About Gasolines and Octanes for Vintage Two-Strokes


For those that like the most detailed historical account on gasolines and octanes, we strongly recommend the two following documents from Wikipedia.

http://en.wikipedia.org/wiki/Gasoline         http://en.wikipedia.org/wiki/Octane_rating

   The vintage two-strokes that we work on at Klemm Vintage are primarily from the late1960s to late1980s genre.    During the late 1960’s and early 1970’s, premium pump gas octanes of 98-102 were commonly available everywhere at name brand gas stations,  During the later 1970s, premium octane ratings came down to the middle 95-98, and by the middle 80’s name brand premium came down to 94-95 octane.

Most high performance vintage two strokes were built to run on the premium octane fuel of the day.  However those octane numbers are long gone.  In addition, today’s pump gas is also laden with varying percentages of ethanol and oxygenates that did not exist back in the day.  The result is that any vintage two stroke (being ridden aggressively) needs to be modified to live happily with the increased detonation risks of today’s pump gas … or be mechanically prepared to operate reliably on a well thought out mix of “higher octane” options. 

  It bears noting that today's gasoline blends are not an answer to the needs of the latest high performance road machinery. They are, instead, an answer to the emissions mandates put forward by the EPA. The EPA has no interest in accommodating the performance nor longevity needs of any fossil fuel engines...they are only interested in reduced emissions…. Period.  Getting reliable service from a vintage two stroke running today’s premium pump gas is doable … but does require some tuning considerations.

The Details of Combustion - To really understand the importance of different quality gasolines, it's necessary to take a detailed look at exactly what's happening during combustion.

  As the piston starts it's upward movement from bottom dead center, the fuel/air mixture is in the process of filling the cylinder and pushing out the 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 tuning setup that consistently maintains the fuel charge at the ideal level of instability for that crucial moment of ignition. Unfortunately, we do not live (or ride vintage two-strokes) in a perfect world. The temperatures in a high performance two-stroke combustion chamber vary greatly. These temperature variations can have a big impact on the instability of the fuel charge. It sometimes happens that combustion chamber temperatures get so high that a hot spot in the combustion chamber can 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 wants 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.

OCTANE. What is it...really - Gasoline octane is not a function of how much lead is present in the gasoline. The "octane rating" refers to the fuel's resistance to pre-igniting under very high temperature conditions. In the early 1900's chemists learned that they could control detonation and pre-ignition 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 of modern engines struggle to make engines more completely burn each charge, chemists have been given the job of increasing the "octane rating" of our gasolines while using less environmentally  toxic substances. This has been no easy task, however the end result has been “somewhat” affordable 91-93 octane retail unleaded fuels. Since two cycle pwc engines do not gain any side effect benefits from the lead in “old school” fuels, the absence of lead is no problem.

Knowing when You Need Higher Octane -  There is an abundance of owners of high-performance vintage two-strokes that want the reliable power afforded by the high octane fuel of the 70’s …. While running the 91 octane pump fuels of today …. And in large part, that is very doable.  For the average amateur vintage MX racer, or the two-stroke street “sport-rider” enthusiast, today’s premium pump gas can work very well, and offer acceptable detonation resistance.

  However, the “octane rules” change for riders that run their machines consistently at high rpms for more than 5 minutes at a time.  Long-term high-rpm operation induces what engine builders call a “heat soak”.  In short, a heat-soaked engine is generating excess heat faster than the fins (or cooling apparatus) are dissipating the heat away.   The heat-soaking is further increased if there is very little “throttle-off” time (as would be the case in road racing)  As the high-rpm operation gets longer, the heat-soaking gets worse, and the risk of engine-killing detonation increases.

 The best technical solutions to stave off this detonation from heat-soak are  A) Retard ignition timing, B) Reduce Compression, C) Richen high-speed jetting, or D) Increase fuel octane.  Since ignition retarding, compression reduction, and over rich jetting all reduce overall power …. Increasing octane looks pretty good.  Below are some good options on how to do that … and some options to avoid.

About Race Gas -  100oct- 120+ High octane racing fuels have been around for a long time.  Very sadly, they have become a lot more expensive and a lot harder to acquire in recent years.  In truth, not every high-rpm two-stroke needs “pure” race gas … only those that experience heat-soak.  As an example, very few drag race engines ever experience heat-soak because their “fully loaded” operation time is so short.  The same could be said for most vintage motocrossers and street bikes that have lots of throttle-off time, and spend very little time at “full-throttle / high-gear”.  The low level heat-soak of these engines can often run on 91-octane pump gas, or a mix of pump gas with race gas.

Race Gas “Mixes”  -  One very easy way to slightly increase the octane of pump gas is by mixing in percentages of high octane race gas.  The simple math of averaging works when figuring these mixed.  That is, mixing a gallon of 91-unleaded with a gallon of 101-oct race gas nets 96 octane fuel that has “great” detonation resistance.  A mix like this would be adequate for 90% of vintage MX racers and aggressive “sport level” street bike riders.  However expert level 125 MX riders, and AHRMA road racers experience a much higher level of heat-soak that does require a 100% load of 101+octane race fuels.  It bears noting that octanes over 115 have been shown to offer no better engine performance nor detonation resistance in a two-stroke application.

About Aviation Fuels - Aviation gasoline (or "av gas") is blended specifically for use in small aircraft. It's also commonly used by many high performance two-stroke owners because of it's high stated octane rating (usually 100oct) and the relatively low price compared to racing fuel.  But it’s importance to understand some details about Av-gas.

