An engine’s power potential depends on how much air it breathes and the fuel it burns. Engine modifications such as larger bores and longer strokes add displacement, which increase airflow and power. Bigger valves, CNC ported heads and a hotter camshaft with increased lift and duration also increase airflow for more power as engine rpms go up.
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A big CFM carburetor or multiple carbs also allow more air and fuel into the engine. Need more power? A supercharger or turbocharger can stuff even more air and fuel into an engine. Just turn up the boost pressure and you can make even more power.
But regardless of what kind of mechanical modifications you make to an engine, or how much air and fuel you tray to cram into the cylinders, eventually the point is reached where the engine is maxed out in terms of how much horsepower it can make on pump gas. The limiting factors are the octane rating of the fuel and how much oxygen is required to burn the fuel.
For street performance applications, most premium grade unleaded pump gasoline carries an anti-knock octane rating of 91 to 93. In some areas you can get higher octane pump gas, but availability may be limited. Unleaded 93 pump gas is certainly better than regular unleaded 87 octane gas, but it isn’t high enough to handle the compression ratios or boost pressures commonly used in many racing engines.
Because of this, most naturally aspirated street engines are typically limited to a static compression ratio of 10-1/2 to 1 or maybe 11 to 1. With boosted engines, the limit may be 8 to 11 psi of boost with a static compression ratio of maybe 8 or 9 to 1. Intercooling and water injection help but there is a limit as to how much compression and boost an engine can safely handle without risking detonation on unleaded pump gas.
Understanding Octane Numbers
The octane rating of fuels can be confusing because of the different ways octane can be determined. Octane is a numerical rating of how well a fuel resists detonation and/or preignition under high temperature and pressure. The higher the octane rating, the better the fuel resists detonation and the more compression and/or boost pressure it can safely handle.
Octane ratings are determined three different ways. One method is the Research Octane Number (RON). The performance of the test fuel is compared to a standard base fuel (a mixture of isooctane and heptane) in a special single cylinder engine that has a variable compression ratio while running at idle speed (600 rpm).
Another method is the Motor Octane Number (MON), which is also done in a lab by running the test fuel in an engine at a higher temperature and rpm, while varying ignition timing. The MON number typically turns out to be about 8 to 10 point lower than the RON number due to the more demanding nature of the MON test.
The third method is the Pump Octane Number or Anti-Knock Index Rating (AKI). This is a numerical average of the RON plus MON octane ratings divided by two (R+M/2). The pump octane number is the one that is displayed on fuel dispensers at filling stations.
The highest octane number is the RON value, and the lowest is the MON value. So if a fuel supplier only quotes the higher RON octane number for a product, it may give the impression that the fuel has a higher octane rating than a competitor’s fuel. So when comparing the relative octane ratings of different fuels, look at the Pump Octane Numbers.
Gasoline is not a single chemical compound but a soup of up to 150 or more different hydrocarbons and other additives. The energy content and octane rating of gasoline will vary with the refining process and the final recipe of the fuel. The energy content of gasoline can therefore vary somewhat, but is generally around 125,000 BTUs (British Thermal Units) per gallon.
Some areas of the country have different fuel requirements such as various versions of “reformulated gasoline” to reduce air pollution. Fuel volatility and energy content can also vary with the season and geographic location. “Winter” gas typically contains a higher percentage of lighter hydrocarbons so it will evaporate more easily than “summer” gas for reliable cold weather starting.
The most often quoted air/fuel ratio for gasoline is 14.7 to 1, but air/fuel ratios of 12.5 to 12.8 to 1 typically produce the most power. But this can also vary somewhat depending on the fuel blend. That’s one reason why racing gas is a better alternative to pump gas for performance applications. Racing gas is blended to specific product specifications so its performance characteristics, energy content and tuning qualities are much more consistent than pump gas.
Leaded Racing Gas
Many racing fuels are leaded. Tetraethyl lead (TEL) is a great octane booster when added to gasoline. It only takes a tiny amount of TEL to boost the octane rating of gasoline substantially. As an added benefit, TEL helps lubricate hot exhaust valves to reduce valve and seat erosion and wear. But lead was phased out as a fuel additive for pump gas back in the 1970s and 1980s because it is a neurotoxin and it fouls catalytic converters and oxygen sensors. It was replaced with other octane boosters such as MMT, MTBE and ETBE.
More recently, the use of ethanol alcohol as a fuel additive has been growing not only to reduce our overall usage of gasoline but to also boost the octane rating of unleaded gasoline. Many premium octane pump gasolines currently contain up to 10% ethanol, and the EPA has recently approved blends of up to 15% (E15) for widespread use in 2001 and newer engines. Blends that contain up to 85% ethanol (E85) are also available for “Flex-Fuel” vehicles that can burn either gasoline or ethanol.
