MGB Supercharger: The SU HIF 44 Carburettor Choice

The MGB, a quintessential British roadster, has long captivated enthusiasts with its timeless design and engaging driving experience. While many classic car owners seek to preserve originality, a growing number are embracing forced induction, specifically supercharging, to inject new life and performance into these beloved machines. Yet, when it comes to fuelling a supercharged MGB, Moss Motors made a fascinating choice: rather than opting for modern fuel injection, they decided to retain the classic appeal by utilising a highly tuned SU HIF 44 side-draft carburettor. This decision speaks volumes about balancing heritage with enhanced performance, presenting a unique engineering challenge that ultimately delivers a truly distinctive driving experience.

The Enduring Appeal of the MGB and Forced Induction

The MGB, produced from 1962 to 1980, is celebrated for its simplicity, robust mechanics, and accessible nature. For decades, owners have enjoyed its spirited, if not outright powerful, performance. However, in the quest for more exhilarating acceleration and greater torque, supercharging has emerged as a popular and effective upgrade. A supercharger forces more air into the engine, allowing for a significantly larger bang from each combustion cycle. This increase in air volume, however, demands a commensurate increase in fuel delivery – a task traditionally handled by carburettors in these vintage vehicles.

Understanding the SU HIF 44 Carburettor

The SU (Skinner-Union) carburettor is an iconic piece of British automotive engineering, known for its ingenious constant vacuum (or constant depression) principle. Unlike fixed-choke carburettors, the SU features a variable venturi, where a spring-loaded piston moves up and down in response to engine vacuum and airflow. Attached to this piston is a tapered needle that slides within a fixed jet. As the piston rises, it simultaneously increases the venturi area and exposes a wider part of the needle, allowing more fuel to flow. This clever design inherently provides a relatively consistent air-to-fuel ratio across varying engine speeds and loads.

The HIF (Horizontal Integrated Float) series, introduced in the late 1960s, was an evolution of earlier SU designs. The '44' in HIF 44 refers to the choke size (1.75 inches). Its key innovation was the integration of the float chamber directly beneath the main body, leading to a more compact unit and improved fuel level stability, especially during cornering. This side-draft configuration means the carburettor mounts horizontally, drawing air in from the side, a common layout in British sports cars like the MGB.

The Supercharger's Demands: Why the SU HIF 44 Excels (When Tuned)

Supercharging fundamentally alters an engine's breathing. With more air being forced into the cylinders, the engine requires a precise and often much richer fuel mixture to prevent detonation and ensure optimal power. One critical demand of a supercharged engine is the need for a large shot of fuel each time the throttle is opened rapidly. Without this immediate enrichment, the engine can momentarily run lean, leading to a dangerous condition known as a lean backfire. A lean backfire occurs when the air-fuel mixture is too lean to burn efficiently, causing it to ignite prematurely or even after the exhaust valve opens, often with damaging consequences to the exhaust system or even the supercharger itself.

While the SU HIF 44 doesn't have a conventional accelerator pump like many fixed-choke carburettors, its constant vacuum design, combined with its unique dashpot damping system, can provide this crucial enrichment. The dashpot, filled with SU dashpot oil, contains a damper that slows the upward movement of the piston. When the throttle is snapped open, the sudden increase in airflow momentarily outpaces the piston's rise. This momentary delay keeps the needle lower in the jet for a fraction of a second, effectively providing a richer mixture when the engine needs it most. This characteristic, when coupled with a specifically chosen needle profile designed for boosted applications, makes the HIF 44 surprisingly adept at handling the transient fuel demands of a supercharged engine.

Moss Motors' Engineering Philosophy: Vintage Flavour Meets Modern Boost

Moss Motors' decision to use the SU HIF 44 with their supercharger kit for the MGB was a deliberate one, rooted in a philosophy that prioritises maintaining the car's original character. While modern electronic fuel injection (EFI) systems offer unparalleled precision and adaptability, they fundamentally alter the under-bonnet aesthetics and the driving feel of a classic car. EFI systems typically involve numerous sensors, an Electronic Control Unit (ECU), a high-pressure fuel pump, and injectors, which can look out of place in a vintage engine bay and detract from the vintage flavour that MGB owners cherish.

By opting for a meticulously tuned SU HIF 44, Moss has provided a solution that delivers significant performance gains while preserving the mechanical simplicity and the tactile, analogue driving experience of the MGB. It's a testament to the versatility of the SU design and the skill of engineers who can adapt classic technology to modern performance demands. This choice appeals to purists who want more power but refuse to compromise on authenticity.

Carburettor vs. Modern Fuel Injection for Supercharged MGBs

The Art of Tuning: "Highly Tuned" for Supercharged Performance

When Moss Motors states their SU HIF 44 is "highly tuned" for the supercharger, it means significant calibration efforts have been undertaken to ensure optimal performance and reliability. This isn't just a stock carburettor bolted on; it's a meticulously set-up unit. Key aspects of this tuning include:

• Specific Needle Profile: The most critical component. The standard SU needle taper is designed for naturally aspirated engines. For a supercharged application, a custom or significantly modified needle profile is essential to provide a richer mixture across the engine's operating range, especially under boost. This ensures adequate fuel delivery at higher airflow rates to prevent lean conditions and detonation.
• Jet Size Selection: While the needle largely controls the mixture, the jet size must be appropriate to allow sufficient fuel flow.
• Dashpot Oil Viscosity: The oil in the dashpot plays a crucial role in damping the piston's movement. For a supercharged engine, the correct oil viscosity is vital to ensure the piston doesn't rise too quickly (leading to a lean condition on acceleration) or too slowly (leading to an overly rich condition or bogging down). This contributes significantly to the "large shot of fuel" effect.
• Float Level Adjustments: Maintaining the correct fuel level in the float chamber is paramount. If too low, the engine can starve for fuel under high demand; if too high, it can run rich or spill.
• Fuel Pump Selection: While not part of the carburettor itself, a supercharged engine requires a fuel pump capable of delivering a higher flow rate to keep up with the increased demand. The standard MGB pump is typically insufficient.

