Fuelish Myths

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Myths of all sorts arise from applying simple intuition to mysterious happenings. It’s no different with racers than it was in the days of the ancient Greeks. One of my favorite motorhead myths was the tale of a justly famed top tuner who, unable to get peak revs down Daytona’s back straight from his two-stroke twin, got in the van, drove out through the old tunnel, and bought 5 gallons of regular from the Union 76 station that used to be on the corner of Bill France Boulevard and 92.

The Mysterious Case of “Slow-Burning” Gasoline

Back in the paddock, he set aside the high-tech race gas he’d been running in the bike and mixed up a batch of good old regular. He was rewarded in the next practice with full performance.

Intuitive conclusion? This is racing. Right? And in racing, things have to move real fast. Therefore, regular gas must burn faster than race gas. That slow-burning race gas slowed his bike down, but fast-burning regular sped it back up.

The Truth About Gasoline Combustion Speed

A momentous discovery…except it’s not true. Flame speed in engines results mainly from mixture turbulence. By itself, a chemically correct mixture of gasoline and air burns at about one foot per second, not nearly fast enough to make internal combustion engines possible. But stir up that mixture with turbulence and combustion accelerates by a factor of between 50–250 times.

Related: What Gasoline Is And Isn’t, And Why Motorcyclists Should Care

If slow combustion wasn’t the problem, what was it? Gasoline is not a pure substance in which all molecules are identical, like distilled water or ethyl alcohol. Instead, gasoline is a cocktail of maybe 200 different molecular structures, all with differing levels of volatility, the latter describing their ability to evaporate. The most volatile part of the fuel is called its “front end” and consists mainly of fast-evaporating isopentane. Its function is to assist fuel droplets in evaporating by turning them into vapor as the droplets heat up on their way into the engine. This evaporation of isopentane inside the fuel droplets physically swells and tears them apart, exposing more fuel surface area to accelerate and if possible complete its evaporation. The result is easy ignition and normal running.

Intake air enters your engine carrying fuel in three forms:

1. Evaporated vapor
2. Unevaporated droplets of various sizes
3. “Wall wash”—a layer of liquid fuel that creeps along the inner walls of the intakes

When you try to start your lawn mower in the springtime, if it’s full of last year’s gas, the engine won’t fire because too little of that old fuel is evaporating to form vapor, and vapor is what the spark wants to ignite. The fuel’s front end has evaporated through the gas cap’s breather and flown south for the winter months ago like some chemical migratory waterfowl.

Understanding that, now it’s time to consider what I call “the butter effect.” When I do our grocery shopping, I have learned to avoid the fancy-name specialty butters and instead buy the plain-Jane store brand. Why? Because turnover of specialty butter is slow, and since it has often been in the display case for days, it has that metallic, refrigerator taste. But the store brand, which more people buy, turns over much faster and so is much fresher and better tasting.

Going back to our original example, the regular gas at that Union 76 station had the fastest turnover and was therefore sitting in the retailer’s underground tanks for the shortest time.

Race gas, despite its high octane number and good anti-knock behavior, loses some of its all-important front end each time the drum or pail is opened. The boys have unloaded the bikes and tools, so now it’s time to gas up and air up for first practice. Two men muscle the fuel drum (half-full and left over from last season, like our nasty old mower gas) onto the electric tailgate of the truck, where the Florida morning sun begins the process of heating it and its contents. Agitating the fuel by pouring and splashing causes even more evaporation. Now you open the bung to put in the pump. PSHHH! That is the sound of isopentane flying north now like a homing pigeon, departing forever and leaving the fuel in the drum just a little bit less volatile. You work the pump and fill a pail to transfer the gas to your racebike through your funnel/filter. Every instant that fuel is exposed to the atmosphere, more isopentane evaporates and is lost.

Reid Vapor Pressure and Volatility

There is even worse news: the race gas you buy may already have lost much of its front end in normal handling. A simple measure of fuel volatility is its Reid Vapor Pressure, or RVP (a value measured under specified conditions). Alert dyno operators, noticing unexplained variations in the performance of the many engines passing through their shops, have taken to measuring RVP. Instead of finding the value stated in the fuel’s descriptive literature, they quite often find less than 20 percent of the claimed RVP.

Related: VP Racing T4: It’s a gas

When I decided to research the distribution of pump fuels to commercial gas stations, I found that RVP loss is such a common problem that extra isopentane may be added to fuel at the depot and distribution level to correct for evaporation.

The result can easily be that the expensive race gas you buy in a 5-gallon can is less able to cold-start your engine, provides sluggish throttle response, and may because of incomplete evaporation at higher rpm not allow your engine to reach peak revs on any jetting or fuel mixture. Yet when you run to the nearest gas station and pick up 5 gallons of regular, the missing performance may return—as our tuner discovered.

Detonation and Octane Number

If your race engine is limited by detonation, meaning its compression ratio is so high that it may detonate on regular pump gas, the temptation is to feed it the very highest octane “turbo gas” available. Yet if your engine is not turbocharged, that approach can backfire, because the fuel blender has to use more of the heavier and less volatile fuel components to reach that high octane number. That’s not a problem in a turbocharged engine because the heat of compression through the turbo takes care of fuel evaporation. But in an atmo (non-turbo) engine, that heat source doesn’t exist. So your engine receives more fuel as unevaporated droplets and less as vapor, causing the problems of poor cold starting, sluggish throttle response, and what feels like high-rpm lean-out. Better therefore to switch to a higher volatility race gas that provides normal operation and adequate octane number to prevent detonation.

As long as that gas, like my butter purchase, is fresh.

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