Rajan Wilcox
Mechanical fuel injection (MFI) is a system that combines a throttle-controlled air valve with a fuel pump, which typically runs at half the speed of the engine. The size of the main bypass jet is a key factor in determining the overall fuel flow to the engine. A larger jet will reduce the richness of the fuel mixture, while a smaller jet will increase it. In racing applications, a high-speed bypass may be used to further shape the fuel curve, particularly when the engine is naturally aspirated. The high-speed bypass is typically activated when the engine reaches a certain RPM level, and it works in conjunction with the main bypass jet to control the fuel flow. While there is limited information on the 075 jet flow specifically, it is mentioned that a high-speed bypass jet size of around 0.070 to 0.075 inches in diameter is common in certain racing engine setups.
| Characteristics | Values |
|---|---|
| Main Bypass Jet | Controls overall fuel flow to the engine |
| High-Speed Bypass | Shapes the fuel curve |
| Jet Size | 0.070-0.075-inch diameter |
| Jet Restrictor | Controls the amount of fuel delivered to the engine |
| Barrel Valve | Controls the amount of fuel and reduces fuel flow at idle |
| Nozzle Venting Vents | Allow air to premix with fuel for better atomization |
| Mechanical Fuel Injection Setup Cost | A few hundred dollars to tens of thousands of dollars |
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Mechanical fuel injection (MFI) history
Mechanical fuel injection (MFI) has a long history in automotive racing and has been used in various formats, including drag racing, circle track racing, boat racing, and top-speed events. Its development dates back to the early days of automotive racing, and it is still utilised today.
MFI was first introduced in the 1940s, with pioneers like Stuart Hilborn of Hilborn Fuel Injection making significant contributions. In April 1948, Hilborn became the first driver to surpass 150 miles per hour at El Mirage Dry Lake, utilising a self-designed constant-flow mechanical fuel injector. This early development set the stage for the integration of MFI in racing and passenger vehicles.
During the 1950s, MFI gained traction in automotive applications. The Rochester Ramjet FI, designed by Zora Arkus-Duntov and John Dolza, debuted on the C1 Corvette and full-size Chevy sedans. This system featured a fuel meter, an air meter, and an intake manifold, ensuring a consistent fuel supply. Despite its relative simplicity compared to modern systems, the Rochester Ramjet FI was a groundbreaking engineering achievement.
In 1954, a Bosch mechanical direct-injection system was utilised in the Mercedes-Benz W196 Formula One racing car. The following year, Mercedes-Benz introduced the first four-stroke direct-injection petrol engine in the 300SL sports car, although lubrication issues prompted a switch to manifold injection. Lucas Industries also developed a fuel injection system, which was employed in Jaguar racing cars, leading to their success at the 1957 24 Hours of Le Mans.
The late 1950s witnessed the emergence of electronic fuel injection (EFI) systems. The American Bendix Electrojector, introduced in 1957, utilised analogue electronics for control but faced reliability issues. In 1958, the Chrysler 300D, DeSoto Adventurer, Dodge D-500, and Plymouth Fury became the first cars known to adopt the EFI system.
Over time, MFI technology evolved, with Bosch playing a significant role in its advancement. Bosch acquired the Electrojector patents and developed the D-Jetronic system, produced from 1967 to 1976. This was followed by the introduction of the K-Jetronic and L-Jetronic systems in 1974, offering continuous and pulsed fuel injection, respectively. The L-Jetronic system, in particular, gained widespread adoption in European cars during the 1970s and 1980s.
MFI continues to be relevant today, with applications in various racing formats. It is known for its compatibility with different fuels and its ability to provide instant throttle response in naturally aspirated engines, making it a preferred choice for racing enthusiasts.
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MFI components
Mechanical fuel injection (MFI) has been used in various racing formats, including drag racing, circle track racing, boat racing, and top-speed events. MFI components play a crucial role in the performance and efficiency of these racing vehicles.
One of the key MFI components is the fuel pump, which typically runs at half the engine speed. It pulls fuel from a vented fuel tank and delivers it through a barrel valve. This barrel valve, controlled by the position of the air valve, regulates the amount of fuel supplied. Tuning can be achieved through simple jetting changes, allowing control over the amount of fuel entering each cylinder.
MFI systems also utilize a main bypass circuit, which serves as a fuel return circuit. This circuit includes a jet restrictor that controls the fuel flow to the engine. By adjusting the size of the main bypass jet, the engine can be tuned to be either leaner or richer. This adjustment is crucial for maintaining the optimal air-to-fuel ratio without affecting the other engine nozzles.