High octane leaded racing gasolines are available in various blends and octane ratings but for offroad and racing use only. Leaded fuels are not street legal. Yeah, some street racers may sneak some leaded racing gas into their fuel tanks to get an edge, but it is rather expensive fuel for everyday driving. And if they use more than a few tankfuls of leaded racing gas in a late model street vehicle, they would probably foul their oxygen sensors and catalytic converter. For street use, a street legal unleaded racing fuel would be required.
The main advantage of using a leaded racing fuel in an engine is that it allows the engine to safely handle more compression or boost pressure. If an engine is optimized for a high octane racing fuel by increasing the engine’s static compression ratio, it can make more power than it would on lower octane pump gas.
A leaded racing fuel rated at 107 pump octane can typically handle compression ratios in the 12 to 1 range. Run the engine on 110 pump octane leaded racing fuel and the compression ratio can be bumped up to 13 to 1. Use 112 pump octane racing fuel and the engine can be built with up to a 15 to 1 compression ratio. These compression ratios may be overly conservative for some engine builders, and we know people who successfully run even higher static compression ratios (up to 17 to 1).
A lot depends on the size of the combustion chamber (small versus large) and valve overlap and timing. Even so, the higher the static compression ratio, the greater the thermal efficiency of the engine and the more power it can squeeze out of every drop of fuel. That’s why diesel engines are so efficient compared to gasoline engines (and the fact that diesel fuel contains more BTUs per gallon than gasoline).
The speed at which a fuel burns also affects engine performance and power. The composition of the fuel will determine the rate at which it burns. According to the fuel suppliers we spoke with, fast burning fuels are not the best choice for blown motors or engines that are using an additional power adder such as Nitrous Oxide (NOx). A slower burning blend works best in these kinds of applications.
Another variable to consider is the Reid Vapor Pressure (RVP) rating of the fuel. This refers to the temperature and pressure at which the fuel evaporates. The more volatile the fuel, the easier it evaporates and the higher the RVP rating. But in a hot motor on a hot day, you want a lower RVP rating so the fuel doesn’t boil in the fuel line or carburetor and cause vapor lock. For this reason, some racing fuel suppliers blend their fuels to keep the RVP down to about 6 psi or less.
The best advice for choosing a particular racing fuel is to follow the fuel supplier’s recommendations. Products that meet very specific requirements are available for different types of racing, and the best results are usually obtained by using a specific racing fuel blend rather than a broader general purpose racing fuel.
Some leaded racing fuels are “oxygenated” as are most unleaded racing fuels. Oxygenating a fuel means adding compounds (typically methanol, ethanol, other alcohols or propylene oxide) that contain additional oxygen. More oxygen in the mix means more bang when the fuel ignites. Additives such as propylene oxide can increase power 7 to 8% with no changes in carburetor jetting.
Fuel suppliers say oxygenated fuels can typically make up to 3 to 7% more power than ordinary fuels. The higher the oxygen content of the fuel, the greater the potential power gains it can deliver.
One fuel supplier said you can estimate the gain in power by dividing the oxygen content of the fuel by 3. For example, if a racing fuel is 12% oxygenated by weight, it should produce about a 4% increase in horsepower over a non-oxygenated fuel with a similar octane rating.
One engine builder said he gained an additional 80 horsepower in a 1100 hp drag motor just by switching racing fuels.
Another oxygenate that can be mixed with gasoline to increase power is nitropropane. It is similar to nitromethane in that it contains its own oxygen to help the fuel blend produce more power. A carburetor does have to be rejetted richer as the percentage of nitropropane is increased to prevent detonation. A typical mixture is to use 10 to 20% nitropropane with gasoline to realize about a 5% gain in horsepower.
Methanol has long been a popular racing fuel because of its increased power potential. Methanol is widely available and can be made from natural gas, coal or other feedstocks. Its chemical formula is CH3OH. It contains oxygen and is relatively inexpensive as far as fuels go.
But it contains only about half the BTU content of gasoline (64,600 BTUs/gallon) so it requires a much richer air/fuel mixture and a higher flow capacity fuel system (larger carburetor jets, high volume fuel pump and/or higher flow fuel injectors). Methanol likes RICH fuel mixtures (from 4.5 to 1 to 6.0 to 1 for peak power) so it takes twice as much methanol to produce the same power as gasoline.
Methanol has a high octane rating: 129 RON, 103 MON, or a pump AKI rating of 113. Because of its higher octane rating (which allows more compression) and the added oxygen content, the overall power gain with methanol can be 10% to 20% or more over pump gas depending on the compression ratio and how the carburetor is jetted.
The purity of methanol can vary depending on where it is sourced and how it is handled so some suppliers certify the purity of their methanol racing fuel to guarantee consistent performance. Methanol is generally sold in sealed steel containers and should be kept in sealed steel containers for safety purposes. Methanol is corrosive so it requires stainless steel fuel lines and a fuel tank with a methanol-compatible bladder. Methanol is also toxic (it will kill you if you drink it) and produces nasty exhaust fumes.
Pure methanol burns with a clear flame, making fires difficult to detect when fuel is spilled in an engine compartment, a pit area or as a result of a crash. Blending gasoline with methanol (M85 which is 85% methanol and 15% gasoline) creates a safer fuel because the gasoline makes the flames visible.
