Extracting this much power from such tiny, lightweight motors, requires great care. The following will help you to make the most intelligent fuel choices for your favorite propulsion.

Excerpted from Group K Technical Documents. See also Alcohol in Fuel,   Does AvGas Run Cooler?

Combustion Details

To really understand the importance of different quality gasoline’s, it’s necessary to take a trip with the piston.

We join the piston just as it starts moving upward from bottom dead center. The fuel/air mix pushes out previously fired exhaust gases. Still moving upward, the piston finally closes off the exhaust port to trap these fresh gases in the upper part of the cylinder bore. From this point on, each millimeter of upward motion increases the amount of pressure in the cylinder. Along with this pressure increase, comes a significant increase in the fuel/air mixture’s “instability.”

Instability refers to how volatile or how “ready to burn without actually burning” the mixture is. Under ideal circumstances, the spark of the plug should ignite this super unstable gas charge just a few milliseconds before it ignites on it’s own. By igniting the charge at this ideal moment, the combustion takes place with the maximum amount force in the minimum amount of time. That is, the flame front moves through the combustion chamber instantly in one big perfectly timed bang that drives the piston downward with the maximum amount of force.

All of this perfect mechanical wonderfulness hinges on an engine combination that consistently maintains the fuel charge at the ideal level of instability for that crucial moment of ignition. Unfortunately, we do not fly in a perfect world. Combustion chamber  temperatures vary greatly—variations that affect the instability of the fuel charge.

Pre-Ignition & Pinging

Combustion chamber temperatures can peak enough to cause a hot spot that will prematurely ignite the unstable fuel charge without the spark of the plug. This is known as “pre-ignition”. It also sometimes happens that the shock waves caused by the first milliseconds of ignition can detonate the very unstable “end gasses” at the outer diameter of the combustion chamber. This is called “detonation” (or pinging).

While pre-ignition and detonation are technically different problems, they are both promoted by the same problem. That problem is a gasoline that is ready to explode before the optimum moment. The problem for fuel chemists is to come up with a gasoline that resists pre-ignition and detonation, yet still burns instantly when ignited. Chemists have learned that they can accomplish this by “raising the octane rating” of the fuel.


Gasoline’s “octane rating” refers to the fuel’s resistance to pre-igniting under very high temperature conditions, not the amount of lead it contains. Early 1900’s chemists learned that they could accomplish this task easily and inexpensively by blending in varying amounts of “tetra-ethyl lead”. This additive is why high octane fuels were referred to as “Ethyl”. The lead in these gasoline’s not only acted as an octane rating enhancer, but (in four cycle engines) it also acted as a lubricant for valve stems and a cushion for valves seats. However, as we know today, the lead resulted in unacceptably toxic exhaust emissions. While engineers struggled to make engines more completely burn each charge, chemists have been given the job of increasing the “octane rating” of gasoline’s with less toxic substances. This has been no easy task, however the end result has been affordable 92 octane (and premium priced 105 octane) unleaded fuels. Since two cycle engines do not gain any side effect benefits from leaded fuel, the absence of lead is no problem.

PUMP GAS ENGINES…RACE GAS ENGINES What’s the difference? – In years past, high performance engines required race fuel. The engine builders of that time prepared engines with unnecessarily radical port layouts. This poor style of porting resulted in very high peak rpms, and very poor low end power. Compression was then hiked up to restore some of that lost low end. The result of these two wrongs was not a right…it was a piston eating engine. In fact, in many cases, even race gas couldn’t solve the terrible temperature problems that these engines had. Race gas just made the problem less noticeable.

