Unravelling the McGee Injection System Mystery

In the intricate world of high-performance internal combustion engines, the method of fuel delivery is paramount. While modern vehicles predominantly rely on sophisticated electronic fuel injection, older or specialised racing applications often turn to more robust, albeit less refined, mechanical systems. One such system, the McGee injection setup, represents a fascinating chapter in automotive engineering. This article delves into the core mechanics of McGee injection, addressing common queries such as the number of injectors, and explores unique modifications found in a specific setup for a Repco crossflow head, as recently uncovered by an enthusiast.

Mechanical fuel injection systems, like those designed by McGee, operate on principles distinct from their electronic counterparts. Instead of electronic sensors and computer control, these systems rely on mechanical pumps, valves, and linkages to meter and deliver fuel. They are renowned for their simplicity, directness, and ruggedness, making them suitable for demanding environments like drag racing or speedway, where precise, real-time adjustments might be secondary to sheer fuel delivery capacity.

The Anatomy of a 'Normal' McGee Injection System

To truly appreciate the nuances of a modified McGee setup, it's essential to first understand the workings of a standard system. At its heart, a mechanical injection system is designed to provide a continuous flow of fuel to the engine's cylinders, with various bypass circuits controlling the amount of fuel actually delivered to the injectors. The primary components typically include:

• The Fuel Pump: This is a mechanically driven pump, often directly off the engine, responsible for supplying fuel under pressure. Its speed, and thus its output, is directly proportional to engine RPM.
• Primary Bypass: During cranking, when the engine RPM is very low, the primary bypass poppet valve remains shut, directing all available fuel to the injectors. This ensures the engine gets fuel to fire. Once the engine starts and the pump speeds up, the poppet opens, allowing excess fuel to be returned to the tank. This maintains a more consistent pressure at the injectors while preventing over-fuelling at higher RPMs.
• Barrel Valve: This component is the primary means of controlling fuel delivery from idle to wide-open throttle (WOT). As the throttle is opened, the barrel valve rotates, allowing more fuel to flow to the injectors. While effective for idle and WOT, its metering in the mid-range is often described as 'rough' due to its linear nature, which doesn't perfectly match engine fuel requirements across the entire RPM range.
• Secondary Bypass: This circuit comes into play during high-speed, off-throttle conditions, such as gear changes in drag racing or cornering in speedway. When the engine is spinning fast but the throttle is closed, the pump is still delivering a large volume of fuel. The secondary bypass opens to return this excess fuel to the tank, preventing the engine from being flooded or 'loaded up' with fuel.

These components work in concert to ensure the engine receives fuel, albeit with a relatively simplistic approach to metering compared to modern electronic systems. The system prioritises maximum fuel flow when needed and bypasses excess when not, rather than precisely tailoring every drop.

How Many Injectors Does a McGee Set Have?

The question of how many injectors a McGee set has is a common one, and the answer lies in the fundamental design principle of mechanical fuel injection for internal combustion engines. A 'set' of McGee injection, or any similar mechanical injection system, is designed to provide one fuel injector (often referred to as a 'nozzle') for each cylinder of the engine it is intended for. Therefore, the number of injectors directly corresponds to the engine's cylinder count.

For instance, if a McGee injection set is specified for a four-cylinder engine, it will typically include four injectors. For a six-cylinder engine, it would have six, and so on. The term 'set' implies a complete complement of components necessary to feed all cylinders. The enthusiast's discovery mentioned in the original post refers to a 'set of McGee injection to suit a Repco crossflow head'. While the exact number of cylinders for this specific Repco crossflow head is not stated in the provided information, it is understood that the injection set would provide one injector per cylinder of that engine. The additional 'bits' mentioned, such as 'nozzles', confirm the presence of multiple fuel delivery points, one for each cylinder.

