While many hot-rodders have chosen the mechanical mixer's side in the war of Carb vs. Fuel Injection, many remain divided as to whether or not to use vacuum or mechanical secondaries. But the fact is that both have their place in the automotive realm, especially if you're willing to put the time in to tuning them correctly.
A mechanical secondary -- aka "double pumper" -- carburetor uses a solid linkage between the primary barrels and the secondary barrels to open them both at the same time. A vacuum secondary uses a vacuum diaphragm to keep the secondary throttle plates closed as long as the engine has vacuum, meaning at low rpm and under part-throttle conditions. Under wide-open-throttle conditions, vacuum in the manifold drops and the engine gets its full dose of air and fuel.
While oft-derided as little more than a fuel economy booster, the vacuum carb does have a few performance advantages over the mechanical secondary. Because it feeds the engine only as much air and fuel as it'll take, the vacuum secondary carb has the ability to self-adjust -- to some extent -- to the size of the engine. It's more forgiving in terms of tuning and helps to boost engine torque, fuel economy and engine efficiency by maintaining maximum air velocity through the carburetor venturis.
If the vacuum secondary carb has one major drawback, it's that it has to respond to the engine's demand for air and fuel before it opens -- and that takes time. The vacuum secondary's ability to instantly open all air and fuel passages in direct accordance to throttle position means that it can offer instant throttle response without the slight lag associated with vacuum secondaries. In addition, you'll never have to worry that the vacuum secondaries aren't opening all the way.
Mechanical secondaries will work just fine as long as you put in the required time and effort in terms of tuning, if you've got a very powerful engine in a light car and if you have numerically high gearing to keep the carb from bogging down. Given these factors, it's a small wonder that the reliable double-pumper remains a darling of racers. The vacuum-secondary has all the outright performance potential of a mechanical secondary, albeit at a slight loss to throttle response. But the payoff is a boost in fuel economy and driveability.