During the middle 1980’s, Group K technicians were the first to develop personal water craft (pwc) engine kits that were “octane specific”. We quickly learned that low octane did not mean low performance. In time we found the few variables that permitted 92 octane engines to produce lots of usable power. We found that with a given octane fuel, reliable operation depended on the correct combination of three “operating temperature” variables. They are peak rpm, compression ratio, and ignition advance. Most stock engine formats could safely tolerate significant increases of any two, but not all three. In time we learned that the best results in performance and reliability were available by a well chosen balance of all three. With a balance of this kind, these pump gas engines were able to operate at nearly full output all the time, without any detonation or pre-ignition. However we also learned that any significant increase in just “one” of these variables would immediately create temperatures that required the need for cooler running race gas.

It bears noting that upgraded cooling systems on 92 octane engine formats could offer only slight decreases in excessive operating temperatures in this situation. The technically preferred solution is always to reduce the excessive heat at the source…the combustion chamber. At a certain point, no cooling system on earth can exchange away the excessive engine heat as rapidly as it is being generated.

Maintaining a “temperature balance” with a group of matched modifications permits an engine to produce much more overall horsepower with excellent long term reliability. That’s why all Group K modifications are designed and sold as packages or kits. Just about any mix of miscellaneous bolt on parts can result in a performance gain of some kind. However, an accidental wrong mix of incompatible parts and mods can result in a string of chronic temperature related engine failures. Group K 92 octane and 105 octane kits net the maximum performance and reliability because they are designed to approach all the limits of temperature without ever stepping over the “excessive heat” limit.

Aviation Fuel

Aviation gasoline (or “avgas”) is blended specifically for use in small aircraft. It’s also commonly used by many high performance engine owners because of it’s high stated octane rating (usually 100-110) and the relatively low price compared to racing fuel. Unfortunately this fuel is not all it appears to be. Avgas octane is rated on a different scale than gasoline’s intended for ground level use. What is 100 octane “av”, is not necessarily 100 octane “ground level”. Besides this, there is also a big chemical difference. Normal ground level race fuels are made up of gas molecules that have a “light end” and a “heavy end”. The light end of the molecule ignites easily and burns quickly with a low temperature flame (as a piece of thin newspaper would burn). The heavy end of the molecule is not so easily ignited, but it burns with a much more intense heat (as an oak log would). This heavy end of the gasoline molecule is responsible for the hotter, more powerful part of the combustion process.

Small aircraft are constructed as very weight conscious vehicles. That’s because their somewhat weak engines often have difficulty taking off with any extra weight. To help reduce this weight problem, aviation gasoline’s are blended with no heavy molecule end. This makes a gallon of avgas weigh substantially less than a gallon of ground level fuel. Since small plane engines turn very low rpms and produce so little power, the omission of the heavy end is not a horsepower issue. However, for high output pwc racing engines, there is defiantly a compromise in power. This, despite the fact that many pwc owners experience the desirable cooler operating temperatures that avgas offers. In addition, some blends of avgas will quickly separate from some oils used in premix situations. For the above reasons, we do not recommend the ongoing use of 100% avgas, and we will not prepare any “avgas” engine kits.

Despite all this bad news, running avgas (accepting the slight power loss) is usually a better choice than burning down a high output engine on regular pump gas. In this situation, the best choice is usually a 50/50 mix of pump and avgas. That provides “some” heavy molecule ends for the engine.

Marine Gas

The unusually high price of marina gas does not reflect some higher quality, rather its a function of the captive customers. The normally low grade of marina fuels are no better than regular auto fuel. Unless you have personal knowledge of the marina’s fuel, the Group K folks recommend against using it.

Octane Boosters

Octane booster additives cannot turn a gallon of average quality fuel into a gallon of racing quality fuel. They are essentially flame retardants. They raise the octane by making it resistant to burning not by improving the high temperature stability.

Pump gas, with an octane additive, can permit you to run a high performance engine without damaging it but with a noticeable power loss. Group K recommends the use of octane boosters only in very extreme or emergency situations. In those situations we recommend using as little as possible (no more than 3 oz. per gallon).

For more technical articles related to 2-stroke engines, please visit www.GroupK.com.