5500 rpm, at a very low compression ratio. The squish thickness varies from 1mm to 1.5mm between the cylinders, the transfer ports are mismatched inside the casting and sleeve as much as 2mm.Now let's go through the normal hop-up procedure as followed by many owners. The first step is to install an aftermarket exhaust system which gives about a 10 hp gain and widens the power band by around 500 rpm. An excellent choice. The next step is to whack 1-2
mm off the cylinder head for an increased compression ratio. In the case of the Yamaha, this is probably a step in the right direction, unless you get greedy and mill too much material off the head and render it physically useless.
However, if the same principle is applied to a Kawasaki 750, you're in for a surprise. This engine has been designed with a relatively tight squish band from the beginning, and a move in the wrong direction will fail miserably, sooner or later. What I'm saying is that it is mandatory to know exactly what needs to be done before attempting any modifications. Okay, back to the Yamaha.
As you mill the head and maybe do some porting, you change the compression ratio, which in turn changes your engine's fuel requirements. At this point, I refer to the following chart to help me find the correct fuel for my adjusted compression ratio. (Note that the horsepower readings may vary, so hold on to your letters.)
What I want to emphasize here is that fuel selection is as important an aspect of engine building as any, and that it must be taken into account. As you get further and further away from stock motors into all-out race machines, the room for error vanishes. That's why we should respect those who know their business when it comes to building good, reliable equipment. And those who know their business will take into account the type of fuel available in your area, and integrate that into the work they do for you. There is no substitute for expertise, precision work, integrity, and honesty.
George Grabowski HPT Sport USA