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Holley Carburetor Operating Principles

By about 1975 it looked as if fuel injection was about to spell the demise of the carb as a prime performance fuel delivery system. But, even today, the carb is a dominant feature of engines used for street performance and racing. So why are they still here and, if my guess is right, for at least the next 25 years? Because they work  extremely well. 

A gasoline mechanical fuel injection system was always an expensive and usually finicky deal. Then along came the microchip and electromagnetic injectors and everything supposedly got easier. That was probably the case if you were computer savvy,  but in the mid 1970s most car nuts were very much mechanically oriented and computers just did not figure into their engine building plans. However, mainstream soothsayers predicted the nearly total demise of the carburetor for pretty much all applications other than lawn mowers and classic cars.


Fig. 2.1 This AED-modified Holley Dominator may be expensive, but it drives like a $2,500 fuel-injection unit and makes more power without the complexity of fuel injection.


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As we know, it did not happen. Instead, carburetor manufacturers upped the ante with new, more refined designs. In terms of power potential, what small gap there might have been, put them ahead when raw power was the only criterion. Today, the carb is still very much alive and well for one simple reason: For all its underlying simplicity, a correctly sized and calibrated carb can make horsepower numbers as big as any more complex fuel injection system and do so on a lot less money.

So why has fuel injection almost universally taken over on OE production lines? Primarily because a computer controls it, and therefore fuel injection lends itself to solving emissions-related problems. Outside of having socially acceptable tailpipe emissions, a carb supplies a means to performance, which ranks about equal to any fuel injection, but without the complexity and cost. But tapping into that performance potential requires you to understand the basic operating principles to enable optimal calibration.


Basic Function

Correct calibration of the carb is mandatory in order for power to be made from a carbureted engine. To acquire any great proficiency at this, a sound understanding of the function of the carb is required. Most carbs fall into one of two groups: constant-vacuum and fixed-jet or choke (“choke” here refers to the venturi not the cold-start system). An SU carb such as used on many British cars from the early 1900s through the mid 1980s is a prime example of a constant-vacuum carb. However, more than 90 percent of all carbs used on American V-8 engines are the fixed-jet/choke type, such as typified by a regular Holley carb or the fixed-choke primary paired with a vacuum secondary such as with a  Quadrajet.


Fig. 2.2 The dyno shows that an SU carb is a very precise means of delivering a well-atomized mixture under all driving conditions, from idle to maximum RPM.


Fig. 2.3 Unlike most carbs, opening the butterfly does not directly allow more air into the engine. When the butterfly opens, it communicates engine vacuum to the piston (see  Figure 2.4), and thus opens it to supply just the amount of air the engine needs.



Fig. 2.4 Here you can see the SU’s vacuum-actuated piston. As the piston rises, it pulls a tapered needle out of a jet.


Exploring Venturi Properties

All fixed-choke carbs depend on the properties of a venturi for their function. As air is drawn through a venturi, it speeds up and the pressure drops at the venturi’s minor diameter (see Figure 2.5). This suction effect draws the fuel up from a reservoir, which, in the case of a carb, is the float bowl, and discharges it into the airstream. The greater the airflow, the greater the amount of fuel drawn into the venturi.


Fig 2.5 As the air speeds up at the venturi’s minor diameter, the pressure drops. This drop in pressure draws fluid from the reservoir source.


At this point you may wonder why the pressure drops as the velocity increases. In essence, any given volume of air possesses a finite amount of energy in various forms. These are temperature, pressure, and kinetic energy from any velocity involved. When the velocity increases, the kinetic energy increases as dictated by the formula:

Kinetic Energy = 1/2 MV2


M = mass

V = velocity

Note that the kinetic energy goes up as the square of the speed (V2). This means that unless some other form of energy drops, the volume of air contains more energy than it started with. In reality this cannot happen, so for a given mass to have only the same energy, the pressure drops. It is just that pressure drop caused by the air passing through the venturi that we use to draw fuel out of whatever reservoir it is stored in and deliver it to the engine’s induction system.


Applying the Venturi Effect

If ever there was a misused term it is “venturi effect.” I have heard it misused in relation to intake ports, rocket nozzles, and a ton of other stuff. When I ask what might be meant by it, I usually get nothing more than a blank look or a “Don’t exactly know” response. The venturi effect is illustrated in Figure 2.5. It is a suction caused by the reduced pressure in the high-speed section of the necked-down section of a tube.

