Racing and Trackday FuelsFrom TrackpediaWhile many people look at endless items when upgrading their car for on track performance they typically overlook a key element in engine performance. Fuels can make or break the way a car performs on a track day or racing weekend. This page will dispel some of the rumors about different fuels types and help you select the best one for your car. Race Fuel Overview
Fuel Terminology & DefinitionsOCTANEOctane is rated in Research Octane Numbers, (RON), Motor Octane Numbers, (MON) and Pump Octane Numbers (R+M÷2). Pump Octane numbers are what you see on the yellow decal on your local gasoline pump and represents the average of the (RON) and (MON). (RON) is generally the higher number and is the number many companies choose to use advertising their products, however (MON) is the octane number more relevant to racing as it is one of the factors that represents the fuel's ability to resist detonation. Other important factors related to a fuel's ability to make power and resist detonation and preignition are listed below including BURNING SPEED, ENERGY VALUE and COOLING EFFECT. SEE OCTANE REQUIREMENTS BELOW. Motor Octane (MON)(a) CFR Tested @ 900 rpms, timing is varied with compression ratio, fuel is preheated to 300 degrees Farenheit, intake air is preheated to 100 degrees Farenheit. R+M÷2 Octane (R+M/2)(a) This method of measurement consists of adding together both the MOTOR and RESEARCH Octane numbers and then dividing by two. Research Octane (RON)(a) CFR tested @ 600 RPM's, fixed timing at 13 degrees BTDC, fuel temp is not controlled and intake air is varied with the barometric pressure. Specific Gravity (SG)This is the Density of the fuel, relative to water, water being at 1.00. If race fuel is .750 (specific gravity) it would weigh ¾ the weight of water. If water at 60 degrees Farenheit weighs 8.125 pounds per gallon (ppg), then race fuel at .750 will weigh 6.09 (ppg). Do not confuse SG with viscosity! Viscosity may be independent of specific gravity. A lower viscosity fuel will flow more easily through an orifice (jet or injector) and a higher (SG) or viscosity will flow less. This is most commonly an issue when oil is added to the fuel, as with 2-stroke engines. Stoichiometric A/F RatioOften overlooked is that combustion is the oxidation of fuel with AIR (specifically oxygen, O2). The Stoichiometric A/F value is ratio of air mass to fuel mass necessary for the oxidation equation to balance. Since all racing fuels are a composition of numerous chemicals, and each chemical has its own A/F ratio, the specific blend you use will have a distinct A/F ratio. Do not assume that this value is 14.7, as has often been asserted. Since fuel metering is typically a function of volume, both the SG and A/F of the fuel (and to a lesser extent, viscosity) will affect the amount of hydrocarbon you are presenting to the engine, even if you have made no jetting or injection changes! Also keep in mind that a rich fuel mixture means you are providing a lower A/F ratio than chemically correct. The amount of enrichment can be approximated by (A/Fideal / A/Fsupply). Beware that lambda sensors that show values of A/F are almost always incorrect, unless they have compensated for the A/F of the fuel itself. Far better to use Lambda as an indicator. Lead Content (LC)Amount of Tetraethyl Lead in grams per gallon. When lead is added to fuel, the knock resistance (anti-knock value) is increased. Lead is also used to increase the octane to a higher number. Reid Vapor Pressure RVPThe tendency of the gasoline to evaporate. Specifically, it is the expected pressure rise in a sealed container when the fuel is raised from 40degF to 100degF. Too high of a RVP may cause the fuel to boil or evaporate in the pump, lines or carburetor at a given temperature which can lead to "vapor lock". Too low of a RVP and the fuel will not vaporize and could cause difficulty starting the engine when cold. Low RVP fuels may also vaporize inadequately at high engine speeds, resulting in combustion mis/malfires. Most Racing Fuels have an RVP in the range of 5 to 7. ColorSimply the physical appearance color of the fuel. The addition of 2 stroke oil may also significantly change the color. The color has nothing to do with performance characteristics. Dielectric Constant (DC)The electrical charge of the molecules within the fuel. These are approximated numbers of the fuel dielectric value when using a HDE G-01 Fuel Analyzer. A Digatron DT47 meter may also be used at times. The accuracy of the meter is subject to fuel temperature, and condition of the reference fluid (typically cyclohexane). When two stroke oil is added to the fuel the meter reading will be INCREASED, depending on the type and ratio of the two stroke oil. Castor-based oils move the meter more positively than modern mineral/synthetic oils. Fuel additives will also change meter reading from that of untreated fuel. Dielectric readings are becoming more controversial, as some sanctioning bodies have associated higher readings with dangerous/illegal substances. This is unfortunate, as many legitimate materials may raise the DC value. (For an experiment, look up the DC for water!) Now, with the addition of alcohols and other oxygenates in street fuels, DC values are higher than ever. A higher or lower DC value, by itself, has no bearing on the perfomance capability of a fuel in a given engine. Nonetheless, DC is used for quality control among responsible suppliers. ConsistencyIt is very important that the fuel you use maintains the purity and consistency which the refinery strives for. Try to purchase fuel that has been stored in sealed containers (steel drums or cans, NEVER plastic). Open