Some say methanol may eventually be phased out or made illegal by the EPA due to its toxic nature. Others say the small amount of methanol fuel that is used for racing has no significant impact on air quality at or near race tracks, so there is no reason for the EPA to go after it. Even so, others say ethanol is a “greener” option to methanol and may eventually replace it as the alcohol of choice for racing venues that allow alcohol fuels.
Ethanol is the up-and-coming race fuel as far as some people are concerned. Indy cars and NASCAR are now using it, as are many circle tracks as an alternative to methanol or racing gasoline. Ethanol is promoted as the “good” alternative to these other fuels because it can be made from renewable feedstocks, it reduces our dependence on petroleum, and is relatively benign compared to gasoline or methanol as far as its health and environmental impacts are concerned. Critics say ethanol uses food crops that should be used for food rather than fuel. But higher crop prices help support American farmers.
Ethanol is mostly made from corn, though it can also be made from sugar beets, potatoes or just about any plant that contains sugars or starch. It is made by fermenting sugar with yeast, then cooking and distilling the mash to separate the ethanol from the water vapor.
Ethanol is the same kind of alcohol that goes into alcoholic beverages, so it isn’t inherently poisonous (in limited quantities). But you can’t drink ethanol racing fuel because federal law requires the fuel to be “denatured” with gasoline or other chemicals so it cannot be consumed.
As mentioned earlier in this article, pump gas that contains up to 10% ethanol (E10) is commonly available in many areas of the country, and E15 blends have recently been approved by the EPA for use in newer vehicles. E85 is another blend that is available for flex-fuel vehicles (but should NOT be used in ordinary vehicles because of the richer fuel mixture that E85 requires).
Pump E85 is commonly used as a “poor man’s” racing fuel, but the percentage of ethanol in pump E85 can vary quite a bit depending on the time of year and the blend provided by the distributor. For this reason, a number of companies have introduced E85 and E90 racing fuels that are blended to exact specifications. The packaged E85 race fuel costs more than pump E85 but is a far more consistent and reliable product. Depending on the other additives in the product, it may also deliver up to 5% more power over a typical pump E85.
Adding ethanol to gasoline boosts the fuel’s octane rating. The improvement depends on the blend and the octane rating of the base gasoline to which the ethanol is added. Straight ethanol has an octane rating of 130 (RON), 102 (MON) and a pump AKI rating of 116. Ethanol racing fuels, which are typically available as either E85 (85% ethanol) or E90 (90% ethanol) blends with gasoline, carry octane ratings of 110 to 116 depending on the other ingredients in the fuel.
Ethanol contains about 84,600 BTUs/gallon, which is about 27% less than gasoline, but about 30% more than methanol. Consequently, a race engine converted from methanol to ethanol typically uses less fuel (30 to 40% less) to make the same level of power. As an added benefit, the engine runs cooler and reduces the risk of overheating.
As a race fuel, ethanol is a clean burning fuel that leaves few deposits in the combustion chamber or on the pistons. It is also non-corrosive to metal but can attack some plastics so fuel system components have to be alcohol compatible.
Ethanol is harder to ignite than methanol or gasoline, so it takes a fairly robust ignition system and a hot spark for reliable ignition. Ethanol can also handle a few additional degrees of spark advance compared to gasoline because it burns slower than gasoline. Ethanol also likes colder spark plugs as a rule, and a somewhat wider spark gap (.040 inches or more).
The air/fuel mixture for maximum power with ethanol can be as rich as 6.5 to one, and engine builders who have had experience optimizing engines for ethanol say a compression ratio of 14.2 to 1 works best with E85 and E90 racing fuels.
Nitromethane is the top dog of all racing fuels. It powers funny cars and Top Fuel dragsters. What makes it king of the hill is the fact that it contains twice as much oxygen as methanol so it can be used with extremely rich air/fuel mixtures. It only takes about 1.7 pounds of air to burn one pound of nitromethane.
The chemical formula for nitromethane is CH3NO2. The high oxygen allows the carbon and hydrogen to burn with little or no additional oxygen needed. Nitro contains fewer BTUs per gallon than gasoline or methanol (only 47,000 BTUs/gallon), but because the air/fuel mixture can be run so rich, the total energy produced is far greater and can multiply an engine’s power output up to 2.5X or more depending on the fuel blend.
Nitromethane is usually mixed with methanol alcohol, and the resulting percentage of each fuel in the blend determines the fuel’s performance properties. More nitro means more power. Track rules often limit the amount of nitro that can be blended with methane in various classes.
Nitro has some drawbacks, however. One is that it is very expensive (up to $80 per gallon). Another is that it produces nitric acid in the exhaust so the driver and engine tuner have to wear a respirator for protection while the engine is running. The fuel also burns slower than gasoline, which means the fuel is still burning when it exits the exhaust pipes. This produces a lot of popping and flames but it can also be hard on the exhaust valves.