Next, we must look at how to translate this basic venturi effect into something resembling the main jet circuit (as opposed to idle, cruise, acceleration, and cold start) of a simple carb. Doing so results in what is seen in Figure 2.6. This also shows what is potentially the first calibration problem associated with a simple carb. Ideally, the fuel level in the reservoir needs to be at the same level as the point of discharge in the venturi. This means that as soon as air starts to flow so does the fuel.


Fig. 2.6. This is a functioning carb in its most basic form. It might be barely functional for a stationary engine running at one speed, but that’s about all. To deal with a typical engine many fuel-to-air calibration fixes are required. The main jet is situated just where the fuel enters the delivery tube in the fuel reservoir.


Fig. 2.7. Here air is shown in red and fuel is shown in blue. A mixture of air and fuel is shown in purple. The main jet restricts the amount of fuel delivered along the passage to the discharge point in the venturi. As RPM and air demand increase, the main jet becomes more effective at delivering fuel so the air/fuel mixture becomes too fuel rich to burn effectively. To compensate, air is leaked into the system through an air corrector jet. Allowing air into the system prior to the venturi discharge point bleeds off some of the suction (signal) to the main jet, thus eliminating the tendency to become overly rich. It also dilutes the fuel delivered from the main jet with air. This not only helps correct a mixture problem but also contributes to better fuel atomization.


Unfortunately such a setup would mean that any movement of the carb as a whole would spill fuel into the engine whether it were running or not. To avoid this, the level of the fuel is set below that of the discharge point. This is called the “spill height” and is usually between 1/4 and 3/8 inch. Also, fuel flow (drawn into the venturi by the depression it  creates) increases faster than the airflow; so a simple jet/venturi system produces a mixture that becomes progressively richer. The basic fix for this is called “air correction.”


Air Correction Function

Air correction works by introducing air into the fuel prior to it reaching the point of discharge in the venturi. In simple terms the fuel is being diluted with air prior to exiting the discharge point in the main venturi. While most of the world calls these “air correction jets” Holley and most Holley users refer to them as “air bleeds.” Their function, in its simplest form, is shown in Figure 2.7. The air corrector jet (or air bleed) becomes more effective as engine speed (RPM) and air demand increases so, under steady-state airflow conditions, it can, for most practical purposes, cancel out the main jet’s tendency to deliver an increasingly richer mixture.

In the real world the air demand created by an engine is anything but steady state, even in a V-8. Because of this, the air-corrected main jet may still not supply the desired ratio of air to fuel at all points of operation. To compensate, engineers devised an ingenious system that not only addressed the reshaping of the fuel curve that would otherwise exist, but also fuel atomization. This is called the emulsion well or emulsion tube.


Fig. 2.8. Exotic carbs, such as Webers and Dell’Ortos, usually have the main jet/emulsion tube/air corrector jet as a single assembly. The main jet (far right) plugs into the end of the emulsion tube as does the air corrector (second from far left). The holder (far left) screws into the carb body emulsion well and holds everything in place.


On an engine employing a single barrel of carburetion connected to each cylinder the emulsion tube design is critical for the accurate delivery of the carb’s air-to-fuel ratio. As more cylinders are connected to the carb so the airflow gets nearer to steady state and the emulsion tubes function as a fuel mixture curve trimming device becomes less critical. Figure 2.9 shows how the emulsion tube works.

Although it has never been a big issue with the type of carbs traditionally used on V-8s, getting the emulsion tube right for a one-barrel-per-cylinder installation, such as a set of Webers or Dell’Ortos, is often considered to be a black art or a trial-and-error process. The good news here is you are about to get the Vizard method for complete and simple emulsion tube “reading.”   

Calibration Considerations

Calibration components considered so far were the main jet, emulsion tube (or in the case of a Holley, the emulsion well), and air corrector. A larger main jet makes the mixture richer, just as a smaller main jet makes the mixture leaner. With the air corrector the reverse is true. The bigger it gets, the weaker or leaner the mixture becomes, with the effect being more pronounced at higher RPM.

