The Secret Life Of Carburetors

Mechanical fuel injection was always an expensive and finicky deal. Then along came the microchip and electronic injectors and everything got easier. Mainstream soothsayers predicted the total demise of the carburetor for all applications other than lawn mowers and classic cars. Well, here we are 25 years down the road, and the carb is still very much alive-and for one simple reason: It works. For all its underlying simplicity, a correctly sized and calibrated carb can make power numbers as big as any fuel injection system and do so for a lot less money. So why has fuel injection taken over on OE production lines? Primarily, because it can be controlled by a computer; fuel injection lends itself to solving emissions-related problems. Outside of having socially acceptable tailpipe emissions, a carb will supply performance equal to any fuel injection, but without the complexity and cost.Yet, to tap into that performance, you have to know how to select and calibrate one. This is what we are going to delve into here.

Basic Function
If a carburetor is going to make big power, then the correct calibration is essential. Great proficiency at this requires a basic understanding of the function of the carb. Most carbs fall into one of two groups: constant vacuum and fixed choke. (The "choke" here refers to the venturi, not the cold-start system.) An SU carb, such as used on British cars from the mid 1980s, is a prime example of a constant-vacuum carb, but 99 percent of all carbs used on V-8 engines are the fixed-choke type, such as typified by a regular Holley carb. This is the type we will deal with here.

The Venturi
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 in doing so, the pressure drops at the venturi's minor diameter (Fig 1). This suction effect draws the fuel up from the float bowl, and discharges it into the airstream. The greater the airflow, the greater the amount of fuel drawn into the venturi.

Translating the basic venturi function into something resembling the main fuel circuit of a simple carb results in what we see in Fig 2. This also shows what is potentially the first calibration problem associated with our 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 would mean that as soon as air started to flow, so would the fuel. Unfortunately, such a setup would mean that any movement of the carb as a whole would spill fuel into the engine whether it was 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 .25 and .375 of an inch. This, and the fact that fuel flow (drawn into the venturi by the depression it creates) increases faster than the airflow, means that a simple jet/venturi system produces a mixture that becomes progressively richer. The basic fix for this is called air correction, and it works by introducing air into the fuel prior to it reaching the point of discharge in the venturi. While most of the world calls these air-correction jets, Holley and most Holley users refer to them as air bleeds. The function of these, in its simplest form, is shown in Fig 3. The air bleed becomes more effective as 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 for this, an ingenious system was devised called the emulsion tube. On an engine employing a single barrel of carburetion connected to each cylinder, the emulsion tube design is critical for the effective function of the carb. As more cylinders are connected to the carb, the airflow gets closer to steady-state, and the emulsion tube's function as a curve-trimming device becomes less critical. Fig 4 (see p. 85) shows how the emulsion tube works. The good news is you are about to get the PHR method for simple emulsion tube "reading."

Calibration Considerations
The calibration components considered so far have been the main jet, the emulsion tube, and the air bleed. Starting at the main jet, we find that a larger main jet makes the mixture richer, and vice versa. The effect of the emulsion tube will depend 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 will affect the top-end of the rev range. Holes in the middle will trim the mid-rpm range, and holes at the bottom, the low-rpm range. Where there are no holes, the mixture will be rich. Where there are holes, the mixture will be leaned out. Just how much the mixture is leaned out by the presence of holes depends on how many, and how big. The more holes present, the more the mixture is leaned out at that point. Because it is fed with air from the air bleeds, the emulsion tube's overall function is influenced by the air bleed size. A larger air bleed leans out the mixture, but at low rpm and small throttle openings, the air bleed has little influence over the mixture. As the engine's demand for air increases due to an increase in throttle opening and rpm, so the air bleed's influence increases. At high rpm, just a few thousandths change in the air bleed diameter can have a significant effect on mixture.

One other aspect of the emulsion tube and well is that they act 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, 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 tube, the easier it is to atomize at the venturi. With an understanding of how it is achieved, let us now look at what we need the main circuit to deliver in the way of air-to-fuel ratio.

