The design of the booster can have dramatic effects on the engine’s output and fuel consumption. Having an understanding of boosters can put you in the position of being able to enhance high-RPM output without sacrificing low-speed output. Never one to avoid controversy, I have stated on many occasions that there is almost no such thing as a carb that is too big. Although a slight exaggeration, it’s not as far off reality as you might think. As an example, the constant vacuum of an SU pulls the air valve open just far enough to satisfy the engine’s air demand. Therefore, at low speed, it is a small carb; at high speed, it is a large carb. This in turn means that you can put a giant SU on a small engine with little fear of having a carb that is too big. With a fixed-jet carb the situation changes. A big Holley on a small engine means that at low RPM, WOT produces a venturi speed that is too slow to meter or atomize the fuel sufficiently well. This, in turn, compromises combustion efficiency. But just how much “too big” is depends greatly on how effective the boosters are.
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If the boosters have a really high gain (i.e., step up) over the main venturi depression by 300 to 400 percent, a big carb is far less of a low-speed liability. A couple examples illustrate this: A small-block Chevy 350 with a Holley flowing 985 cfm (which produces big torque numbers from idle up) and a similar 350 with a 1,020- cfm Holley for a Trans-Am. I realize much of this flies counter to conventional wisdom. But the reality is that carb manufacturers (in this case Holley) make that statement to simplify carb calibration for the typical consumer. But it doesn’t tell the entire story.
You may want to install a highly functioning carb right out of the box, but to get the most out of it, you need to properly set it up. It takes some time and inclination to learn about and properly set up a carb. Don’t use a carb that is too big unless you know how to select or design a booster that still gives an appropriate signal at low speed. Again, if you have a working understanding of boosters, it can put you several steps ahead of your competition at the race track. Knowing what works and how to get there allows the use of greater carb CFM before you experience any negative impact on low-speed drivability and torque. Also there is no universally “best” booster. It is always a case of selecting whatever is best for the job at hand.
When Holley seriously committed to racing in the 1960s, it became apparent that making bigger, highflow carbs also meant increasing booster design research. Even to this day, much of that knowledge remains in the possession of only a few professionals within the industry. To many performance enthusiasts, booster science looks more like booster black art. But in this book, I begin dissipating the fog of confusion. Chapter 2 introduced a brief description of booster function; this chapter goes into more detail on the booster’s workings and interaction with the rest of the carb’s functions.
A booster’s function hinges on the events that take place as air is drawn through a venturi. Take a look at Figure 7.2. The suction (partial vacuum) of the engine draws air through the venturi. As the air passes through the venturi’s minor diameter it speeds up. The red curve on the graph below the venturi illustration depicts the increased air speed. When this happens, the air pressure drops (blue line). As the air expands in the exit area of the venturi, it slows and a pressure recovery takes place. The greater the airflow through the venturi, the greater the pressure drop at the minor venture.
On a running engine, the volume of air flowing through the venturi depends on the amount of suction, which is created by the pressure drop at its exit end (i.e., the intake manifold end). By installing a smaller (booster) venturi into the main venturi as shown in Figure 7.2 you get a certain effect. If the smaller venturi has its exit end precisely at the smallest diameter of the larger venturi, it experiences a pressure drop greater than that seen by the main venturi. This comes about because the main venturi is subjected to the suction produced by the engine while the greater depression that occurs at the main venturi’s minor diameter dictates the booster venturi flow.
As a result the airflow through the booster (P3) is faster and the resultant pressure drop at its minor diameter is greater. Because air is being sucked in not by the smaller pressure drop at P1, but by the much larger pressure drop at P2, the signal at the main venturi is amplified (or “boosted”) at P3 by the booster venturi. Hence the name “booster.” The importance of the boosteramplified pressure drop is that it is the signal used by the carb’s fuel system to not only meter the amount of fuel for a given amount of air, but is also a key factor in producing sufficient fuel atomization for effective combustion. If either metering accuracy or fuel atomization are off by too much, power output suffers.