or bulk fuel storage tanks may have a tendency to sweat and produce moisture, therefore changing the consistency or purity of the fuel. They are, by law, vented, and are subject to bacterial contamination at the HC/water line. If at all possible buy your fuel in a factory sealed drum. Though the cost is generally higher, the quality may well be worth it in the long run. You are playing russian roulette with your engine if you buy from a pump and hope everything will be OK. Burning SpeedBurning Speed is the speed at which fuel releases its energy. In a high speed internal combustion engine there is very little time (real time - not crank rotation timing) for the fuel to release its energy. Peak cylinder pressure should occur around 16-20 degrees ATDC. Fuel that is still burning past this point will contribute less efficiently to crankshaft torque, as the volume of the chamber has grown very swiftly by this point. Nearly all fuels burn at approximately 30m/s. For most well developed engines, this is approximately 60deg of crank angle. Burn speed is often confused with burn/ignition delay. The delay from ignition until the flame kernal is sufficiently large to sustain itself may be from 12-18 crankshaft degress. If a fuel evaporates very quickly, it will begin burning more readily at the spark plug, causing this delay to be shorter. THIS IS JUST LIKE ADVANCING THE TIMING. Thus, a racing fuel parodox: faster evaporating fuels may require less ignition advance, not more, to obtain best power. This flies in the face of the old adage about advancing the timing when using higher octane fuels. Energy ValueEnergy value is an expression of the potential energy in the fuel. This energy is measured in BTU's (British Thermal Units) per POUND, not per gallon. This data is important as Air Fuel (AF) ratio is measured by weight, not volume. Nearly all fuels have approximately 18-19,000BTU/lb. To be more precise, you really care about the BTU's per pound of Air consumed, as air supply is the limiting power factor in most engines. By dividing the BTU/lb of fuel and dividing it by the stoichiometric A/F ratio, you get the BTU/lb of air. Cooling EffectCooling effect of the fuel is related to the heat of vaporization. The higher the heat of vaporization, the better its effect on cooling the intake mixture for a denser charge. This is of some benefit in 4-stroke engines, but can be a large source of gain in 2-stroke engines. More important than charge density, for 2-strokes, is the cooling effect the fuel may have on the surface and underside of the piston. A fuel that cools insuffiently may permit unchecked piston growth, reulting in seizure. Knock - Detonation - PingIntense pressure within the cylinder due to inability of fuel to have a controlled burn. This could be caused by too low of an octane fuel for the application, poor combustion chamber/squish design, or incorrect timing procedures. The sound you hear is from the actual vibration of the cylinder walls or the fuel exploding micro seconds prior to timed ignition firing which may cause two intense high pressure waves to collide or clap together and the colliding energy wave produces the sound you hear. This is hard on an engine. It is just like taking a ball peen hammer to the pistons. Use of proper fuel and tuning techniques will control this situation. Note that running an engine lean will NOT cause it to detonate. Many performance engines are set too rich, with far too much ignition advance. This causes longer ignition delay (see above). Then, when the mixture is set less rich (though not yet "lean"), the ignition delay shortens appreciably, resulting in peak pressure far too early. This may result in detonation, if the octane is insufficient for these conditions. Pre - IgnitionPre-ignition is the premature ignition or lighting of the mixture in the cylinder. This condition takes place prior to the spark plug firing. It is usually caused by a deposit or object glowing in the combustion chamber. A overheated spark plug or incorrect tuning parameters can cause preignition. This condition occurs while the piston is traveling up during the compression cycle. Pre-ignition may cause the piston to attempt to change directions which can cause major mechanical damage within the engine. Pre-ignition is much different that detonation although they both can reak havock on your engine. Pre-igntion is a special case of auto-ignition. Auto-ignition occurs in many engines, though we do not realize it, because the auto-ignition point is after the spark plug firing point. Diesel engines depend upon auto-igntion. Ever found your spark ignition (SI) engines continues running even though the igntion is turned off? That's autoignition. Octane Requirements - Fuel NeedsThe required octane is one that provides a controlled burn throughout the power cycle of the engine. As noted above octane, is not the only factor within the fuel that controls the burn and there are many factors within the engine that affect the burn rate which then affect the amount of octane or burn control required. These factors include, but may not be limited to, engine compression, camshaft timing, ignition timing, combustion chamber design, spark plug location, valve adjustment, engine operating temperature, fuel mixture, and weather. Extremely high octane is not necessarily best for performance. The other characteristics dominate performance. It is better to error to the higher side of the octane instead of the lower. Normally a margin of safety is desired just in case the air gets really dense or the engine temperature climbs a little high to prevent detonation in these instances. Q&A
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