High-speed bypass jets are another important component in MFI systems. These jets come into play when the engine reaches a certain speed, reducing the fuel supplied to the engine at higher RPM levels. They are particularly useful in maintaining performance when volumetric efficiency decreases. The combination of the high-speed bypass valve and jet restriction determines the fuel curve reduction as the engine speed increases.
Additionally, MFI systems use individual nozzles to control the fuel flow to each cylinder. These nozzles can be adjusted to fine-tune the fuel delivery, ensuring optimal performance. Port nozzles, often used in racing engines, are connected to a pressure poppet, providing more fuel pressure at low RPM for improved response.
The simplicity and adjustability of MFI systems make them well-suited for racing applications, especially with naturally aspirated or forced-induction engines. The ability to handle various fuel types, from gas to nitro blends, further contributes to their versatility.
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MFI cost
Mechanical fuel injection (MFI) setups can vary significantly in cost, ranging from a few hundred dollars for simple single-nozzle systems to systems that cost tens of thousands of dollars. The cost of an MFI setup depends on various factors, including the engine type, size, RPM range, compression ratio, cam, cylinder heads, racing type and class, fuel type, vehicle weight, and drivetrain specifications.
The cost of an MFI system can also be influenced by the type and quality of its components, such as the fuel pump, barrel valve, nozzles, and jets. For example, a Hilborn Fuel Injection system requires detailed information about the vehicle to assemble a compatible system with the appropriate throttle bore size, barrel valve, nozzles, pump size, main jet selection, and type and setting of bypass units.
The main bypass jet plays a crucial role in determining the overall fuel flow to the engine. It controls the fuel mixture by diverting excess fuel away from the engine and back to the fuel tank. The size of the main bypass jet can be adjusted to increase or decrease engine enrichment, which refers to the air/fuel ratio. A bigger jet reduces engine enrichment, resulting in a leaner engine, while a smaller jet increases enrichment, making the engine richer.
Additionally, high-speed bypass jets come into play at higher engine speeds, further reducing the fuel supplied to the engine at higher RPM levels. These jets are commonly used in racing applications, such as drag racing or sprint car engines, where they help optimize fuel delivery and engine performance. The size of the high-speed bypass jet is typically around 0.070 to 0.075 inches in diameter, and it works in conjunction with the valve to determine the fuel curve reduction as engine speed increases.
When considering the cost of MFI, it's important to factor in the potential need for replacement parts and maintenance. Components like the fuel pump, barrel valve, intake, and jets can be expensive to replace, and proper maintenance, such as periodically replacing the jet sealing O-ring, is necessary to ensure optimal performance.
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MFI setup
Mechanical fuel injection (MFI) has been used in various racing formats, including drag racing, circle track racing, boat racing, and top-speed events. It works with a throttle-controlled air valve and a fuel pump, delivering fuel through a barrel valve that controls the amount of fuel based on the air valve's position. Tuning can be achieved by changing the size of the main bypass jet, which controls the overall fuel flow to the engine. Increasing the size of the main bypass jet reduces engine enrichment, while decreasing its size increases enrichment.
The MFI setup also includes high-speed bypass jets that come into play at higher engine speeds, reducing the amount of fuel delivered to the engine as volumetric efficiency may decrease at these higher RPM levels. The combination of the valve flow restriction and a high-speed jet, typically around 0.070 inches in diameter, determines the fuel curve reduction as engine speed increases.
For a typical methanol drag or sprint car V-8 engine, the high-speed bypass flow is approximately equivalent to a 0.070 to 0.075-inch diameter jet size. If the bypass flow is too much, causing a curve that is too flat, a restrictor jet is added to reduce the high-speed bypass flow.
Overall, MFI provides instant throttle response, making it well-suited for racing applications, especially with naturally-aspirated or forced-induction engines.
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MFI sizing
Mechanical fuel injection (MFI) has been used in various racing formats, including drag racing, circle track racing, boat racing, and top-speed events. MFI sizing is a critical aspect of its performance and efficiency.
Additionally, MFI systems may include a high-speed bypass jet, which comes into play at higher engine speeds to further control fuel flow. This high-speed bypass jet works in conjunction with the main bypass jet to shape the fuel curve. The size of the high-speed jet can range from 0.070 to 0.075 inches in diameter, depending on the engine configuration and desired fuel restriction.
The sizing of the MFI components is crucial to achieving optimal engine performance. By adjusting the size of the jets and bypass pathways, the fuel flow can be fine-tuned to match the specific requirements of the engine and its intended use.
It is worth noting that while MFI sizing is important, the setup of the entire fuel system also plays a significant role in the overall performance. Factors such as fuel pump capacity, nozzle placement, and throttle control all contribute to the effectiveness of the mechanical fuel injection system.
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