   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 gasolines are blended with no heavy molecule end. This makes a gallon of avgas weigh measurably 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 significant horsepower issue.  Running 100% Avgas in a vintage two-stroke is not a good idea because there is a measurable loss of power resulting from the absent “heavy molecule ends”

   All that said, for most vintage two-stroke applications, a 50/50 mix of 91-octane premium, is a mix that offers a big boost in octane, along with “enough” heavy molecule ends” to make good power.  We consider it a very functional and inexpensive way to greatly reduce temperatures, and improve detonation resistance (over straight 91 octane unleaded).  We would tend to recommend against it’s use in a sustained high rpm application (like road racing or pro-level 125s motocrossers).  However in any vintage two-stroke that is not being run “wide open all the time” … it is a fuel that works great.    It bears noting that some blends of av-gas will quickly separate from castor-based oils used in premix situations.  If you are per-mixing bean oil … av gas is not a good idea.

   About Octane Boosters - Octane booster additives cannot turn a gallon of average quality fuel into a gallon of racing quality fuel. These additives are essentially flame retardants. That is, they raise the octane rating of a fuel by making it resistant to burning ... not by improving the high temperature stability.  The end result is that they reduce detonation risk by reducing power output.  An octane booster can make 86 or 87 octane fuel into a 89-90 equivalent (from a detonation resistance standpoint).  However it can only raise 91 octane by one octane point or less.  Given all this, we strongly recommend against using any octane boosters in a high performance two-stroke.

About Ethanol as a Fuel – In the day of vintage two-strokes, there was no ethanol (or other oxygenates) blended into pump gas fuels.  Today’s pump gas comes with a varying percentage of ethanol blended in as an oxygenate.  The addition of ethanol itself is an small annoyance, but not a big problem.  What is a big problem is the varying percentages of ethanol across the country, and from one retailer to the next.  While the fuel pumps “state” a certain percentage, in truth, that percentage can vary a lot.  Since ethanol has a much lower specific gravity than gasoline, it make jetting slightly leaner. 

 Ethanol (aka Alcohol) is a fuel that many racers have some experience with. 100% methanol used to be permitted in some forms of dirt track racing and is still employed by many go cart racers. It's use was abolished in many forms of racing because it's flames are virtually invisible. This very dangerous quality makes for obvious safety problems.

Inside the engine, 100% methanol burns very clean and yields very low operating temperatures. Unfortunately it's low specific gravity means that the jetting in the carburetors had to be doubled. Jetting was very difficult because so few carbs could pass fuel at double the rate in all circuit ranges.  In addition, ethanol is “very” unfriendly to non-metal fuel system parts and rubber crank seals.  The final death blow for methanol use in most forms of racing was that burning twice as much fuel means that you can't cover much distance on a tank of gas. It is strictly a "short distance" fuel.

Ethanol Percentile Variations - In automobiles (and vintage two-strokes) a 10% content of methanol is definitely enough to make the carburetion noticeably leaner. Correcting for this lean condition is not difficult. However there are fuel refineries across the country that will occasionally blend in bigger percentages of the inexpensive and very plentiful methanol. It is said that some of these blends can contain as much as 15 to 20% methanol. Is this legal?...probably not. However in a world of discount gas stations located in outlying areas, a don't ask don't tell policy is in place when the price is right. If an unknowing motorist buys a load of 15+%  methanol fuel, his car starts a little harder and runs a little lazier. This same load of fuel, in a high output modified two-stroke, can cause a lean condition with more serious results. How can you know the true ethanol content?...you can't.  The ethanol content of “brand-name” fuels tends to be much more consistent than discount retail gas stations.  This underlines the better wisdom to buy fuel for your high performance two-stroke at a name brand retailer.

Ethanol Solvency -  Another complication of methanol is the high solvency. That is, it has a lesser lubricicity than the fuel it is being blended with. This higher solvency literally washes internal engine surfaces that require a presence of lubrication. In 100% methanol race engines, this solvency is so profound that mixes between 15:1 and 20: 1 are commonly used to provide ample lubrication for high rpms (our old methanol half milers ran on 12:1 Klotz...they loved it). Most stock oil injection systems yield a 40:1 - 50:1 equivalent mix at full throttle. For normal use in a two-stroke street bike, this is probably okay. However,  we believe that this is questionably enough oil for extended high rpm use...particularly in 15%+ blends of ethanol may be at hand. In the case of a highly tuned two-stroke running on oxygenated pump gas, fuel/oil mixes between 40:1 and 32:1 should be used. If your oil injection cannot deliver that, the rest should be added to the fuel.

  The refined ethanol blended into today’s gasoline is actually 95% ethanol, and 5% water.  Refining out this 5% water is considered not cost effective.   This “included”  water passes through every part of your fuel system and engine.  In addition to that, ethanol “attracts” water from it’s surrounding atmosphere.  That is to say that it literally draws water out of the surrounding atmosphere into the fuel. This drawn-in water then enters the engine along with the fuel and oil. While the percentage of water content is low, it further contributes to the internal "washing" of surfaces needing lubrication. Blends that contain more methanol can attract more water.

Back in the day when 100% ethanol was used in racing two-strokes, it was standard practice for owners  of those race engines to purge out all the methanol after each race day by running it briefly on conventional premixed gasoline. Doing so assured that the methanol would not draw any damaging water content into the carb circuits and engine internals.

What About “E-85 Fuels” -  E-85 is 85% ethanol, and 15% gasoline.  For all the terrible things you can say about ethanol based fuel, current (federally subsidized) E-85 is very affordable and has an impressive 104 -108 octane.  A small number of two-stroke owners have converted their machines for E-85 use with great short term results.  The negative luggage (besides the fuel consumption rates), is that ethanol is very chemically unfriendly to non-metallic carb and fuel system components, and the moisture attracting qualities are nearly the same as running the admittedly impractical 100% methanol.  In an engine that was run for very short distances, and torn down often for inspection, it might make sense.  However for 99% of vintage two-stroke owners, it is not a very “user friendly” option.