Mixture Requirements
To achieve optimal operation under all normal circumstances, a carburetor must deliver an air/fuel ratio appropriate for prevailing conditions. For maximum power, 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. Normally, air/fuel ratios are quoted in terms of pounds of air per pound of fuel.

If maximum power is the goal, then the mixture ratio must fall within well-defined limits. The graph (Fig 5) shows how the power changes as the mixture is changed. We can see that power drops off faster on the lean side of the graph than it does on the rich side. Also, if we are to achieve better than 99 percent of the power potential of the engine, the mixture needs to fall between 12.5 and 13.

When the vehicle is cruising, the mixture needs to lean out considerably if good mileage is to be achieved. With most carbs, we are likely to be dealing with a power-enrichment circuit, activated by a vacuum-sensitive power valve. This usually takes the form of 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 what can best be described as an additional main jet that supplies the extra enriching fuel. This additional main jet in any Holley-style carb is commonly known as the power valve restriction channel, or PVRC for short.

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 and ultimately it will be the size of the power valve that dictates the full-throttle mixture. In practice, this is rarely done, as most power valve restriction channels are of a fixed size. But most does not mean all. Many high-performance Holley-style carbs now make most circuits, including the emulsion tube wells, with easily changeable small size jets.

Boosters
Maximum-output carburetion must have sufficient airflow to completely satisfy the engine's demand at peak rpm. This calls for a carb selection that is bigger than if low- and mid-speed power was the primary goal. When a fixed-jet/fixed-venturi carb is sized with high output in mind, the booster design becomes more critical for operation over an acceptable rpm range. Before delving into advanced booster design, it's worth taking a look at this aptly named carb component to get a basic understanding of how it works. Fig 7 details this (see p. 88).

Before Holley could introduce its high-flow Dominator-series carb, it had to come up with booster designs having far more gain than before. 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 we see today are booster forms that can cover a wide range of applications. Fig 8 shows (see p. 89), in the order of gain, the characteristic form of the main variants. For instance, at a typical wide-open throttle pressure drop, the number one booster amplifies the main venturi signal by about 1.8, while a number five with all the casting flash removed and a clean-up on the entry and exit delivers an amplified signal about four times that of the main venturi. Fig 9 (see p. 89) will give a good perspective of the difference in signal strengths of these five booster styles as tested in one barrel of an 850 Holley carb.

For a high-cfm carb to deliver over a wide rpm range, the booster gain needs to be high. But it can be too high. If the fuel is over-atomized, too much of it will vaporize in the intake, which will cut the engine's volumetric efficiency and consequently cut power. Getting the booster's characteristics just right for the application is a key factor toward making torque and horsepower from any carbureted engine. That is why the relationship between sizing and booster selection is so important.

Idle and Transition System
As important as the wide-open throttle power circuits may be, none of them will be worth a nickel if the idle and transition circuits don't work. Fig 10 (see p. 90) 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.

Because the idle/transition circuits are the most used during normal driving, time spent calibrating them pays big dividends. Other than the idle adjustment, the main point for achieving the desired goal is to 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 four-barrel carb.This works fine when there is plenty of intake vacuum (12 or more inches), but when bigger cams are used, it takes more butterfly opening to supply the engine's idle needs. This means that the butterfly, in its wider open position, leaves less of the transition slot available for doing its job. The first fix is to use a four-corner idle system where both the primary and secondary barrels supply the engine at idle. This leaves more transition slot length to do what it is supposed to do-deal with the engine's transition needs. It may also be necessary to put a hole or holes in the butterflies to allow further throttle closing in an effort to gain more transition slot use.

