Honda F1 V8 cutaway, courtesy American Honda.
or something similar, right? In fact, the truth of the matter is that all of those F1 engines ran with fuel octane in the 95-102 range as tested using the RON standard. This would be approximately 91-98 octane using the AKI method as is common here in the USA. Most F1 engines use closer to 96 RON, or approximately 94-octane using the AKI method. It may sound crazy, but there are other factors at play. More important than octane are the volatility profile of the fuel, the power components and the burn rate. And this is true for NASCAR, F1, or any other racing you care to name, including your own chosen form of motorsport that you participate in.
genous manner. Liquids don’t burn, so the fuel must be vaporized. The more complete the vaporization, the more efficient the engine (less fuel used per hp/hour, also known as lower Brake Specific Fuel Consumption, or BSFC). After spark ignition of the fuel, the spread of the flame must be almost instantaneous. Theoretically the fuel burn should be instantaneous to put the most work on the piston possible, but in reality this would cause detonation. In other words, we need a fuel that burns as quickly as possible without generating detonation or engine knock. (For more information about detonation and knock, click here to read the article). Choosing the right fuel and then tuning your engine for use with that fuel requires a little bit of knowledge. In the following you'll learn about Octane Rating, Burn Rate, Specific Gravity, Reid Vapor Pressure, Latent Heat of Evaporation, Heating/Energy Value, Consistency and a few other details. We'll begin with Octane Rating as that's the most commonly understood, and yet misunderstood, characteristic of fuel.
Gasoline is made up of various hydrocarbons. Octanes are a family of hydrocarbon that are typical components of gasoline fuel. The octane-rating of a fuel is a measure of how likely the fuel is to self-ignite. The higher the octane rating, the less likely the fuel is to self-combust. The octane rating was developed by chemist Russell Marker. He chose 2,2,4-trimethylpentane (what is commonly called octane) and n-heptane to be used as a reference standard to benchmark the tendency of gasoline to resist self-igniting. For his bench-marking, Russell Marker decided that 2,2,4-trimethylpentane (we’ll just call it octane from here on) would have a rating of “100”, while its test counterpart, n-heptane, would be “0” on the octane rating scale of 0-100. Fuels tested for octane rating would be compared to how susceptible they were to knocking compared to the mix of octane vs. heptane that produced a similar result. These days, the octane-rating of a fuel is measured using a special single-cylinder dyno-lab test engine as defined in the octane rating procedures set forth by the American Society for Testing and Materials (ASTM). The fuel tested is defined by comparison with the mixture of octane and heptane that would have the same anti-knocking capacity as the fuel under test, quite like Russell Marker showed us many years ago. The percentage, by volume, of octane in that mixture is the octane number of the fuel. For example, if the fuel being tested had the same knocking characteristics as a mixture of 95% octane and 5% heptane, that fuel would be said to have an octane rating of 95. If the fuel tested had the same knocking characteristics of a mixture of 87% octane and 13% heptane, that fuel would be said to have an octane rating of 87. Obviously a fuel may have a Research Octane Number (RON) greater than 100, because octane is not the most knock-resistant fuel component available. Racing fuel blends and alcohol fuels may have octane ratings significantly higher than 100, such as the 116 octane fuel you see used in some drag- and oval-racing cars.
To determine the octane rating of the fuel, two tests are used; one for Research Octane Number (RON) and another for Motor Octane Number (MON). The RON test results in a higher octane value than the MON test as the MON test runs the engine under higher load and speed ranges, thus putting the engine and fuel under greater stress. If you average the RON and MON results, you get an octane value called AKI (meaning Anti-Knock Index). It's often labeled (R+M)/2 at your local gas pump (here in the USA) to inform consumers that the indicated octane rating is the average of the RON and MON values. It's not uncommon to hear that MON is more important than RON because the MON test is performed under higher temperature and engine speed conditions as mentioned above, but then lab tests are never an exact test of the real world and laboratory test conditions can’t be expected to account for all conditions and scenarios, so it’s best to think of octane rating as an indicator, rather than as an absolute. This being the case, don’t allow yourself to get hung up on octane rating alone while being dismissive of the other qualities of the fuel in question.
