"What's that?" you ask. "Vacuum secondaries on a full-sized race carb?" Is it heresy, or might there just be a method to the madness? We know that vacuum secondaries are a feature designed and aimed at the streets, and we also know that the King Demon line is aimed squarely at maximum track performance, and until now the two have lived separate lives that have never crossed. Like many components initially developed for the track, there is a certain element in the street scene that just must have one.
While in some cases the need might be more perceived than actual, there are some trends in the performance world that make a huge carburetor like the new Barry Grant Road King Demon (racer net $1,222.55) quite an appropriate choice in some street applications.
Aside from those who are after the "large-frame" 4500-series carb for sheer image and mystique, the increasing popularity of "plus-sized" engines really demands a monster-sized carb to fully quench the airflow appetite of all that displacement. Take for example the 572 cubic inch capacity, a popular size for serious street Mopars, both wedge and Hemi, as well as big-block Chevrolets. These engines are possible because of the revised blocks that are now available, and are some 30 percent bigger than the largest production big-blocks they are based on. Now, we probably wouldn't cause an argument by saying an 850-cfm vacuum secondary carb is a reasonable choice for a 440 Mopar or 454 Chevy. Upsize that 850-cfm carb some 30 percent to keep up in proportion to the displacement, and you are looking at better than 1,100 cfm, or right in the range of the King Demon's capacity. Starting to make sense? Now let's look at how the vacuum secondaries play.
One of the defining characteristics of a carb is the manner by which the secondary barrels are actuated. With a mechanical secondary carb, the link is direct, and operated by the linkage to your right foot. A mechanical carb can have a progressive or staged linkage system at the carb, varying the point at which the secondaries begin to open relative to the action of the primary barrels. What doesn't change in a mechanical system is that once your foot is planted to the floor, all four throttle butterflies are standing at attention, and wide open. Vacuum secondaries are operated by a vacuum canister linked to the secondary barrels, with the vacuum source positioned so that activation is via airflow through the carburetor. Counterbalancing the force of vacuum is a spring opposite the diaphragm, which provides a handy tuning point, allowing springs of various rates to be substituted to tailor the opening rate.
Mechanical secondaries are used in conjunction with a separate accelerator pump circuit for the secondary barrels of the carb. This is to compensate for the lag in fuel flow through the secondary main metering system relative to the airflow when the secondary barrels are snapped open. Vacuum-operated secondaries are much more progressive in operation, since they are controlled by air velocity through the venturii to open progressively as demand builds, negating the need for a secondary pumpshot. Actually, a mechanical pump shot at the secondary will be met by a closed throttle plate; it doesn't take a Ph.D. to understand that won't work.
As a bonus, vacuum secondary carbs provide better fuel economy.So, what is the reason for vacuum-operated secondaries? The vacuum opening system provides more flexibility in situations where the airflow velocity through the booster is insufficient for good throttle response, a situation most commonly encountered in street applications. On the street there are compromises, with equipment such as low-stall converters with automatic transmissions, high gear ratios, heavy vehicle weight, and low-rpm engines. These things favor the progressive nature of vacuum secondaries. That's not to say that a vacuum secondary carb cannot be a strong performer. The virtue of vacuum secondaries is that they can extend the operating range of a given carb size downward to a lower rpm than would otherwise be practical, and at the same time extend the useable carb capacity upward. Simply put, because the secondaries do not come into play until they are needed, a vacuum secondary carb acts like a smaller carb under low-airflow demand conditions, but has the capacity to do the job when high flow is needed.The RS line of carbs refers to Demon's patented removable venturii sleeves.
Interchanging versions of the venturii in the appropriate size allows the actual working capacity of the carb to be varied over a range from 795 to 1,195 cfm, utilizing the same carb body. This allows street users to refine their engine tuning by sizing the carburetor to their specific engine requirements. The boosters, airbleeds, and metering block bleeds--practically every tuning orifice--are all replaceable, allowing an unprecedented degree of tuning flexibility. Like all King Demon RS carburetors, the new vacuum secondary carb features three metering circuits (high-speed, intermediate, and low-speed) for the fuel curve flexibility required under a wide range of operating conditions. While all of the tuning possibilities may represent more adjustments than the typical street user can master, the experts at Demon carbs have already put in the calibration to target these carbs for the street. The vacuum secondary RS even features street-friendly annular discharge boosters for better signal and atomization.
This a part we are not accustomed...
This a part we are not accustomed to seeing on a King Demon carb, but in some cases is a very welcome addition. The secondary diaphragm displaces the normal mechanical linkage to engage the abundant secondary airflow as needed.
Vacuum in the venturi begins...
Vacuum in the venturi begins to pull on the secondary diaphragm as airspeed begins to rise through the carburetor; counterbalancing this force, the secondary spring controls the rate and rpm at which the diaphragm reacts and opens the secondary barrels. The access plug in the vacuum housing allows spring changes in just seconds, facilitating secondary tuning.
Absent from the secondary...
Absent from the secondary side of the carb are the usual accelerator pump and associated linkages. Instead, a blanking cover is used at the bottom of the float bowl, and the discharge nozzle position above the secondary bores is plugged. An accelerator pump is incompatible with vacuum secondaries.
Older race carbs of the large-frame...
Older race carbs of the large-frame configuration (4500-series flange) typically were drag-race only with two metering circuits, basically allowing on-and-off fine tuning. Three-circuit carbs add an intermediate circuit, making it possible to tune a fuel curve for a broader range of operating requirements. The three air bleeds per barrel is the giveaway identifying the three circuits. Of course, all three air bleeds are replaceable for tuning.