Achieving this level of precision often involves extensive testing on a rolling road (dynamometer). A dynamometer allows tuners to accurately measure horsepower and torque while monitoring air-fuel ratios across the entire RPM range, ensuring the engine runs safely and efficiently under boost.

Maintaining Your Supercharged SU HIF 44

Owning a supercharged MGB with an SU HIF 44 carburettor requires specific maintenance considerations to ensure its longevity and performance:

• Dashpot Oil: Regularly check and top up the dashpot oil (typically 20W-50 engine oil or specific SU dashpot oil). This is vital for the carburettor's proper operation, especially for transient enrichment. Low or incorrect oil can lead to hesitation or a lean backfire.
• Air Filter Maintenance: The supercharger demands a lot of clean air. Keep the air filter clean or replace it regularly to prevent restriction and protect the supercharger and engine.
• Fuel Quality: Always use high-octane premium unleaded fuel, as recommended by the supercharger kit manufacturer. Boosted engines are more susceptible to detonation with lower octane fuels.
• Vacuum Leaks: Regularly inspect all vacuum lines and manifold gaskets for leaks. Leaks can introduce unmetered air, leading to a lean mixture and unstable idle, which is particularly detrimental to a supercharged engine.
• Fuel System Integrity: Check fuel lines, connections, and the fuel pump for any signs of leaks or degradation. Ensure the fuel pressure is within the specified range.
• General Carburettor Service: Periodically, the carburettor may benefit from a thorough cleaning, inspection of the needle and jet, and adjustment of the float level.

Troubleshooting Common Issues

Even with a well-tuned SU HIF 44, issues can arise. Here are some common problems and potential causes:

• Rough Idle/Stalling: Could be due to vacuum leaks, incorrect idle mixture, sticky piston, or incorrect dashpot oil level.
• Hesitation on Acceleration: Often indicates a lean condition. Check dashpot oil level, ensure the piston moves freely, and verify the correct needle profile is installed and seated correctly. This is where the risk of a lean backfire is highest.
• Poor Fuel Economy/Rich Running: Could be caused by a sunken float, incorrect needle, worn jet, or an overly thick dashpot oil.
• Backfiring (through exhaust): Can indicate a rich mixture, ignition timing issues, or occasionally a lean condition in specific circumstances.
• Overheating: A lean-running engine can lead to excessive engine temperatures. Ensure the carburettor is delivering enough fuel, especially under boost.

Frequently Asked Questions (FAQs)

Is the SU HIF 44 reliable with a supercharger?

Absolutely, when it is correctly tuned for the supercharger and maintained properly. Moss Motors' kits are designed to ensure reliability, but regular checks and adherence to maintenance schedules are crucial.

Can I convert my standard MGB to use this supercharged carburettor setup?

The SU HIF 44 used in the Moss supercharger kit is part of a complete system. It's not a standalone carburettor upgrade but rather an integral component designed to work specifically with the supercharger, manifold, and other kit elements. Converting requires the entire supercharger system.

What type of fuel should I use in a supercharged MGB?

It is almost always recommended to use high-octane premium unleaded petrol (typically 97 RON or higher) in a supercharged MGB. The increased compression and cylinder pressures generated by forced induction make the engine more susceptible to detonation if lower octane fuel is used.

How often should I service the carburettor on a supercharged MGB?

Regular checks of the dashpot oil should be done every few fuel stops or with every oil change. A more comprehensive service, including checking float levels and cleaning, might be beneficial annually or every 5,000 miles, depending on usage. Always refer to the supercharger kit's specific recommendations.

What is the main advantage of using a carburettor over fuel injection for a supercharged MGB?

The primary advantage is the preservation of the MGB's vintage flavour and aesthetic. It allows for significant performance gains while maintaining a period-correct appearance under the bonnet and the mechanical, analogue feel of driving a classic car. It's a choice for enthusiasts who value heritage alongside performance.

Conclusion

Moss Motors' decision to pair their MGB supercharger kit with a highly tuned SU HIF 44 carburettor is a brilliant example of engineering ingenuity meeting classic car passion. It defies the conventional wisdom that modern performance demands modern fuel delivery systems, proving that with meticulous tuning and a deep understanding of the constant vacuum principle, a vintage carburettor can indeed meet the complex demands of forced induction. This setup delivers exhilarating performance, ensures the necessary large shot of fuel to prevent dangerous lean backfires, and most importantly, preserves the timeless vintage flavour that makes the MGB such an enduring icon. For those seeking boosted power without sacrificing authenticity, the SU HIF 44 supercharged MGB offers a truly unique and satisfying driving proposition.

If you want to read more articles similar to MGB Supercharger: The SU HIF 44 Carburettor Choice, you can visit the Automotive category.