The effect the emulsion tube has on the mixture curve depends on the “hole” pattern. Here is how to read it. First, hold the emulsion tube upside down and inspect the hole pattern. Holes at the top of the emulsion tube affect the top end of the rev range. Holes in the middle trim the mid-RPM range. Holes at the bottom affect the low-RPM range. Where there are no holes, the mixture is rich.


Fig. 2.9. The booster in the carb’s main venturi develops suction and draws fuel (blue) up through the main jet into the emulsion well. In addition, air is drawn in through the air corrector jet into the emulsion tube within the well. This air then percolates out through the holes in the emulsion tube into the fuel in the emulsion well. The fuel, which now contains many small air bubbles, is emulsified.


Fig. 2.10. The emulsion tube/well for a typical Holley metering block (left) is very simple, and it usually takes the form of one or two holes drilled into a passage connecting the emulsion well to the air corrector passage. The high-performance versions of most Holley-style carbs have emulsion tube/well holes (left), which are calibrated with brass screw-in jets (right).


Fig. 2.11. The air corrector jets for Holley-style carbs are positioned on either side of the booster leg. This carb has replaceable air correctors, but most run-of-the-mill Holleys have press-in ones. The outer ones are for the idle circuit and the inner ones are for the main jet circuit.


Where there are holes, the mixture is leaned out. Just how much the mixture is leaned out by the presence of holes depends on how many there are and how big they are. The more holes, the more the mixture is leaned out at that point. Because it is fed with air from the air correction jets, the emulsion tube’s overall function is influenced by the air corrector size. A larger air corrector leans out the mixture, but at low RPM, and small throttle openings, the air correction has little influence over the mixture. As the engine’s demand for air increases, due to an increase in throttle opening and RPM, the air corrector’s influence increases. At high RPM, just a few thousandths change in the air corrector size can have a significant affect on mixture ratio.

As mentioned earlier, one other aspect of the emulsion tube and its well is that it acts not only as a means of calibration but also as a control element for fuel atomization. By emulsifying the fuel prior to it reaching the booster located in the venturi the fuel is easier to shear into fine droplets at the point of discharge. Generally the more it is emulsified with air in the emulsion well/tube, the easier it is to atomize at the venturi.

With an understanding of how calibration is achieved let us now look at what the main circuit needs to deliver in the way of air-to-fuel ratios.


Mixture Requirements

To achieve optimal operation under all normal circumstances, a carburetor must deliver an air/fuel ratio appropriate for the prevailing conditions. For maximum power on gasoline, an air/fuel ratio of around 13:1 is needed. Under part-throttle cruise conditions, fuel economy (rather than outright power) is the major issue. During cruise, the engine’s fuel efficiency can be improved considerably by leaning out the mixture. Typically, air/fuel ratios are quoted in terms of pounds of air per pound of fuel. Figure 2.12 shows the physical dimensional proportions of the range of air/fuel ratios you’re likely to be dealing with.

If maximum power is the goal,  the mixture ratio required must fall within certain well-defined limits. Figure 2.13 shows how the power changes as the mixture is varied. You can see that power drops off faster on the lean side of the graph than it does on the rich side. Also, to achieve better than 99 percent of the power potential of the engine, the mixture delivered needs, for typical gasoline, to fall between .5 and a 13.4:1.


Fig. 2.12. Air/fuel ratios are typically quoted by weight, but this drawing shows the typical weights of fuel and air, and what they look like in terms of their volume. The fuel occupies a relatively small space compared to the air space. If the air-to-fuel ratio is “chemically correct,” all the oxygen in the air plus the fuel are 100 percent utilized during the combustion process. For gasoline, this mixture is typically around 14.7:1. A rich mixture has too much fuel for the amount of air and a mixture that is lean has too little fuel. The ratios shown here are typical for maximum power and lean cruise.


Fig. 2.13. The mixture must fall into the narrow operating range shown here to achieve maximum horsepower. This requires precise calibration of the carburetor over a wide range of RPM and airflow situations.


Fig. 2.14. This cutaway shows the functions of all components (main jet, air corrector, emulsion well, etc.) that interact during WOT operation. The enlarged section shows the open power valve in greater detail. The power valve’s chamber is connected to the intake manifold on the downstream side of the butterfly, so it experiences manifold vacuum. When the throttle is opened wide, the manifold vacuum drops and allows the spring on the power valve to lift the valve off its seat. This allows fuel to flow from the float bowl into the power valve and out of the PRVC into the emulsion well.


