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By Rick Sieman

What you pour in your gas tank is important, especially if you race a vintage bike. Older bikes used to run on better gas than we have in our pumps today.


Everyone complains about the new generation pump gasoline we burn in our cars, trucks and bikes. You know what! They're right! Take a gallon of leaded regular gas, for example. In the good old days, a gallon of decent regular leaded gas contained about two and one-half grams of tetraethyl lead. Today, you can't find leaded pump gas. Old-fashioned high-test (premium leaded) gas used to come with about 4.27 grams of lead per gallon. Hot stuff! So we now have to deal with a future of unleaded gasoline.

This wouldn't be so bad, except that the new gasolines suffer from more than just the removal of lead. Among other things, modern gas does the following:

* Builds up varnish and deposits much faster than "good gas."

* Won't tolerate high-compression engines.

* Stores poorly.

* Reacts poorly to the presence of oil, as in a two-stroke pre- mix situation.



The octane rating of a gasoline is the measure of the anti-knock quality of the fuel. Knock is just that...a hammering sound inside your engine.

When a charge of fuel is introduced into an engine and ignited, the flame spreads out from the spark plug to the cylinder wall. If this happens all at once, you get detonation, or knock. A steady, even, slow burn is desirable.

Octane (or an increase in the octane level) will not increase your horsepower, no matter what your buddy tells you. However, insufficient octane can cause an engine to lose horsepower. All the octane you need is what your engine demands to keep from detonating.


We got a lesson in how to figure out octane ratings from Bruce Conrad, owner of F & L Fuels and Lubricants.

Some theory is needed here. Without get ting into upper trig, you can assume that the higher the compression ratio of an engine, the higher its octane requirement. The mechanics behind compression ratios is simple, and creates problems for high-performance engines. The smaller the space you try to squeeze the burning fuel into, the more power you'll develop...all other things being equal.


The number you see on the gas pump is supposed to tell the octane reading of the gasoline. It's not quite as simple as that. Here's how it all adds up.

The Research, or "R" Method, of rating octane is done on a standard test engine in a lab. The air temperature is controlled on this to a strict 125 degrees Fahrenheit. Mild loads are put on the engine, just about the loads you might put on your family car under normal driving conditions. This is called an F1 test.

The next rating test is called the Motor Method. Here, another engine is used on a dyno, but the air is introduced into the inlet at a hot 300 degrees Fahrenheit. Heavy loads, similar to full-throttle acceleration and passing on uphills, are dialed into the engine. The engineer notes ping and detonation at certain load conditions and the octane rating is determined by its ability to control the detonation. This is called the F2 test.

The number you see on the face of your gas pump is known as the Road Octane rating and is a combined average of the Motor and the Research Methods. In fact, if you take a close look near the posted octane number, you'll more than likely see: M0+R=92 OCTANE, or whatever the actual octane rating is of that particular gas. For example, if the F1 test showed 94 octane and the F2 test indicated 86, then the Road Octane rating would be 90.


Bruce says that checking race gas with a specific gravity tester is the
mosst important thing you can do when choosing a race fuel.

For you folks who drive in Baja, here is the scoop. Pemex is the government-controlled gasoline, and it's not as bad as the stories would have you believe. However, the octane rating is in the low 80s and that's why most engines will ping and detonate badly when using it. In some parts of Baja, there's new high test gas available that's rated at 92 octane and it's quite good. Look for the red pumps.

Mexican gasoline feels very oily compared to United States stock. That's because its refining process is not as elaborate as ours and there are less light ends and volatilities than in the typical U.S. gas. Also, contamination and the presence of water is a very real problem.

If you buy Mexican gas in a busy city location, chances are it will be of decent quality, even though the octane rating is low. Buy your gas along the road at a small station, and you'll more than likely buy a fuel that's been stored for a considerable time and suffers from contamination. If you drive in Mexico, a good fuel filter is a must and it should be checked and cleaned regularly.

One last word of warning: the very low octane rating of Mexican gasoline can create a lean condition in an otherwise properly jetted carb. Consider going larger on the jetting, or even backing the ignition timing off if you must run this gas for any extended period of time in a high performance engine.


You've seen them on the shelves at your local bike shop and on the counter at Local Discount Auto Parts: cans of octane booster in every color of the rainbow, ranging in cost from a buck to five dollars.

Do they work? Sure! The good ones do. First, let's take a look at what a good octane booster will do to your gasoline. Please make note of that word "good," because there is a lot of trash out there on the market.

An ounce of an effective octane booster should raise your octane rating by 2-1/2 numbers. That is, if you start with 86- octane gasoline, one ounce of booster to a gallon will take it to 88-1/2 octane. One word of caution. Octane boosters decrease in effectiveness as their quantity increases. Just because one ounce gives you 2-1/2 numbers more, ten ounces will not give you 25 numbers. Here's a good scale of just about how decent octane boosters will work:

One ounce/per gallon ¼ 2-1/2 numbers increase

Two ounces/gallon ¼ 4-1/2 numbers increase

Three ounces/gallon ¼ 6 to 6 1/2 numbers increase

Four ounces/gallon ¼ 7 to 7 1/2 numbers increase

A good rule of thumb is that anything over three ounces per gallon is wasted. Some crazies dump in a whole can of booster, thinking they're going to get radical gains. Actually, they'll get a decrease in performance, as excessive ratios can cause the volatiles in the gas to deteriorate and excessive aromatics can make the engine run "heavy," or rich.