The Accelerator Pump
Under idle and cruise, a considerable amount of vacuum exists in the intake. 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 helps fuel distribution considerably during idle and cruise. When running down the freeway at 2,000 to 3,000 rpm with 15 inches of vacuum, a lot of 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 transitions rapidly from a high value to near zero, fuel held in vapor form now condenses into liquid onto the manifold walls. Although a fresh charge of air is entering the engine and carrying its associated fuel, the engine, for a moment, still goes very lean. This is due to the fuel that was contained in the air suddenly clinging to the manifold walls, and for a moment at least, going nowhere. This causes an enormous flat spot that the engine simply will not drive through. To offset fuel condensing on the walls, an accelerator pump system is added. This squirts additional fuel into the intake to cover the would-be hole. A basic schematic of a typical pump system is shown in Fig 11 (see p. 91). 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 carried out by jets to control the rate at which it goes in; various springs, cams, and diaphragm sizes are used to control the amount that is injected and the duration of the injection phase.

Carb CFM: How Much Do You Need?
The first step toward installing the best carb is to make a preliminary selection based on the engine's displacement. Next, modify this result by factoring in relevant engine details such as the heads and cam. For the initial calculation, we need to determine the amount of airflow the engine is likely to inhale if it was able to breathe at 100 percent volumetric efficiency (VE). To do this, multiply the displacement by the rpm the engine is likely to turn. Be realistic when making the rpm estimation. Start by estimating where peak power is likely to occur and then add 500 rpm for over-speed. Because we're dealing with a four-cycle engine, we need to divide the CID x RPM result by 2. The resulting number tells us the cubic inches of air displaced per minute. To change that to cubic feet, divide by 1,728.

Our calculation assumes the engine has a 100 percent VE. For a race engine, where exhaust scavenging is a factor, the VE can exceed 100 percent by a big margin. For instance, a well-built 350 with no regulatory race restrictions can reach 115 percent VE. This means that as far as the carb is concerned, it looks the same as 400 inches. At the other end of the range, we find that a stock street engine may have a VE of 80 percent at best. This means from the carb's point of view, it's only about 300 CID. Our carburetor selection needs to take this into account. The airflow depends primarily on the cam and the breathing capability of the heads. Assuming the compression ratio and exhaust system are appropriate for the engine, the heads and cam are left as the most influential on carb sizing. As cams get longer, so the engine's VE improves. The VE also improves as the cylinder head flow improves. The chart, Fig 12 (see p. 92), gives a correction factor (CF) to take into account cam duration and cylinder head flow. Using this correction factor, we come up with a simple formula that gives a good prediction for carb CFM. This formula is:CFM = CID x RPM x CF 2 x 1,728

An example will show how this works. We will use the UNCC Motor Sports big-block build from a few issues back [see "The Street Beast," Sept. 2006-ed.]. We were targeting peak power at 6,500 rpm, so our max rpm figure would, at 500 over that, be 7,000 rpm. The CF for the COMP Cams street roller (248 degrees @ 0.050) with the basic race-ported Dart Iron Eagle heads (Green curve in Fig 12) came out to 1.065. Putting all this into the equation we have:CFM = 482 x 7,000 x 1.065 = 1,040 CFM 2 x 1,728

The carb selected was a 1050 Holley Dominator and, as our dyno testing showed, the choice worked out well. Here's another example, this time a 5.0 small-block Ford I built for my road-race Mustang. The engine displaced 306 inches, had race-ported Dart heads, a big COMP Cams solid street roller with 254 degrees duration @ 0.050, and peak power rpm targeted at 7,250. Using the green curve, the CF came out to 1.07. Putting the numbers into the equation:306 x 7750 x 1.07 = 735 CFM 2 x 1728

The carb used was a 750 Barry Grant Demon and this pump-gas burning 306 turned out 485 hp and 392 lb-ft of torque.

The carb sizing we have just gone through is a little on the conservative side as it makes no allowance for the fact that a tricked-out carb with high-gain boosters can successfully use greater CFM. This will allow a little more power to be developed without sacrifice in the lower rpm range. Going this route does mean you have to know your carbs or work with a carb specialist such as AED or the Carb Shop, to name two.

What's Available?
Now that you have the basics of carb function, let's look at some of the options available. Up until the fuel-injection era, the most commonly seen performance carb was the Quadra-Jet. This carb was designed with both fuel economy and power in mind. It featured small primary barrels with a multiple high-gain booster and very large secondary barrels that opened progressively as the engine's airflow requirement went up. They worked well, but compared to a Holley, they were a little more difficult to calibrate for modified engines. There are still several million of these carbs in use. If you are restoring an older musclecar and want to stick with the Q-Jet, then a rebuild by a specialist shop such as the Carb Shop is a good bet.