Power valves are available at various vacuum actuation values. For most street and street/strip uses, the power valve needs to actuate at between 4 and 7 inches of manifold vacuum.


Fig. 2.15. The arrow indicates one of the two PVRCs, which feed extra fuel into the main jet well to enrich the full-throttle mixture. This particular metering block is from a high-performance Holley carb, and utilizes screw-in jets for optimum calibration.


When the vehicle is cruising down the highway the mixture needs to lean out considerably if good mileage is to be achieved. Most of the carbs you are likely to be dealing with use a power-enrichment circuit activated by a vacuum-sensitive “power valve.” This is usually a vacuum diaphragm, which senses how much intake manifold vacuum is present. Opening the throttle causes the intake manifold vacuum to decrease to near zero. This allows the power valve to open and operate as an additional main jet, which supplies the extra mixture-enriching fuel. This additional main jet in any Holley style carb is commonly known as the power valve restriction channel (PVRC).

Traditionally, a Holley-style carb is calibrated with the main jet, but the introduction of a power valve in the circuit means that the main jet now calibrates the cruise mixture. The size of the power valve jet dictates the full-throttle mixture. In practice this is rarely done because most PVRCs are a fixed size. Many high-performance Holley carbs now have most circuits, including the emulsion tube/wells, easily calibratable with small replaceable jets.



Maximum-output carburetion must have sufficient airflow to completely satisfy the engine’s demand at peak power RPM and a little beyond. This calls for a carb that is bigger than if low- and mid-speed power were a primary goal. When a fixed-jet/choke and venturi carb is sized with high output in mind the booster design becomes more critical for operation over a typically acceptable range of 5,000 rpm.

Before delving into advanced booster design, review Figure 2.16. It details the basics of how this aptly named carb component works.


Fig. 2.16. A booster allows the signal (pressure drop) at the minor diameter of the main venturi to be increased as measured at the minor diameter of the booster venturi. Here, the blue line shows the pressure drop through the main venturi; the red line shows the velocity.

The engine’s suction (P1) causes a pressure drop, which dictates the amount and velocity of the air flowing through the main venturi. This is not true for the booster. The air flowing through the booster is actually dictated by the much greater pressure drop occurring at the minor diameter of the main venturi (P2). This brings about a much higher pressure drop and velocity at P3.

In simple terms, the booster has amplified the signal generated at the main venturi. A high-gain booster can increase the main venturi signal by as much as 400 percent.


Fig. 2.17. Holley’s big-CFM Dominator was only a viable wide-band power proposition because of its intermediate cruise circuit and the use of high-gain annular discharge boosters.


Before Holley introduced the high-flow Dominator series of carbs, it had to come up with booster designs with far more gain than had been previously used. The new design needed to take a relatively small signal generated at the minor diameter of the main venturi and amplify it into a strong, useable signal for the purposes of metering and atomization.

What you see today are booster forms that can cover a wide range of applications. Figure 2.18 shows, in the order of gain, the characteristic form of the main variants. For instance, at a typical WOT pressure drop, the number-1 booster amplifies the main venturi signal by about 1.8 while a number-5 with all the casting flash removed and a cleanup on the entry and exit delivers an amplified signal about four times that of the main venturi. Figure 2.19 provides a good perspective of the difference in signal strengths of the five booster styles tested in one barrel of an 850 Holley carb.

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Booster Gain—How Much?

For a high-CFM carb to deliver over a wide RPM range, the booster gain must be high.

But it can be too high. If the fuel is too finely atomized too much of it vaporizes in the intake manifold, which reduces the engine’s volumetric efficiency (breathing efficiency) and consequently reduces power.

Getting the booster’s characteristics just right for the application is a key factor in making torque and horsepower from any carbureted engine. That is why the relationship between sizing and booster selection is so important.


 Fig. 2.18. Booster 1 is primarily used for street applications. Booster 2 is commonly used in high-performance carbs as is booster 3, which is a dog-leg booster as is booster 2 with a step machined into the underside. This enhances fuel atomization. Booster 4 is a stepped annular discharge design and booster 5 is a similar annular discharge style but without the step. Booster 4 and booster 5 are high-gain types, which are most often used in big-CFM carbs.


