Converting a vacuum secondary carburetor to a true mechanical secondary unit is generally not a straightforward or recommended modification for optimal performance. While minor alterations can be made to influence the vacuum secondaries, a full conversion to a "real" mechanical double pumper is highly challenging due to fundamental design differences.
Understanding the Core Differences
To understand why a direct conversion is difficult, it's essential to recognize the distinct operational mechanisms of vacuum and mechanical secondary carburetors:
| Feature | Vacuum Secondary Carburetor | Mechanical Secondary Carburetor |
|---|---|---|
| Opening Mechanism | Actuated by engine vacuum and airflow demand. Opens progressively as airflow increases. | Directly linked to the primary throttle linkage. Opens instantly or almost instantly with the primaries. |
| Response | Smoother, more gradual transition, which helps prevent bogs on street applications. | Immediate, aggressive throttle response, often preferred for racing. |
| Fuel Delivery | Relies on vacuum for secondary opening, reducing the need for an additional secondary accelerator pump shot. | Requires a dedicated secondary accelerator pump to provide an immediate fuel shot, preventing a lean bog when secondaries open instantly. |
| Primary Use | Street performance, general driving, better fuel economy. | Racing, drag racing, high-performance applications where instant power is crucial. |
Why a True Conversion is Difficult
The primary reason a vacuum secondary carburetor cannot easily be turned into a true mechanical double pumper is the absence of a secondary accelerator pump. Mechanical secondary carburetors require this additional fuel shot to cover the lean condition that would otherwise occur when the large secondary throttle blades open instantly. Vacuum secondary carburetors don't need this because their secondaries open gradually as engine airflow naturally increases, drawing more fuel.
Beyond the accelerator pump, the throttle linkage designs are inherently different, making a direct mechanical connection challenging to implement effectively and reliably.
Attempts to "Quickly" Open Vacuum Secondaries
While a full mechanical conversion is impractical, some enthusiasts have tried to modify vacuum secondary carburetors to make their secondaries open more quickly, resembling mechanical action.
One common, though often problematic, method involves:
- Modifying the Linkage: A rudimentary approach is to place a bolt in the linkage of the vacuum secondary carburetor.
- Effect: This modification aims to force the secondary throttle blades open sooner, typically about a quarter-inch quicker than they would naturally with vacuum.
- Consequence: However, this typically leads to a significant bog as the engine receives a sudden influx of air without the corresponding immediate fuel enrichment (which a mechanical secondary carburetor's accelerator pump would provide). This bog can severely hinder performance and driveability.
This type of modification is more of a hack to speed up vacuum secondary opening, rather than a genuine conversion to a mechanical secondary system. It does not address the fundamental fuel delivery requirements of a truly instant-opening secondary.
Practical Recommendations
Given the complexities and potential performance drawbacks of trying to convert a vacuum secondary carburetor:
- Purchase a Dedicated Mechanical Secondary Carburetor: If your application truly demands the instant response and characteristics of a mechanical secondary carburetor, the most effective and reliable solution is to invest in a carburetor specifically designed for mechanical secondaries. Carburetors like Holley's 4150 HP or Dominator series are built from the ground up to provide this performance.
- Optimize Your Vacuum Secondary Carburetor: For street and many performance applications, a properly tuned vacuum secondary carburetor can provide excellent results. Focus on optimizing its spring rates, metering, and power valve to match your engine's needs, ensuring a smooth yet powerful transition.
Attempting to convert a vacuum secondary carburetor to a mechanical one often results in a carb that performs worse than either a well-tuned vacuum secondary unit or a purpose-built mechanical secondary unit.