Rather than diminish the variety of carbs, the onset of the fuel injection did the reverse. From mainstream suppliers we have a wide range of options plus some exotics from well-known overseas manufacturers.

Let's start with Edelbrock's carbs. These are a cost-conscious, evolutionary version of the now discontinued Carter Thermo-Quads, and are available from 500 to 800 cfm. [For more info on tuning the Edlebrock AVS, see "Thunderstruck," March, 2004.-ed.] They use the same functional flow-on-demand and needle/jet calibration method that the Quadra-Jets have. As such, they can be accurately calibrated for good all-round performance. Unlike a Holley-style carb, many of the Edelbrock's principle 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. In the event some calibration adjustments are required, visit Edelbrock online. With cutaway drawings and simple instructions, they make it practical for even a first-timer to calibrate this carb.

For any of us under 110 years old, Holley seems to have been around forever and offers a wide range of carbs. If your engine is relatively short-cammed, you can use a basic Holley to good effect. The nice thing about this is the price. Many hot rodders will opt for a mechanical secondary carb because that's what racers use. For the street, a vacuum secondary is, for the most part, a better deal. The often-perceived reduction in performance because the secondaries do not open right away is mostly 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.

When the cam is more radical, then some concessions need to be made. First, with a cam of more than about 275 degree of seat duration, you should consider using a secondary metering block (as opposed to a vastly cheaper metering plate) and a four-corner idle system. Also be aware that to get the transition circuit working, you may have to drill a small hole in each butterfly or open up the ones that are already there.

Superchargers have escalated in popularity over the last 15 years to the extent that there are now millions on the road. When thinking superchargers, most of us tend to think in terms of fuel-injection. Granted, going to fuel-injection eliminates a lot of problems carbs can have. But the sheer volume of blowers out there has meant that developing carbs specifically for use with blowers has become a viable proposition.

There are two distinctly different ways a carb can be used in a supercharged application. The easiest to calibrate is a draw-through system. Here, the carb operates in almost the same manner as it does on a normally aspirated application. The other option is the blow-through setup. Here, the carb is pressurized and this makes for some radical spec changes if accurate mixture ratios are to be delivered under all circumstances. For what it's worth, it is easier to get good calibration when using a turbine-type supercharger such as a Vortech or Pro-Charger. Part of the advantage of a blow-through system is that it can make for a much easier-to-build and more compact installation.

A new carb hitting the scene is the Braswell carb. David Braswell, who has an enviable Cup car win record, is best known within the industry for his contributions to the design of Holley's horsepower series of carbs [the same one featured in our opening lead image-ed.]. 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. Not only does it embody all the features of a new millennia carb, but also its all-aluminum construction saves weight. Models for drag, road race and oval track are available.

Barry Grant is another carb manufacturer who came out of the ranks of the Holley specialists. About eight years ago, BG started to manufacture complete carbs. Today, they have an extensive line ranging from small two-barrel carbs to the monster King Demon four-barrels, which can flow 1,300 cfm. Currently, their most popular style is the Demon, which can be had in various models from a regular street deal with a choke and vacuum secondaries to a race Demon. These carbs are a direct replacement for a 4150-series Holley, so whatever manifolds the Holley fits, so will these. The RS version of this carb and its bigger brother, the King Demon, has 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, the race Demons also have replaceable boosters. This means that just about any aspect that can benefit from fine tuning can be fine tuned.

The latest offering from the BG stable is the vacuum-secondary King Demon. [For the complete story on Barry Grant's Road Kind Demon, see "King of the Street," July 2006.-ed.] This, mounted on a two-plane intake such as Edelbrock's Air Gap Performer, may just be the ticket for a really streetable, high-output big-block. It has enough flow to supply an air-hungry big-block at the top end while retaining the ability to deliver the required characteristics for a strong off-idle performance.