Fig. 2.19. This graph shows the signal strength for each of the booster styles depicted in Figure 2.18. Note the big difference between the lowest and highest.


Idle and Transition System

As important as the WOT power circuits are, none of the assets are worth a nickel if the idle and transition circuits don’t work as they should. Figure 2.21 shows the basic function of these two circuits in a Holley-style carb.

Although they may look quite different this mode of function is common to most types of carbs, whether Holley or an exotic carb such as a Weber. Some carbs, such as the Weber and Dell’Orto carbs, use a series of holes rather than a slot for the transition circuit. The mode of function, however, is the same.

Because the idle/transition circuits are most frequently used during normal driving, time spent calibrating them pays big dividends in producing good street drivability and fuel economy. Although idle circuit adjustment is a prime criterion, the first step toward achieving a good idle and subsequent low-speed cruise performance is to first select a suitable carb. For a short-cammed street machine, the idle circuit of a Holley-style carb need only be on the primary side of a 4-barrel carb. This works fine when there is plenty of intake vacuum (12 or more inches), but when bigger cams are used it takes a larger butterfly opening to supply the engine’s idle needs. (See Chapter 8 for details on how idle and low-speed vacuum are affected by cam selection and the type of intake manifold.)


 Fig. 2.20. The idle discharge port is the hole in the throttle-body wall. 


Fig. 2.21. The idle/transition circuit relies on the high vacuum at small throttle openings. This vacuum draws fuel from the float bowl through the idle jet. It then communicates with a passage that goes from the idle air corrector jet to the idle mixture adjustment screw.

In the passage, air from the air corrector and fuel mix to form an emulsion. In addition to air from the air corrector, air is also drawn in through the transition slot. This, at idle, is above the butterfly where it experiences typical air pressure. The idle mixture screw, when appropriately adjusted, meters sufficient fuel emulsion into the intake airstream to meet the idle needs.

As the throttle opens beyond the idle position, the butterfly begins to uncover the transition slot. The slot then begins to experience the intake manifold vacuum. As a result the slot progressively stops drawing in extra air and instead discharges extra fuel emulsion from the idle circuit to meet the needs of the extra incoming air from opening the throttle.


Anything that reduces vacuum (such as a bigger cam) means that the butterfly needs to be open wider to supply the idle airflow called for by the engine because the vacuum (suction) by the engine is less. To meet airflow requirements under these conditions the butterfly must be opened wider. In this position, less of the transition slot length is available for doing its job. The first fix is to use a four-corner idle system where the primary and secondary barrels supply the engine’s idle air demand. This leaves more total transition slot length to do what it is supposed to do: deal with the engine’s transition needs.

At a certain point, the cam may be so big that even four idle circuits are not enough to smooth the transition. Under these circumstances, it may also be necessary to put one or more holes in the butterflies to allow further closing of the butterflies in an effort to gain more transition slot use.


Accelerator Pump System

Under idle and cruise conditions, a considerable amount of vacuum exists in the intake manifold. This vacuum reduces the boiling point of the fuel causing it to vaporize much easier under the prevailing high-vacuum conditions than under low vacuum. This useful characteristic considerably helps fuel distribution under idle and cruise situations. When driving down the freeway at 2,000 to 3,000 rpm with 15 or so inches of vacuum most, if not all, of the fuel being drawn into the engine is vaporized well before it reaches the cylinders.

Standing on the gas pedal completely changes the situation. When the vacuum almost instantly transitions from a high value to near zero, fuel that was held in vapor form condenses into liquid on the manifold walls. Although a fresh charge of air is entering the engine and carrying its associated fuel, the engine momentarily goes very lean. This is due to the fuel that was contained in the air within the manifold condensing on the manifold walls, and for a moment at least, going nowhere. This causes an enormous lean-mixture flat spot that the engine simply does not drive through.


Fig. 2.22. For Holley-style carbs, an accelerator pump diaphragm is located at the bottom of the float bowl. Jet size, pump stroke, and pump capacity set the calibration.


Fig. 2.23. Fuel for the accelerator pump is drawn in from the float bowl via a non-return valve. When the throttle is rapidly opened fuel is injected through the discharge passage, past the check valve, and out through the jet located just above the venturi.