To achieve optimum top-end horsepower, use a carb with sufficient airflow to fully meet the engine’s needs at peak RPM. This inevitably calls for a bigger carb than required if power at low and mid speed was the main criterion. The bigger the carb, the more critical the booster design becomes if the carb is to operate over an acceptable RPM range. Before Holley could introduce the Dominator series of carbs with their big barrel/venturi sizes, the company had to come up with booster designs that produced sufficiently high gain. In other words, the needed boosters had to take a relatively small signal as generated at the minor diameter of the main venturi and amplify it into a sufficiently strong signal for the purposes of metering and atomization.
Over the years Holley has broken much new ground in terms of booster design. In Figure 7.4 you see the characteristic form of the main variants. These are shown in approximate order of gain. For instance, at a typical WOT flow and pressure drop, booster 1 amplifies the main venturi signal by about 1.8. However, booster 5 (with all the casting flash removed and a cleanup on the entry and exit) delivers an amplified signal nearly four times that of the main venturi. Figure 7.5 shows how each of these booster designs reacts when tested in the 1.5-inchdiameter venturi barrel of an 850 Holley carb.
If you need a carb that delivers good performance over a very wide RPM range, the bigger the carb is for a given engine size, the higher the booster gain needs to be. With a very high gain booster, you expect a largeCFM carb to produce nearly flawless low-speed performance. If such a booster is used, it is entirely practical to use a 1,000-cfm carb on an engine that typically required about 650 cfm. The advantage of the bigger carb is that it allows the engine to make more top-end power while the high-gain boosters still produce an adequate signal to meet the metering and atomization requirements for low speed. So, for typical performance, there is almost no such thing as a carb that is too big; only one with an inadequate booster signal.
Variable CFM Design
Getting an adequate booster signal by utilizing a high-gain design is not the only way to get a 4-barrel engine to perform at the top and bottom ends of the spectrum. All too often, street rodders make the mistake of assuming if the racers use it they should too. In the real world, that usually means big-CFM carbs with mechanical secondaries. Such an arrangement just doesn’t make it at the low end, but how about a variable-CFM carb? That is small at low RPM and big at high RPM. It is readily available and commonly known as a vacuum secondary carb. Holley offers many smaller carbs, such as the 4150 and the like. But if you are into big-blocks just think what a useful piece a vacuum secondary Dominator would be!
To see by how much it is possible to “cheat,” the commonly accepted “too-big carb” philosophy is shown in Figure 7.8. Here a basic stock cam/ valvetrain Chevy small-block 350 equipped with pocket-ported production-line 186 head castings and dyno headers was tested with a singleplane race manifold. The carburetion for this test involved the use of a 650 carb and an 850 carb. Typically a conservatively spec’d engine, such as this, is equipped with no more than a 650-cfm carb. From the graph in Figure 7.8 you can see how an 850, with first a stock booster and then high-gain boosters, stacks up against a stock 650. The first lesson to be learned is to not assume that bigger is better as the stock 650 showed far better results than the stock 850.
Also, when the 850 was equipped with boosters that delivered about the same signal strength per CFM as the 650, a different pattern emerged. With the 850 the low-speed output was almost the same as the 650 but the top end was significantly better.
From what we’ve learned so far, it seems that the more gain a booster has, the better a carb works. And that’s true over a wide operating band. But if it’s a race engine that you’re optimizing for output over, say, 2,000 rpm, things are a little different, and additional booster gain is not desired because it is possible to have a booster that brings about a too finely atomized charge.
The smaller the fuel droplets, the more readily they evaporate into a vapor. When a droplet becomes vaporized to a gas, it takes up much more room in the intake manifold and reduces the volumetric efficiency of the induction system. Although a charge of vaporized fuel and air displays about the best combustion characteristics, it doesn’t produce the best power because there is less air in the cylinder with the vaporized fuel than with liquid fuel droplets of a suitable size. Although a charge with fully vaporized fuel delivers the best drivability and fuel efficiency, a certain optimum droplet size is required to make maximum power.
