How Fuel Pressure Affects Engine Power Output

Increasing fuel pressure can increase the amount of fuel delivered to the injectors, which can lead to higher horsepower. However, it is important to note that simply increasing fuel pressure does not always result in more power. The relationship between fuel pressure and power is complex and depends on various factors, such as the type of engine, the design of the fuel injectors, and the ability of the fuel pump to keep up with the increased fuel demand. Additionally, increasing fuel pressure beyond certain thresholds can lead to inconsistent injector performance and potential engine damage. Therefore, while fuel pressure adjustments can be a tool for performance tuning, it is crucial to have a comprehensive understanding of the entire fuel system and its interactions with other components to make informed decisions.

Injector flow rate

The ideal injector flow rate depends on several factors, including the desired engine horsepower, brake-specific fuel consumption (BSFC), the motor's safe duty cycle, and the number of cylinders. By determining these factors, one can calculate the recommended injector flow rate using the formula: injector flow rate = horsepower x BSFC / # cylinders x safe duty cycle.

Brake-specific fuel consumption (BSFC) represents the amount of fuel consumed per horsepower produced per hour. It varies depending on the type of fuel and engine aspiration method (naturally aspirated, supercharged, or turbocharged). The safe duty cycle refers to the percentage of time the injector valve is open during each combustion cycle, typically ranging from 80% for stock engines to 90% for performance-modified engines.

It is important to note that using fuel injectors with an incorrect flow rate can lead to issues. Injectors that are too large will result in sluggish acceleration, while those that are too small will starve the engine of fuel, affecting its performance. Therefore, it is crucial to calculate the correct injector flow rate to ensure the engine receives the right amount of fuel for optimal performance.

Additionally, effective pressure, which is the pressure differential across the injector, plays a significant role in injector flow rate. When an engine is idling, the vacuum in the intake manifold pulls fuel out of the injectors, increasing the effective pressure. Conversely, in a supercharged or turbocharged vehicle, the pressure in the manifold pushes fuel back into the injector, reducing the effective fuel pressure. Understanding these dynamics is essential for properly setting up the fuel system and ensuring predictable fuelling behaviour.

Returnless vs return style fuel systems

Increasing fuel pressure alone does not increase engine power. However, increasing fuel pressure can be one part of a wider performance modification, which can include installing a higher-pressure fuel pump, thicker fuel lines, a better fuel filter, a fuel pressure regulator, a high-flow fuel rail, and larger-capacity injectors.

There are two generic types of fuel system setups: returnless and return style. A return-style fuel system can be thought of as an endless cycle. The fuel pump constantly pumps gas from the tank, through a regulator, to either the injectors or carburetor. The fuel pressure regulator then sends the excess fuel back to the tank. This is a simple mechanical process.

A returnless fuel system, on the other hand, uses an in-tank pump and regulator. A single fuel line exits the tank and travels to the engine. Pressure is controlled by a computer, which adjusts the pump speed or fuel injector rate to regulate the fuel pressure.

Return-style systems have the advantage of maintaining a constant effective fuel pressure, which can extend the range of fuel injectors and help them function at lower fuel demands. They also mitigate vapor lock and make engine tuning easier. However, they require fuel pumps to work all the time, which can shorten the life of the pump, and they can make your plumbing system more complicated.

Returnless systems are becoming more common in new vehicles, partly due to tightening federal environmental regulations. They are easier and cheaper to design and build a car around, and they reduce the amount of harmful evaporative emissions released from the vehicle. However, retrofitting a returnless system can be expensive, and replacing your OE returnless system may bring regulatory concerns depending on where you live.

Effective pressure

With a return system, the base pressure is set with the engine off but the pump running. For a GM, this pressure is usually set to 58psi (factory fuel pressure in the rail). The vacuum/boost-referenced regulator will help to change the pressure in the rail based on the pressure in the manifold. When an engine is idling, it may be pulling 20 inHg of vacuum, which translates to roughly 10psi. The reference to the regulator will allow it to adjust and lower the pressure in the rail to 48psi, resulting in 58psi effective pressure, the same as the base pressure. When the engine is making 10psi boost, the regulator will adjust and increase rail pressure to 68psi, again resulting in 58psi of effective pressure. The regulator will constantly bleed off pressure inside the rail to maintain the same effective pressure at all operating conditions.