To offset fuel condensing on the walls of the intake manifold an accelerator pump system is added. This physically squirts additional fuel into the intake to cover the would-be hole in the carburetion. Figure 2.23 shows a basic schematic of a typical pump system. In this example, a piston is shown injecting the fuel, but most often, the function of the piston is carried out by a spring-loaded diaphragm, such as in a typical Holley carburetor. Calibration of the accelerator pump system is not only by means of jets to control the rate at which the fuel goes in. The system also uses various springs, cams, and diaphragm sizes to control the amount of fuel injected by virtue of the duration of the injection phase.


Carb Sizing

Although Holley makes choosing carbs appear easy in the literature, there is actually a lot more to it. Also, Holley’s carburetor selection methods produce results on the conservative side. The reason for this is that they are in the carb business to sell you a functional carburetor, not to teach you how to be a carburetor engineer. Still, be aware that Holley has a useful interactive website. It not only helps you effectively choose the carb size but also goes through a list of carbs that meet the criteria called for. It deals with the decision-making process in terms of mechanical or vacuum secondaries very well.

That is the Holley selection method. If you want to tighten up on the accuracy of the CFM selection a little more, maybe the Vizard method as detailed in Chapter 6 is for you. Though still slightly on the conservative side, it is a lot more sophisticated than you are likely to find anywhere else.


Carb Options

Until the fuel-injection era, the most commonly seen performance carb on a GM vehicle was the Quadrajet. This carb was designed with both fuel economy and power output in mind. It featured small primary barrels with a multiple high gain booster system and very large secondary barrels that opened progressively as the engine’s airflow requirement increased. In all, these carbs worked well, but compared to a Holley they are a little more difficult to calibrate and set up for modified engines used in competition. A number of Quadrajet issues, such as float-bowl fuel surge, must be dealt with, especially if cornering at high gs. There are still several million of these carbs in use. If you are restoring an older muscle car and want to stick with the Q-Jet, a good option is a rebuild by a specialist shop.

Rather than diminish the variety of carbs available, the onset of the fuel-injection era did almost the reverse. If you are looking to replace an older carb of a different type, it helps to know both the strong and weak points of what you may be replacing compared with whatever you may feel is a worthy replacement.


Fig. 2.24. From mild to wild. The Edelbrock carb (left) is a 500-cfm street driver. The Endurashine (right) is a finished 800-cfm unit.


Fig. 2.25. This Chevy small-block has an Edelbrock 2x4 carb setup with essentially a dual-plane manifold. It delivers the strong low-end output typically seen with this type of manifold. By utilizing two carbs instead of one, Edelbrock has given this manifold a top-end potential similar to a single-plane, street 4-barrel manifold.



These are a cost-conscious, evolutionary version of the now-discontinued Carter Thermo-Quads and are available from 500 to 800 cfm. They use the same functional flow-on-demand and needle/jet calibration method that the Quadrajets have. As such they can be accurately calibrated for good all-round performance.

Unlike a Holley-style carb, many of the principal circuits can be calibrated without removing the float bowl. The main-circuit calibration needles can be removed without touching much else. For the most part these carbs come with the calibration pretty close for most normal applications. If some calibration adjustments are needed, simply visit Edelbrock’s website. With cutaway drawings and simple instructions they make it practical for even a first-timer to calibrate this carb.   


For anyone younger than 110  years old, Holley seems to have been around forever and to offer an incredibly wide range of carbs. If your engine is a relatively short cammed unit you can use a very basic Holley carb to good effect. One great aspect here is the cost. The model 0-80457C 600-cfm vacuum secondary carb in Figure 2.26 not only looks good on a hot rod but also delivers good mileage and power output at a very friendly price. Many hot rodders opt for a mechanical secondary carb because that’s what racers use.

For the street, a vacuum secondary is usually better. The often-perceived reduction in performance because of secondaries that do not open right away is mostly a myth. Apart from improving street drivability, it is often possible to use a vacuum secondary carb of about 50 cfm more than with a mechanical secondary.


Fig. 2.26. Holley’s 600 cfm (model 0-80457C) is a vacuum secondary carb. It features primary-only idle circuit, metering plate on the secondary, and electric choke. This carb is a low-cost option that works great on short-cammed engines up to about 350 hp.


