![]() By installing a smaller (booster) venturi into the main venturi as shown in Figure 7.2 you get a certain effect. 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). The greater the airflow through the venturi, the greater the pressure drop at the minor venture. ![]() As the air expands in the exit area of the venturi, it slows and a pressure recovery takes place. When this happens, the air pressure drops (blue line). The red curve on the graph below the venturi illustration depicts the increased air speed. As the air passes through the venturi’s minor diameter it speeds up. The suction (partial vacuum) of the engine draws air through the venturi. ![]() 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.Ī booster’s function hinges on the events that take place as air is drawn through a venturi. But in this book, I begin dissipating the fog of confusion. To many performance enthusiasts, booster science looks more like booster black art. Even to this day, much of that knowledge remains in the possession of only a few professionals within the industry. When Holley seriously committed to racing in the 1960s, it became apparent that making bigger, highflow carbs also meant increasing booster design research. It is always a case of selecting whatever is best for the job at hand. Also there is no universally “best” booster. 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. Again, if you have a working understanding of boosters, it can put you several steps ahead of your competition at the race track. 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. It takes some time and inclination to learn about and properly set up a carb. 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. Note the big difference between the lowest and highest. This graph shows the signal strength for each of the booster styles depicted in Figure 7.4. But the reality is that carb manufacturers (in this case Holley) make that statement to simplify carb calibration for the typical consumer. I realize much of this flies counter to conventional wisdom. 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. 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. You can copy and paste this link to share: SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. For a comprehensive guide on this entire subject you can visit this link: LEARN MORE ABOUT THIS BOOK HERE This Tech Tip is From the Full Book, DAVID VIZARD’S HOW TO SUPER TUNE AND MODIFY HOLLEY CARBURETORS. But just how much “too big” is depends greatly on how effective the boosters are. This, in turn, compromises combustion efficiency. 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. With a fixed-jet carb the situation changes. 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. Therefore, at low speed, it is a small carb at high speed, it is a large carb. ![]() As an example, the constant vacuum of an SU pulls the air valve open just far enough to satisfy the engine’s air demand. Although a slight exaggeration, it’s not as far off reality as you might think. 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. Having an understanding of boosters can put you in the position of being able to enhance high-RPM output without sacrificing low-speed output. The design of the booster can have dramatic effects on the engine’s output and fuel consumption.
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