It may seem to be an easy task to develop a booster that delivers the required droplet size and solve the problem. Unfortunately, highperformance engines are rarely that simple. In practice, many factors influence what happens to the fuel after it leaves the booster. The most important of these is how well the fuel stays in suspension in the air and the temperature of the induction system. Ports with a high and uniform velocity and good wet flow characteristics are a good start, but unless you are in the cylinder head business, you don’t have much influence over that factor other than to choose your cylinder head supplier wisely. You also don’t have control of wet flow other than making a knowledgeable purchase.
However, as an engine builder, you can have a significant influence on the induction temperature. What arrives at the cylinder is an air/fuel mixture with a range of droplet sizes from very small to overly large. Fuel separation also leads to a certain amount of fuelforming rivulets and that is really bad for the engine’s output. Fuel entering the cylinder in rivulets (and the larger droplets) produce a poor charge quality. This charge does not ignite very easily and does not become part of the combustion process until the ignitable fuel has created enough heat to vaporize the wet fuel. Indeed, some of the fuel fails to burn at all.
In practice, the presence of liquid fuel means that a certain proportion of the air/fuel mixture entering the cylinder needs to be in an easily ignitable vapor form. Without some vapor, the ignitability and subsequent burn is not very effective. In other words, because finer droplets evaporate easier, the cooler the intake charge becomes, the more finely atomized the fuel delivery needs to be. (If you are building a nitrous engine, see sidebar “Booster Signal and Nitrous” on page 74). Where power is the sole concern, the basic rule is to have low-gain boosters for heated intake manifolds and high-gain boosters for cool manifolds. If you are experimenting with thermal-barrier induction systems and/or artificially cooled intakes, be aware that without reevaluating the booster design you may not see the gains in output you hoped for.
What’s It Worth?
David Braswell and I ran some tests in the 1970s, which illustrate just how much an engine can gain or lose by failing to get the right booster for the job. For this test David built two carbs, one with high-gain boosters and one with low-gain boosters. Each was run on an engine that was first equipped with a hot manifold, then with a cold manifold. The first was an aftermarket manifold that had crossover heat applied.
The second was a single-plane race manifold with no crossover heat and was air cooled. Figure 7.10 shows a comparison between low-gain boosters delivering relatively large droplets and high-gain ones delivering considerably finer fuel atomization. With low-gain boosters delivering larger droplets, the heat applied to the manifold plenum vaporizes a nearly optimal percentage of the fuel. Tests with the heated manifold resulted in the low-gain boosters outperforming the high-gain boosters. This was because the high-gain booster’s smaller droplets resulted in too much vaporization.
Figure 7.11 shows the same two booster styles, but this time the engine is equipped with a cool-running, single-plane race manifold. Bearing in mind that this is the same engine as in Figure 7.10 you see that the carb/ booster combination that worked best on the heated intake manifold did not work so well on the significantly cooler race manifold. Note how the low-gain booster-equipped carb dropped a huge amount of lowend torque. In many instances, this is accepted as a shortcoming of the race manifold’s design. After all these are back-to-back tests of the hot manifold low-gain booster. In reality, many race intakes, such as the Holley/Weiand Strip Dominator, have much more lowspeed potential than they are often credited with. The low-speed potential is just a case of having the appropriate degree of fuel atomization. The golden rule here is: Reduce temperature, increase atomization. If you fail to do that, your cool-intake system can’t meet its full potential. Reduce that intake temperature enough with an indifferent booster design and you may find your efforts are rewarded with reduced power rather than the increase you expected.
Annular Discharge Booster Myth
I have heard it said that annular boosters are not good for a highperformance engine because they block the main venturi, but this is not so. They only appear to block the main venturi because of their bulk when viewed from above. In reality, if it is sharp, the end of the booster has no area at its installed height in the main venturi. In other words, the booster’s knife edge takes zero area away from the minor diameter of the carb’s main venturi. The test for this is on the flow bench. A well-prepped annular discharge booster costs little, if anything significant, in the way of airflow. The moral here? Don’t dismiss the annular discharge booster as a non entity for your performance Holley. There are plenty of real-world performance situations where the annular discharge booster is the best choice.
Written by David Vizard and Posted with Permission of CarTechBooks