Fig. 2.27. The introduction of Holley’s HP series put the company squarely into the value-plus race carb market. This series proved to be very versatile with a comprehensive range of calibration options, including PVRC and emulsion well jetting.


Fig. 2.28. Race engines, packing more than 30 pounds of boost, have successfully used this blow-through-style Holley from the Carb Shop. Part of the deal is a high-fuel-flow dog-leg booster and a high-flow float bowl needle and seat assembly (inset).


Fig. 2.29. Jet extensions, such as these hexagonal ones, are a good investment for the rear-metering block because they compensate for fuel migrating to the back of the float bowl under hard acceleration.


Fig. 2.30. On this draw-through, Holley-carbed, Weiand-supercharged small-block, the carbs operate almost as they do on a naturally aspirated engine. The power valve is accessible from the intake manifold and not from the underside of the carb. This type of installation makes for easier calibration.


Concessions need to be made when the use is more radical. First, with a cam of more than about 275 degrees of seat duration you should consider using a metering block (as opposed to a vastly cheaper metering plate) on the secondary as well as on the primary, plus a four-corner idle system. To get the transition circuit working properly, you may have to drill a small hole in each butterfly or open up the holes that are already there. If the carb is a Holley high-end model, this chore may not be needed because the carb can have an idle air bypass adjuster situated under the filter hold-down stud. (See Chapter 8 for more details on the idle and transition circuit setup.)


Braswell Carburetion

The Braswell carb comes from a name that has been associated with high-performance carbs for nearly 40 years. David Braswell, who has an enviable cup car win record, is best known in the industry for his design contributions to Holley’s HP series of carbs. Having designed just about every aspect of a Holley at one time or another, he felt he was ready to produce a carb from scratch. His creation embodies all the features of a new-millennium carb, and its all-aluminum construction about halves the weight compared to the usually used zinc-based alloy. I did a quick but hardly spot-on flow test of one of Dave Braswell’s bigger 4150-style carbs and it indicated well over 1,000 cfm. Models for drag, road race, and oval track are available.


Fig. 2.31. The newest Braswell carb comes with a whole load of desirable features, including positive throttle stops and billet base plate and metering blocks.


Fig. 2.32. Repitched throttle bores, skinnier but larger-diameter butterflies, flow-developed boosters, and main venturis contribute to the Braswell carb’s high flow.


Barry Grant, Inc.

Barry Grant is another carb manufacturer that came out of the ranks of Holley specialists. I am including the brand here because its carbs are still available on the used market. Production started in the early 1990s and progressively built to an extensive line ranging from small 2-barrel carbs to the monster King Demon 4-barrels, which can flow 1,300 cfm. Before the company’s demise its most popular style of carb was the Demon, which was available in various models from a regular street version with a choke and vacuum secondary to a race Demon. These carbs were intended as a direct replacement for a 4150-series Holley.

The RS version of this carb and its bigger brother, the King Demon, have replaceable venturis. This adds an extra element in the tuning procedure because it allows the carb to be optimally sized for the job. In addition, race Demons also have replaceable boosters. This means that just about any aspect that can benefit from fine tuning can be fine tuned.

Among the last offerings from the Barry Grant stable was the vacuum secondary King Demon. Mounted on a dual-plane intake (such as Edelbrock’s Air Gap Performer), it may just be what is needed for a really streetable high-output big-block. It has enough flow to supply an air-hungry big-block at the top end while having the potential to retain the ability to deliver the required characteristics for a strong off-idle performance.

I once started a big-block project intending to use the vacuum secondary King Demon and made some promising progress. The unfortunate demise of the company also brought an end to that line of investigation.

Like Holley and several other fuel system companies, Barry Grant made some high-volume race fuel pumps. Production pumps are sized to feed production engines, not the high-output ones we are striving to build. As of 2013 you can still find Barry Grant pumps in good used condition at swap meets. Very often you can get two used ones really cheap and rebuild them into one good one. That said, Holley’s latest range of high-output fuel pumps do come at a very reasonable price.


Supercharged Applications

Superchargers have escalated in popularity since 1990 to the extent that there are now millions on the road in the United States. Most are being used with fuel injection, but that is because most blowers are factory-installed units with the Eaton blower topping the list.