A return style system that isn’t variable will maintain a certain pressure inside the rail, regardless of what is happening in the manifold. For instance, take a GM system with the standard 58psi in the rail. No matter what operating condition, pressure in the rail will always be 58psi. When idling at 20 inHg, this means effective pressure will rise to 68psi because the vacuum in the manifold is adding 10psi to the rail pressure. This requires injectors to pulse shorter so as not to overfuel the engine and cause a rich condition. By contrast, when a naturally aspirated engine is wide open throttle, the manifold pressure is not in a vacuum or in boost, so the effective pressure is the 58psi of rail pressure and nothing more. However, a boosted engine at 10psi will be resisting the fuel, causing effective pressure to drop to 48psi from the 58psi in the rail. This lowers the ultimate output of the injectors.

Fuel pump capability

The fuel pump is a critical component of any vehicle, responsible for creating positive pressure in the fuel lines and delivering gasoline to the engine. When it comes to fuel pump capability, there are several factors to consider:

Firstly, the fuel pump must be capable of supplying adequate fuel pressure to meet the engine's requirements. This is particularly important at full throttle and maximum RPM, where higher fuel pressure is necessary for optimal performance. To achieve this, the fuel pump must be large enough to supply more fuel than the maximum amount that the engine may demand. Additionally, when altering the fuel pressure regulator or using a rising rate fuel pressure regulator, it is essential to also increase the fuel flow rate or fuel flow volume of the fuel pump, as it needs to pump against higher pressures.

Secondly, the placement of the fuel pump plays a significant role in its capability. In modern cars, the fuel pump is typically located inside the fuel tank. This has several advantages, including reducing the risk of fire and minimizing the impact of gasoline vapors. By submerging the pump in the tank, it is kept cool and away from the engine, reducing the chances of fuel boiling.

Thirdly, the quality of the fuel pump is essential. It is recommended to purchase fuel pumps from authorized retailers to ensure their reliability and performance. High-quality fuel pumps can deliver a constant flow of gasoline to the engine, with any unused fuel returned to the tank. This not only enhances driving safety but also prolongs the life of the pump and allows for a smaller and lighter device to be used.

Furthermore, fuel pump capability can be enhanced by installing a larger fuel pump or a second fuel pump. A second fuel pump is particularly advantageous for modified street cars, as it can be wired separately to only operate when extra performance is required. This ensures that the fuel flow rate is optimized for both cruising and high-performance driving conditions, reducing the exposure of fuel to heat when cruising.

Lastly, it is important to consider the fuel pickup, especially in modified street race cars with steel fuel tanks. During hard cornering, heavy acceleration, or heavy braking, the fuel level in the tank can drop below the fuel pickup, causing a momentary drop in fuel pressure and engine performance. To address this, a secondary fuel pump can be used to supply a small fuel reservoir or swirl pot, ensuring that the primary fuel pumps always have an adequate fuel supply.

Fuel pressure regulators

There are two main types of fuel pressure regulators: return-style and non-return style. Return-style regulators control fuel pressure by allowing excess fuel to flow back into the tank, ensuring a consistent supply of fuel to the engine. This type of regulator is commonly found in vehicles with fuel injection systems and is known for its ability to maintain a constant effective fuel pressure. On the other hand, non-return style regulators maintain consistent fuel pressure without returning excess fuel to the tank.

Aftermarket fuel pressure regulators (FPRs) are popular among car enthusiasts who want to modify their vehicles for improved performance. However, it is important to note that simply increasing the fuel pressure from the regulator does not always translate to increased horsepower. In fact, the factory-installed original equipment manufacturer (OEM) FPRs are often sufficient for most vehicles, even with upgraded fuel pumps or larger injectors.

When considering an aftermarket FPR, it is crucial to prioritise quality over aesthetics. Cheap, non-branded FPRs with inconsistent fuel pressure can cause expensive engine damage. Reputable brands, such as Aeromotive, Turbosmart, AEM, and DeatschWerks, offer precision fuel pressure regulators with adjustable settings, ensuring the engine receives the exact amount of fuel required for optimal performance.

Frequently asked questions