Carburetors with features that contribute to smoother secondary opening include:
* Vacuum secondaries: These open more gradually based on engine vacuum, providing a smoother power transition than mechanical secondaries. This is generally preferred for street applications. However, they might not be ideal for high-performance applications where a quicker secondary opening is desired.
* Adjustable secondary linkage: Allows fine-tuning of how quickly and forcefully the secondaries open. This is crucial for getting the right balance between smooth operation and power delivery.
* Properly calibrated carburetor: A carburetor that's properly jetted and tuned for your specific engine will always perform better and more smoothly, regardless of the type of secondary operation. This is often overlooked but extremely important.
Brands often associated with good secondary response (but still require proper tuning):
* Holley: Holley carburetors are very popular and offer a wide range of models, from easily tunable double-pumper designs to vacuum secondary options. Their reputation for performance is well-established. You'll need to research specific Holley models to find one suitable for your application. The 600 CFM or 750 CFM double pumpers are popular choices depending on your engine's needs.
* Edelbrock: Edelbrock also makes a variety of carburetors known for their relatively smooth operation, and they are often easier to tune than Holleys for a street application.
Important Considerations:
* CFM rating: Choose a carburetor with a CFM (cubic feet per minute) rating appropriate for your engine's displacement and modifications. Too small, and the engine will be starved for air; too large, and it might be difficult to tune properly. A 750 CFM carburetor is a common starting point for a modified 400 small block, but a properly tuned 600 CFM might be sufficient for a less-modified engine.
* Engine modifications: The level of your engine's modifications (cam, heads, intake manifold) significantly impacts the carburetor choice. A mild street engine needs a different carburetor than a heavily modified race engine.
* Professional tuning: Regardless of the carburetor you choose, proper professional tuning is essential to achieve smooth secondary opening and optimal performance. This is often overlooked, but a properly tuned carburetor makes a huge difference.
In short, there's no single "best" carburetor. Research specific models from Holley and Edelbrock, paying close attention to their CFM rating and secondary type (vacuum or mechanical), and then consult with a professional carburetor tuner to get the best results for your 1976 400 small block.