Harv's Unique McGee Setup: Beyond the Standard

What makes the recently acquired McGee setup particularly interesting are the unique modifications that suggest it may have led 'a few different lives', potentially on a road-going vehicle rather than solely a race car. Two key deviations stand out:

The Vacuum-Operated Valve

Unlike a standard mechanical injection system, this setup includes an additional vacuum-operated valve. In a typical system, the barrel valve handles all fuel metering from idle to WOT, which, as noted, can be 'rough' in the mid-range. The vacuum valve introduces a new layer of control:

• Functionality: At idle and during cruise, when the engine produces a high vacuum signal, this valve opens. This provides additional fuel to the system. Conversely, when the throttle is opened wide (low or no vacuum), the valve shuts, and the system reverts to its standard injection operation.
• Purpose: This modification primarily serves to introduce a 'cruise circuit'. This is highly beneficial for a street car, where the engine spends significant time at part-throttle settings, where the barrel valve's imprecise metering is most noticeable. By providing additional fuel during cruise, it can smooth out performance and potentially improve drivability and fuel economy in a road-going context.
• Implications: The vacuum valve also allows for a larger primary bypass 'pill' (a metering orifice). A larger pill means more fuel can be bypassed at higher RPMs, leading to a leaner top-end mixture. This could be advantageous for fine-tuning performance, perhaps for a specific power band or to prevent rich conditions at high RPMs. This feature is less critical for dedicated drag or speedway applications, where vehicles are rarely at 'cruise' settings.

The Electric Solenoid Shutoff Valve

Another unusual feature of this particular McGee set is the presence of an electric solenoid as the shutoff valve. Most mechanical injection systems utilise a manual ball valve, often cable-operated, positioned before the barrel valve. This manual valve serves as a positive means to cut off fuel flow to the engine, preventing run-on or fuel syphoning into the engine when shut down.

• Deviation: The use of an electric solenoid for this function is a significant departure from the norm. An electric solenoid is an on/off valve actuated by an electrical signal.
• Suggestion of Use: This electric component strongly suggests that the injection set spent time on a road car. In a street vehicle, an electric shutoff can be integrated into the ignition system, allowing for a more convenient and automated engine shutdown compared to a manual lever or cable. It also provides an additional layer of security or remote control if desired. For a race car, a manual, robust shutoff is often preferred for its simplicity and directness.

Advantages and Disadvantages of Mechanical Fuel Injection

While fascinating, mechanical injection systems come with their own set of pros and cons, especially when compared to modern electronic systems.

As the table illustrates, while mechanical injection offers simplicity and robustness, it sacrifices the precision, efficiency, and adaptability that modern electronic systems provide. The modifications found in Harv's system, particularly the vacuum valve, represent attempts to bridge some of these gaps, making the system more amenable to street use by improving mid-range metering.

Frequently Asked Questions About McGee Injection

What is McGee injection?

McGee injection refers to a type of mechanical fuel injection system, primarily used in high-performance and racing applications. It delivers fuel to the engine's cylinders through mechanical means, using a driven pump, various valves, and orifices, rather than electronic control units and sensors.

How does mechanical injection differ from electronic fuel injection?

The fundamental difference lies in their control mechanisms. Mechanical injection uses physical components like cams, linkages, and bypass valves to meter fuel, making it robust but less precise. Electronic fuel injection uses an Engine Control Unit (ECU) with input from numerous sensors to precisely calculate and deliver the optimal amount of fuel, leading to better efficiency, emissions, and drivability.

Why would someone use an older system like McGee today?

Enthusiasts or racers might choose McGee injection for its historical authenticity, its robust and simple design which can be easier to troubleshoot mechanically, or its raw, direct performance characteristics suited for specific racing classes where electronic aids might be restricted.

What are the common challenges with McGee injection?

Common challenges include less precise fuel metering, especially at part-throttle, which can lead to rough idle or poor fuel economy. They can also be challenging to start when cold and are generally not designed for emissions compliance, making them unsuitable for many modern road vehicles without significant modification.

Is McGee injection suitable for a daily driver?

While modifications like the vacuum-operated valve can improve street manners, McGee injection is generally not ideal for a daily driver. Its primary design intent is high-performance, and it lacks the refinement, fuel efficiency, and emissions compliance expected of modern road vehicles. It requires more frequent tuning and is better suited for dedicated race cars or show vehicles.

Conclusion

The discovery of this particular McGee injection set, complete with its unusual vacuum-operated valve and electric solenoid shutoff, offers a fascinating glimpse into the evolution and adaptation of mechanical fuel injection systems. While the exact number of injectors in a McGee set is simply determined by the engine's cylinder count, the modifications highlight efforts to make these robust, albeit unrefined, systems more versatile. It underscores the ingenuity of engineers and enthusiasts in tailoring classic technology for specific needs, transforming a pure racing setup into something capable of handling the demands of street driving, showcasing the enduring appeal of these mechanical marvels in the automotive world.

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