When thinking about superchargers, most understandably think in terms of fuel injection for the fuel delivery system. Going to fuel injection does solve a lot of problems carbs can have when paired with a supercharger. But the sheer volume of blowers out there means that developing carbs specifically for use with blowers has become a viable business proposition in terms of cost to the consumer and production volumes.


Fig. 2.33. Although rated at 850 cfm, this Barry Grant race Demon actually flows about 930 cfm. In about 2005, I built a pump-gas-burning stroked, 351 Windsor with an 850 Demon that’s actually punched out to 418 ci. After fine tuning using the dyno’s oxygen mixture measuring system, the Demon proved its worth. Output was 563 ft-lbs and 610 hp. All this was done with a hydraulic-roller street cam. As this carb is no longer available, a Holley 950 HP Ultra carb makes an excellent substitute.


Fig. 2.34. The Barry Grant Demon and the King Demon carbs were available in an RS version, which featured replaceable venturi sleeves. Because the company is no longer in business, you may have to make your own venturis if you plan on experimenting in this area.


Fig. 2.35. One of the last carbs Barry Grant introduced was the vacuum secondary King Demon. It is just about the perfect carb for a true street, high-output big-block engine.


Fig. 2.36. Reminiscent of high-performance Pontiacs of the late 1960s, the Barry Grant six- shooter uses the center 2-barrel carb for everything up to cruise. If you stand on the throttle, the front and rear carbs kick in. If you are into the nostalgia of three 2-barrel carburetors, check out Holley’s latest offerings.


Fig. 2.37. This represents a top-of-the-line pump, pressure regulator, and output controller from Barry Grant. The controller cuts voltage to the pump during part-throttle use, thereby reducing pump wear and tear, and delivers a steadier fuel level for economy.


There are two distinctly different ways a carb can be used in a supercharged application. The easiest to calibrate is a draw-through system, in which the carb operates in almost the same manner as it does on a naturally aspirated application.

The other option is the blow-through setup, in which the carb is pressurized. This can make for some radical spec changes if accurate mixture ratios are to be delivered under all circumstances. It is easier to get good calibration when using a turbine-type supercharger such as a Vortec or Pro-Charger. Part of the advantage of a blow-through system is that it can be much easier to build and a more compact installation.



Most of the carbs discussed so far are used to feed a plenum-style manifold. There are some exotic carbs such as Webers, Dell’Ortos, and a few others that are designed to run one isolated barrel per cylinder. This sort of setup is known as an independent runner (IR) system. All the basics covered for a typical 4-barrel carb also apply to these seemingly more complex carbs. Although making the manifolds for exotic carbs is more complex and costlier, there are definite advantages to an IR setup, particularly if the carb has the required airflow.


Fig. 2.38. The HP series opened the door to specialist carb shops and made an excellent basis for custom-prepped carbs. This 1,030-cfm unit from AED is a prime example. A lot of precision detail work has delivered extra airflow with no low-speed penalty, compared to a stock 950.


Fig. 2.39. In the 1960s, a set of Weber carburetors on a side-draft manifold on a Pierce setup was one of the key aspects to making the Grand Sport Corvettes into the road race conquerors of the time.


Fig. 2.40. This is a clear plexiglass metering block machined by BLP. It gives an idea of how the fuel circuits are routed within the block. (This photo has a green tint to better illustrate the features of this metering block.)


First, mixture distribution is spot on. Second, there is no inter-cylinder robbing so the negative effect of a long cam on the idle vacuum and idle quality is reduced. This type of setup also makes significantly more low-speed torque so a bigger cam can be used before the loss of low-speed output becomes unacceptable.

Last, they have an awesome induction power roar when the throttles are wide open. All this power potential might make you think they must be gas guzzlers. Not so. You are paying for a carb installation that is about the same cost as a fuel injection setup. As such these carbs, when correctly calibrated, deliver an accurately metered and well-atomized mixture to the engine. I had four 2-barrel downdraft 48 IDA Webers (3,300-cfm in all) on my 350 Chevy work truck. It had stump-pulling torque from 800 rpm on and, even with a 1970s-style 3-speed automatic, it got 18 mpg overall!


Written by David Vizard and Posted with Permission of CarTechBooks

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