Most racing fuels have an initial boiling point of around 100 degrees and a final point of approximately 250-300 degrees. If a fuel’s maximum vaporization temperature is to high for a particular application, all of the fuel may not be vaporized in the combustion chamber leading to an incomplete reaction. If the initial temperature is to low, the engine may be prone to vapor lock. If the initial temperature is to high, the engine will be hard to start, although, this varies somewhat with application. A nitrous application for example, may not need a low initial boiling point because of the additional cooling from the nitrous. Turbo-charged or supercharged applications may require a higher vaporization temperature because of the additional heat in the intake tract. What’s important is that the distillation curve provides a lot of information that can be used to help determine the correct fuel for a given combination.
Reid Vapor Pressure
Another value usually included in fuel manufacturer’s literature is Reid Vapor Pressure (RVP). RVP is a measure of how much pressure will result from heating a sample of race gas inside of a confined space. This information is another way of determining the amount front ends are in a particular blend of race fuel. Pump gas typically has RVP’s in the 7-13 PSI range, depending on season. The RVP will be higher in the winter and lower in the summer. Racing fuels typically have RVP’s in the 5-8 PSI range and don’t change with the seasons like pump gas does. High RVP values suggest the fuels use many volatiles in the blend, which can cause an engine to be prone to vapor lock. Low RVP’s can make the engine hard to start in cold weather.
A fuel’s specific gravity is a measure of how much a volume of fuel weighs when compared with the same volume of a standard liquid. Water is the standard liquid for comparison to fuels. Therefore, if a fuel has a specific gravity of 0.750 it weighs ¾ as much as an identical quantity of water. Racing fuels can have a specific gravity from a low of about 0.660 through a high of about 0.780, which is a significant difference.
The tool used to measure specific gravity looks like a thermometer that partially floats when dropped into a volume of fuel. The specific gravity is read off graduations on the outside of the bulb. These tools are relatively inexpensive and can usually be purchased from well-stocked fuel suppliers or chemistry supply houses like Cole-Parmer.
Knowing the specific gravity of the fuel a racer is using can be important to help determine how to re-jet the carb when a fuel change is made. Since jets meter by volume, if a lighter fuel is used, it will usually require a larger jet to maintain the correct air/fuel ratio. This phenomenon can also be seen by measuring the effects of heat on the specific gravity of a fuel. As fuel becomes warmer the specific gravity decreases. Most racers understand the effects of hotter weather on jetting and a fuel’s specific gravity is another indicator of this relationship.
Another use for measuring specific gravity is to check for stale fuel. If race fuel is stored improperly it can become stale. Racing fuel should be stored in sealed metal containers in a cool location out of direct sunlight. If race fuel is stored properly, it can last indefinitely. However, if fuel is left in fuel jugs inside a racer’s trailer, where temperatures can reach in excess of 100 degrees, the front ends of the fuel can begin to vaporize. When the jug is opened, this vaporized fuel will vent to atmosphere and is lost. As the front ends go away, so does the fuels ability to react the heavy fractions and power will suffer. This loss of front ends will show up as a slight decrease in the fuels specific gravity and is another reason to test this aspect periodically.
A fuel’s heat content, measured in British Thermal Units (BTU) is another number sometimes thrown around in bench racing sessions. However, the reality is that there isn’t a significant difference in BTU content between various racing fuels. Most racing fuels have a BTU content of around 19,000 BTU per pound (surprisingly, this is about the same as pump gas as well). Since race fuels weigh approximately 6 pounds per gallon, this equates to around 114,000 BTU per gallon. Based on percentages, even an increase of 1000 BTU per pound is a relatively small increase in thermal content at the carb jet.
Another common misconception is that heavier fuels, those with a high specific gravity, have a higher BTU content. Again, this is not necessarily true. The chemists that brew racing fuel have ways of juggling a fuel’s specific gravity and BTU content to accomplish whatever goal they are after. Consider the BTU content of the fuels in Figure 1. C-14 is the lightest fuel, based on specific gravity, however it has the largest BTU content.
What is more important than BTU content in making horsepower, is the ability to release as much of the thermal energy as possible. This gets back to knowing how and when the fuel vaporizes relative to ignition and reaction.
Race Fuel Myths
A couple of the myths surrounding race fuel have already been discussed concerning what octane is and isn’t and its relationship to fuel burn rate. However, a couple more myths are worth discussing. Racing fuels are often compared by how they will affect an engine’s exhaust gas temperatures (EGT), suggesting that raising or lowering EGT’s is a good thing. Given that the BTU content of most racing fuels is not significantly different and that when reacted, they will burn at approximately the same temperature, it is very unlikely that a fuel by itself will have significant effect on EGT’s. An engine’s EGT is a function of many different factors; it’s basic design, ability to breath, ignition timing, and valve timing. Additionally, it takes a significant amount of dyno testing to determine what a particular engine’s ideal EGT should be. So to suggest one fuel is better because of its effect on EGT is gross oversimplification.
Another myth is that race gas can be doctored by home “chemists” to make significantly more power. Most racers have heard the old wives tale that a handful of mothballs thrown into the fuel will really wake up an engine. Unfortunately, it usually doesn’t work quite like that. While there are a few commercially available additives that do add horsepower, most come with either significant risks to a racer’s engine or personal health, or are expensive, or both. It’s important to remember that there are a whole bunch of well educated and talented people working for racing fuel manufacturers who are constantly looking for ways to improve their fuels. If there were a safe and reasonably priced additive that would create more power in a racing engine, they would be using it. Trying to doctor fuels at home is taking unnecessary risks. Leave this to the chemists and engineers working for the manufacturers.
How to Choose a Racing Fuel
So far, we covered the science side of racing fuels, but how does a racer choose a fuel for their car? Is racing fuel even required for all applications?
To answer the second question first, serious bracket racers concerned with every thousandth of a second in consistency should consider using race fuel. Even if a particular engine application could run on pump gas, the regional and seasonal variances in the blending of pump gas can lead to inconsistencies in engine performance. Pump gas purchased in a metropolitan area of California in the winter can be significantly different than the same brand purchased in the Midwest during the summer. One of the benefits of race gas is the consistency in the blends and that alone is a good enough reason to use it.
OK, we know we need to use race gas, so which one is best. There are several factors in deciding on a race fuel. First, if you don’t build your own engines, talk to your engine builder and get their recommendation. But don’t stop there. Also talk to the manufacturers of the fuels that are readily available in your area. Fuel manufacturers have tech-support people who know racing and can advise racers on which of their fuels will best meet their needs. They will want to know some things about your engine including compression ratio, whether it’s naturally aspirated or blown/turbo-charged and the type of racing you do (in case some non-drag racers are reading this article). Also be certain that the fuel is readily accessible in your area. Finding the perfect fuel and not being able to purchase it readily can be a significant problem.
Once a racer has decided on a fuel that works for their application, stick with it. Keep adequate reserves of the fuel (stored safely, of course) so that you don’t get caught short. Be careful about depending on trackside suppliers. Most that I have worked with are dependable, but being a trackside supplier is difficult and thankless work with little financial reward, so it’s not surprising when one quits. Additionally, the track owner may change suppliers of the track’s “official fuel” on relatively short notice. Be aware of these things so that you don’t get caught short of fuel on race day.