One of the first decisions made when considering a new carburetor is zeroing in on a size. Performance four-barrel carbs are rated for airflow in cfm, which is the cubic feet (volume) per minute of air that can pass though the carb at wide-open throttle. Carb flow in cfm is usually thought of as a constant, however the cfm reading is meaningless without factoring in the pressure differential at which it is rated. The industry standard for rating four-barrel carburetors is 1.5-inch Hg. depression, or pressure drop. The engine sucking air through the carb by the displacement of the pistons creates this pressure drop.If we can answer the question of how much air the engine needs, the appropriate carb size is fairly easy to determine. Actually, this is simple to calculate, since the engine displacement is a fixed quantity, essentially the size of the engine. Taking the factors of engine size (displacement), rpm, and volumetric efficiency, a simple formula can calculate the max volume of airflow the engine will need at max rpm, given the engine's volumetric efficiency (VE). This formula is given as:
(Engine size (CID) x Max RPM /3,456) x VE = CFM
For an example, a race 446 cubic inch engine turning 6,500 rpm with 103 percent VE (a street engine VE would typically be significantly less) provides the following result:
(446 x 6500/3456) x 1.03 = 863 cfm
In this scenario, it would seem as though an 850-cfm carb would be near ideal, and in the typical street/strip application the formula works out quite well. However, we may find on the dyno, with an all-out racing effort, that a larger carb, like a 950 or 1,000 cfm HP-series Holley may just tack a couple of ponies onto the very top of the power curve; so what gives?
It goes back to the way carbs are rated; remember 1.5-inch Hg.? This is the pressure differential at which an 850 carb will flow 850 cfm, and it represents the vacuum in the manifold required to reach that flow. The carb will actually flow more than 850 cfm if more than 1.5-inch Hg. is acting upon it, and less than 850 if the pressure differential is lower. In our example of the 446 engine at 6,500 rpm, the engine will demand 850 cfm, and the carb will deliver it, however to reach that flow the manifold will show 1.5-inch Hg. of vacuum. Most all-out race engine builders like to see less than 1-inch of manifold vacuum at peak airflow demand at the top of the dyno pull. To supply the same 850 cfm to the engine combo with, say, 3/4-inch Hg. vacuum at peak wide-open-throttle rpm takes a much bigger carb, one that will supply the 850 cfm the engine demands at a lower restriction. These efforts to reduce the pressure differential are targeting the last couple of percent of the engine's potential in race applications.
With either the Holley or Demon carbs, you might notice that several of the models or sizes are offered with different types of boosters. The first question here might be, just what is the booster and what does it do? All conventional carbs work on the venturi principle for delivering fuel flow in relation to airflow. This proportionality is what makes a carburetor possible. The venturi is a portion of a carb barrel at which the cross sectional area diminishes. As the column of air enters the confines of the venturi it speeds up, and Bernoulli's principle states that as velocity increases, pressure decreases. Therefore, the venturi represents a low-pressure area, with pressure drop increasing with airflow velocity. This is pretty handy, since a pipe can be run uphill from the jet to the venturi, and like magic when airflow become sufficient, the low pressure in the venturi will suck the fuel right into the carb throat. The system will deliver more fuel as the airflow through the venturi, and therefore pressure differential, increases.
Now, that's a simple carb, but how about if we put a venturi inside the venturi, right at the end of the pipe? Now we're getting trick with our carb design. Essentially this is a booster venturi--a venturi positioned in the center of the main venturi, with the fuel discharge nozzle located right at the low-pressure area inside. The booster venturi amplifies the venturi effect a second time, making the main fuel circuit far more responsive to changes in airflow. Since the air passing through the center of the booster venturi is moving faster than anywhere else in the carb's barrels, discharging fuel at the center of the booster provides the most effective point for atomization.
There are a number of ways a booster can be designed, and in Holley and Demon carbs, where there is a choice, we typically see three types: straight, downleg, and annular. Straight boosters are generally the type seen in budget carbs, since they are the least involved style to manufacture. Straight boosters are quite effective, although the shape, and requisite longer booster body, produces a greater degree of restriction to overall flow as compared to the downleg booster. The downleg is the general-purpose high-performance design, offering the least flow restriction, and a very effective position low in the main venturi. This provides a strong signal to the main circuit with a shorter, less-restrictive overall booster venturi length. Annular boosters differ from the other two in that rather than a single discharge tube the fuel is discharged through a series of smaller holes positioned around the booster's inner circumference. This aids in atomization and signal, both of which are in short supply with a large carb during low-airflow operation, such as at low rpm. Annular boosters create a small penalty in airflow, but assist lower rpm response, especially when pushing the limits on carb size for a given application.
It may be noted in each manufacturer's line, the further up-market the carb series, the greater the tuning flexibility. The HP series Holley carbs feature provisions for replaceable air bleeds, and four corner idling not found in lesser models. Similarly, the Road Demons provide for additional tuning provisions as compared to the Jr. models, and moving up to the top of the scale to the Race Demon RS series, the tuning capabilities are astounding. While these allowances for tuning are a major asset to a pro racing organization with serious resources available to take advantage of the performance potential offered, for the average street enthusiast the carb's capabilities are beyond the scope of what is required. In fact, with emulsion, air bleed, channel restriction, idle feed, jetting, and more on the table, real expertise is required. This is genuinely an area best left to the very finest experienced tuners.
Fortunately, the manufacturers put a great deal of effort into developing the operating calibrations for all of these units to a very fine degree. As long as these carbs are used in their intended applications, the as-delivered tune will be very hard to beat. The street orientated carbs do have fewer ready tuning points than the racier units, but chances are better than good that you'd be better off not going there anyway. Typically, dialing in with jetting will be all that's required. However, if you've got the kind of grasp it takes to do one better, the provisions are there in the high-end models to really dial it in.
Vacuum or Mechanical
Of all the choices made in selecting a carb, the actuation mechanism of the secondary barrels is critical to defining the character of the carb. Typically, the choice here will be between mechanical-linkage operated secondaries controlled via your right foot, and those operated via a vacuum diaphragm. Edelbrock carbs fall somewhat between the two types, having strict mechanical linkage opening the secondary throttle plates, tempered through the employment of a secondary control device in the form of an air door or velocity valve. In the case of the Edelbrock offerings, the secondary mechanism style is inherent in the design. With either the Holley or Demon carbs you'll have to decide which is correct for your engine.
Typically 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, allowing velocity through the secondary venturi to build more slowly, negating the need for a secondary pumpshot. Generally, vacuum secondary carbs provide better fuel economy.
Vacuum secondary carbs provide more flexibility in situations where the airflow velocity through the booster is insufficient for good throttle response. Examples here primarily apply to street applications. Factors such as low-stall converters with automatic transmissions, high gear ratios, heavy vehicle weight, and low-rpm engines, favor 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 upwards.
Mechanical secondary carbs, sized correctly, will provide better response and power than an equivalent vacuum secondary carb. The precaution here is not to misapply such a carburetor in terms of size and expected rpm range. With a mechanical secondary carb, wide-open-throttle results in all four barrels open--right now. If at the same time the engine is struggling against gear, weight, converter, or an otherwise restricted engine combination at low rpm, the velocity though the boosters, the fuel discharge, atomization, and distribution will all be pretty sorry. A mechanical secondary carb is best in a high performance application, where the combination is free to take advantage of the instantaneous airflow and blast up the rpm range. Here the mechanical secondary carb nearly always wins, making it no wonder that this configuration is used to the near exclusion of all other designs in true race applications.