One common misconception is that higher octane equals greater horsepower. This isn’t always the case as the octane-rating doesn’t indicate in any way the chemical components of the fuel tested nor the energy content of the fuel; only its octane-rating as compared to a reference standard. Switching to a higher octane fuel does not necessarily add more hydrocarbon content or oxygen, so the engine cannot necessarily develop more power. The only way that a higher octane fuel can be relied upon to produce more power from an engine is if the engine spark advance curve or map was calibrated for use with a low-octane fuel that forced less-than-ideal power output from the engine due to the need of limiting knock and detonation. If this is the case, then switching to a higher octane fuel will allow you to re-map the spark advance tables (or adjust the distributor spark curve) and take advantage of the greater knock-resistance of the higher-octane fuel. Many modern vehicles are calibrated for optimum performance using 91-octane fuel but can run perfectly fine using 87-octane as the ECU can adjust spark advance to suit the fuel by way of the knock sensors and associated calibration tables. However, if you pour a bunch of 98-octane fuel into the tank, it’s unlikely you’ll see any benefit from it unless you re-map the base ignition tables to suit the new fuel.
Higher octane fuels ignite less easily due to their higher activation energies, but this should not be confused with the fuel burn rate. A higher octane rated fuel may be necessary for use in a high compression ratio engine to prevent detonation, but this factor is independent of the burn rate of the fuel. A fuel with a lower burn rate (a slower-burning fuel) will require more spark timing advance and will result in a lower overall power output due to the lower efficiency of the engine/fuel combination. A fuel with a faster burn rate can be ignited later in the compression stroke of the engine and produce peak cylinder pressure at the ideal time relative to a slower burning fuel that must be ignited earlier and thus cause cylinder pressure rise that the piston must fight on its way up through the compression stroke. This requires (and thus robs) torque from the crankshaft that could be used to turn the wheels. Considering the above, the total power output potential of the engine is a function of the properties of the engine design and the fuel used. If you want more power from your engine, you can increase compression ratio and valve open-duration timing or increase boost of forced-induction engines, or add more nitrous. All of these typically, (but not necessarily!) require a fuel with a higher octane rating. A faster burn rate, however, is always welcome if you can adjust the ignition timing map to take advantage of the fuel used. Note however, that faster burning fuels are also “lighter” hydrocarbons and don’t contain the same energy content as heavier fuel blends. Some good examples of why octane is not the only fuel parameter to consider for your race engine can be found in top-level motorsports such as NASCAR and Formula-1. NASCAR uses a fuel called Green E15 which is a specially-blended racing fuel that contains 15% ethanol by weight. This fuel has an octane rating of 98 (AKI) and it’s used in engines that have a 12:1 compression ratio and turn at over 9,000 RPM. Formula 1 engines typically had compression ratios exceeding 18:1 and turned in excess of 19,000 RPM during the V10 era, and similarly during the V8 era. Current F1 engines have turbo V6 engines that turn over 11,000 RPM (technically the teams are allowed to run engines to 15,000 RPM, but fuel-flow limits do not necessarily allow this). “Common Knowledge” would lead you to believe that these engines need very high octane race gas,
Fuel: The engine burns it and it makes the wheels go round-and-round. Some folks just grab what they can at the local pump and try going as quickly down the track as possible. Others want to know more about race fuel versus pump gas, octane rating, burn rate, vapor number and a few other things. If that's you, well, you're in luck as that's what we're going to cover here.
Commonly available auto fuel comes in two basic forms; gasoline and alcohol. We’ll cover diesel fuel separately some other time as it’s not a common fuel for the performance industry, but is in fact, capable of high-performance. But back to our two fuels, gas and alcohol. The role of the fuel used is to expend the chemical energy it contains through combustion (by harnessing gas expansion due to heat) to act upon the piston and thus transfer this energy to the crankshaft and turn the flywheel. To do this as efficiently and effectively as possible, the fuel must burn very quickly and thus needs to be vaporized very rapidly in a homo-