Regan Brown
Supercharging an engine is a cost-effective way to boost horsepower. A supercharger is a mechanical air compressor that increases the pressure of the air entering the engine beyond atmospheric pressure, which is 14.7 psi at sea level. This increase in pressure allows more fuel to be mixed with the air, resulting in more power. While superchargers were once common in car engines, they have largely been replaced by turbochargers in the 21st century due to higher fuel economy and power output. However, superchargers are still favoured in applications where throttle response is a key concern, such as drag racing. When it comes to fuel pressure, there are two main types of systems: return style and returnless. The pressure in the rail will usually be maintained at 58 psi, but this can vary depending on the operating conditions and the type of system used.
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Fuel pressure regulators
There are two common types of fuel pressure regulators: deadhead regulators and bypass regulators. Deadhead regulators have a valve that clamps down on the line to reduce pressure, and they work well for fuel pumps that develop less than 22 psi. Bypass regulators divert excess fuel to a secondary line that returns to the tank, allowing fuel to keep flowing and generating less heat in the line.
When setting up a fuel system for a supercharged application, it is important to consider the type of supercharger and how it influences the fuel system components. The blow-through design, for example, mounts the supercharger to the inlet side of the carburetor, which is a pressure-differential device. The simplest way to feed fuel to a supercharged engine is with a carburetor, and modern electronic fuel injection has made tuning easier.
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Fuel pump size
The fuel pump size for a supercharged car depends on several factors, including the engine size, desired fuel pressure, and the type of fuel injection system.
For a supercharged engine, the fuel pump must be able to deliver enough fuel at the required pressure to meet the engine's demands. The fuel pump's flow rate is typically measured in litres per hour (LPH) and the fuel pressure is measured in pounds per square inch (PSI).
For example, if you have a supercharged engine with 19-pound injectors, you may require a fuel pump with a flow rate of around 255 LPH to provide sufficient fuel delivery. However, it's important to note that a larger fuel pump doesn't always mean better performance, as the fuel system also includes a return line that sends unused fuel back to the tank.
The stock fuel pump in some cars, such as certain Mustangs, can work effectively with lower-flow fuel pumps, such as a 155 or 190 LPH pump, without sacrificing performance.
Additionally, the type of fuel injection system used can impact the fuel pump size. Modern electronic fuel injection systems have made tuning a supercharged engine easier, but the traditional carburetor remains a simple and effective way to feed fuel to a supercharged engine.
When selecting a fuel pump, it's crucial to consider the voltage and amp draw. The flow rate of an electric fuel pump increases with voltage, and the amp draw increases with pressure. Most cars with a charging system and appropriate wiring can supply around 13.5 volts to the pump.
In conclusion, when determining the fuel pump size for a supercharged car, it's important to consider the engine's requirements, the desired fuel pressure, the type of fuel injection system, and the electrical specifications of the pump to ensure optimal performance and efficiency.
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Injector sizes
Determining the right injector size for a supercharged engine makes both the Brake Specific Fuel Consumption (BSFC) and duty cycle numbers doubly important to ensure the injectors are adequately sized. The duty cycle is the percentage of time that the injector is on during the engine cycle. The oft-mentioned 85 per cent duty cycle number is the absolute maximum that should be seen on an ECU duty cycle display at max power.
For a 900 hp engine, a 95 to 100 lbs/hr injector would be a good starting point. It is recommended to size injectors 20 to 30 per cent larger than you might think to allow for adding more power without requiring a large increase in fuel pressure.
For a supercharged engine, where cylinder heat increases with boost, tuning shops will often adjust the air-fuel ratio to 12.0:1 at full throttle. This is to achieve a safer tune, as opposed to around 12.8:1 on a naturally aspirated engine.
If larger injectors are installed, an engine tuning shop is usually required to retune the map (the fuel curve) to achieve the correct air-fuel ratio.