Important stuff: "You may not need octane boosters or race gas in your bike. If it runs without detonation on 92-octane pump gas, leave it alone."


There's no mystery here. Most octane boosters are nitrogen carriers. Common ingredients are methanol, isopropyl alcohol, naptha, xylene, toluene (tyline), benzine, hexane, nitro benzine and aniline. Available octane boosters do not have tetraethyl lead in them, as lead is very toxic.

There are lead substitutes for sale also, but very little substantiation and research is available on these.

Aniline is the best octane booster available and is the prime ingredient in the better boosters. Nitro benzine also works well, but is not as effective and costs a great deal. Aniline is made by Dow, and is a very poisonous liquid that can actually be absorbed into the skin on contact.

Nitro benzine smells like shoe polish and castor oil. Alone, it's not too effective, but used in conjunction with other additives, it can do a decent job. You'll also find it used as an additive in some of the more expensive castor bean oils for use in two-stroke engines.


No. This is a misconception. Octane in itself does not add power. However, an engine that's forced to run on fuel with a lower octane than what's needed will run hot, detonate, and eventually lose power. The proper octane level will let the engine run to its full potential, but won't transform it into something special.

When a combustion chamber gets a charge of fuel, the plug sparks, and the fuel is burned. With the right octane level, the burn is clean and even. With too little octane, the burn can be uneven and a hammering can result.



  • Will attack plastics, rubbers and some fiberglass.
  • Discolor and attack most paints.
  • Foam filters will deteriorate if cleaned in a booster-carrying gasoline. So will the glues holding the filter together.
  • Some oils are affected by octane boosters. Most normal oils are not bothered, but if you have any doubt about your favorite brand, check with the manufacturer, to play it safe. Two-stroke users have to pay particular attention to this.
  • Can make an engine run rich. Rejetting may be necessary.
  • Are toxic to the skin, and the fumes can make you sick.
  • Exposure to air can cause a 50-percent breakdown in effectiveness.
  • Ultraviolet rays - that's plain old sunlight- will make octane boosters deteriorate.
  • Will attack gas tank sealants and could plug up your entire fuel system if used together.
  • Cost a lot of money. NOW...THE GOOD SIDE Don't let all of that scare you. Octane boosters have a real place in the world today. Here are some of the good things they can do:
  • Better throttle response. You can actually feel it.
  • Gets better mileage. Also, you can often lean out the carb slightly when using octane boosters, which will give improved mileage.
  • Best performance possible from your engine, short of using race gas.
  • Reduce detonation and pinging.
  • Clean out deposits. A good booster will actually let the engine run cleaner and inhibit carbon build-up.
  • Acts as a gasoline stabilizer when the machine is left to sit for a period. Gas stores longer with a good octane booster in the fuel.
  • Lets you use whatever gas is available at the time.
  • A good booster doubles as an emulsifier and can keep small amounts of water in suspension. Fuel system condensation is a very real problem, especially when the machine sits for long periods of time between use.


    At most any race track in the world, you'll see big drums of race gas. At around four bucks a gallon, it can still be a bargain if you have a highly modified engine.

    The market is loaded with octane additives. Here's one rule of thumb: if the product comes in a clear or transparent bottle, don't even consider it. Ultra violet rays cause deterioration.

    There should be specific directions on the label, i.e., how much octane booster to use to how much gas. And, there should be a listing of how many numbers the octane will be raised per ounce of booster used to each gallon of gas. A good octane booster will raise a gallon of gas by two-and-a-half numbers with one ounce added. If the label isn't specific, don't bother with the product.

    Consider the cost per ounce. You can get a good octane booster to add 2-1/2 numbers per ounce per gallon for around 30 cents per ounce. Some of the cheaper products might not be as efficient as the more expensive ones.

    Make sure the label has a toxicity warning. If it isn't toxic, it isn't going to work. And if it is toxic and there's not a prominent warning, this borders on criminal negligence. Some of the better octane boosters are aniline, nitro benzine and toluene. Additives like acetone and sulfurs can be very corrosive.

    Better octane boosters also have metal deactivators in them. This lessens the corrosive action of the additive on brass. And, as you know, all of your jets are made of brass, as are a number of parts in the carb and fuel system. Traces of brass can destroy volatilities in the gas.


    Ideally, you should only use what you need to stop pinging and detonation. An engine makes the most horsepower and has the best throttle response when there's just a light trace of pinging under a maximum load situation.

    In air-cooled engines, play it safe and allow for heat build-up and potential detonation, as it does happen. A water-cooled engine is much more stable and can be jetted much leaner; a bit of pinging can be tolerated without harm to the engine.

    You're better off starting with the best pump gasoline you can buy and then adding small amounts of octane booster, rather than gettin g low-octane gas and adding a lot of booster. Most gas stations have available unleaded premium in the 90- to 92-octane range. An ounce or two of a good octane booster added to this should be more than enough to handle the demands of a stock engine running hard.


    Here's how the ORC staff goes about it. We normally run unleaded premium with the highest octane we can find. If there's a rattle in the engine under a load, we add a small amount of octane booster, as needed.

    In high-performance bikes, one ounce of octane booster per gallon of 92-octane unleaded premium works well for us. Some of the highly modified bikes demand three ounces per gallon. If the temperature is high, or the bike is raced, we like to add one gallon of real race gas to three gallons of pump gas.

    Naturally, top pro racers cannot afford to take any chances, and you'll see them using only pure racing gasoline. When your livelihood is on the line, you get rid of all the variables and only take chances on the track, not on what goes into your tank.