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Carburetor
Carburettors are a crucial component of a car's fuel delivery system, and their setup and maintenance are essential for optimal performance, especially in supercharged engines. Here's what you need to know about carburettors in supercharged cars and the desired fuel pressure:
Carburettor Basics
The carburettor is a device that mixes air and fuel in the correct ratio to ensure the engine runs smoothly. It is a key part of the fuel delivery system, which starts with the fuel tank. Proper ventilation of the fuel tank is crucial to the carburettor's function, as it replaces the volume of fuel consumed by the engine. Without adequate ventilation, negative pressure can build up, affecting the pump's ability to deliver fuel and potentially damaging pistons.
Fuel Pressure Considerations
The desired fuel pressure for a carburettor in a supercharged engine depends on various factors. If the supercharger is before the carburettor, the fuel pressure must be higher than the boost level to ensure adequate fuel delivery. For example, if your carburettor is set to 3-4 PSI fuel pressure and you're running 10 lbs of boost, you would need around 13-14 PSI fuel pressure at the carburettor.
Performance and Adjustments
The performance of a supercharged engine with a carburettor can be influenced by several factors. Firstly, the angle of the pressure hat on the carburettor can make a significant difference in power output. Additionally, during initial start-up, a slightly rich condition can help prevent engine overheating. Adjustments to the carburettor should be made after start-up by checking the spark plugs for a medium to dark tan colour and then fine-tuning accordingly.
Efficiency and Alternatives
Supercharging a carburettor can lead to increased engine efficiency and power. By cramming more air into the engine, supercharging increases cylinder pressure, resulting in higher efficiency and performance. While modern electronic fuel injection systems have simplified tuning supercharged engines, the carburettor remains a cost-effective and straightforward way to feed fuel to a supercharged engine.
In conclusion, when considering the desired fuel pressure for a carburettor in a supercharged car, factors such as supercharger placement, boost levels, and engine performance come into play. Proper carburettor setup and adjustments are key to achieving optimal fuel pressure and overall engine performance in supercharged applications.
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Fuel type
Superchargers are a form of forced induction that is mechanically powered, usually by a belt from the engine's crankshaft. They are a type of mechanical air compressor that pressurises the air going into the engine's intake manifold. This forces more air into the engine, producing more power for a given displacement.
The type of fuel used in a supercharged car is important as it needs to be able to withstand the higher pressure and temperatures created in the engine. This is influenced by the engine's compression ratio and combustion-chamber design. A higher ratio extracts more power during the expansion stroke, but it also creates higher cylinder pressures and temperatures, which can cause the engine to knock.
Fuels with a higher octane rating are better able to resist auto-ignition and detonation, which can damage the engine. Higher-octane fuels are often used in high-performance engines, such as those found in aviation, to prevent this. For example, the development of 100-octane aviation fuel in the 1930s enabled the use of higher boost pressures and increased power output in aircraft engines.
In a supercharged car, using a fuel with insufficient octane can cause the engine to knock. To prevent this, the ignition timing can be retarded (delayed) or the boost pressure can be reduced. However, using a higher-octane fuel than is required may not always improve performance and can be more expensive. It is important to refer to the manufacturer's recommendations for the correct fuel octane rating to use in a supercharged car.
Some examples of supercharged production car engines that use premium fuel include the BMW M3 straight-six, Saab 9-5 2.3-liter turbo, and the Jaguar AJ-V8 supercharged V8.
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Frequently asked questions
A supercharger is a mechanical air compressor that pressurizes the air going into the engine’s intake manifold. It is a form of forced induction that is mechanically powered, usually by a belt or chain connected to the engine.
A supercharger compresses the intake gas, forcing more air into the engine, which can then mix in more fuel, resulting in more power.
Fuel pressure depends on the operating conditions of your car. However, the factory fuel pressure in the rail for a GM system is usually 58 psi. Atmospheric pressure at sea level is 14.7 psi, so a typical supercharger providing an increase of 6-9 psi boosts the engine's air consumption by about 50%.
Superchargers offer an instant boost in horsepower and are a cost-effective way to improve the performance of an older car.
