4/9/2023 0 Comments Air venturi power boosterNormally, an engine with such a conservative spec would have been equipped with a 600- to 650-cfm carb. If you are into big-blocks, just think what a useful piece a vacuum secondary Dominator would be!To see by how much we could cheat the normally accepted "too big a carb" deal, a basic stock cam/valvetrain 350 small-block Chevy equipped with pocket-ported production line 186 head castings and dyno headers was tested with a Victor Jr. If you like that idea, then the good news is it is readily available and is commonly known as a vacuum secondary carb. Such a deal just won't make it at the low end, but how about a variable-cfm carb? That would be small at low rpm and big at high rpm. All too often, street rodders make the mistake of assuming if the racers use it they should also do so and that usually means big cfm mechanical secondary carbs. Getting an adequate booster signal by utilizing a high-gain design is not the only way to get your four-barrel carbed engine to perform both at the top and bottom ends of the spectrum. 3 are tested in one barrel of an 850-cfm Holley carb. 4 will give a good prospective of the difference in signal strengths when the five booster styles shown in Fig. 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 booster with all the casting flash removed and a clean-up on the entry and exit delivers an amplified signal nearly four times that of the main venturi. These are shown in approximate order of gain. 3 shows the characteristic form of the main variants. Over the years, Holley has laid down some sterling groundwork in terms of booster design. In other words, it had to take a relatively small signal as generated at the minor diameter of the main venturi and amplify it into a strong, useable signal for the purposes of metering and atomization. ![]() Before Holley could introduce the Dominator series of carbs with its big barrel sizes, the company had to come up with a booster design that produced high gain. The bigger the carb becomes, the more critical the booster design becomes if the carb is to operate over any acceptable rpm range. This inevitably calls for a bigger carb than would be required if power at low and mid speed was the main criteria. If either one of these factors are off by too much, power output suffers.īooster GainMaximum horsepower calls for a carb with enough airflow to completely satisfy the engine's need at peak rpm. The importance of the pressure drop is that it is the signal that is used by the carb's fuel system to not only meter the amount of fuel for a given amount of air, but also to produce sufficient atomization for the fuel to burn effectively. So, now you know why it's called a booster. In other words, the booster has amplified, or as we say "boosted," the pressure drop from P2 to P3. This means the air at P3 is going even faster than that at P2, so the pressure drop at P3 will be even higher. This is because it is being sucked in not by the smaller pressure drop at P1, but by the much bigger pressure drop at P2. ![]() By having the exit point of a smaller venturi at the point of lowest pressure of the main venturi, the amount of air going through the smaller (booster) venturi will be faster. As you can see, the pressure drop (P2) at the smallest point of the main venturi is more than the pressure drop (P1) caused by the cylinder's draw. 1 and insert a smaller venturi with its exit end precisely at the smallest diameter of the larger venturi (Fig. Now, let us take that same venturi shown in Fig. ![]() Now we come to the key point of understanding how a booster works: the volume of air flowing through the venturi depends on the amount of suction (pressure drop) there is at its exit end. The greater the airflow through the venturi, the greater the pressure drop at the minor venturi. As the air expands in the exit area of the venturi, it slows, and a pressure recovery takes place. When this happens, the pressure of the air drops in the manner shown by the blue line. ![]() As the air passes through the minor diameter of the venturi, it speeds up, as depicted by the red curve on the graph. What you see here is air being drawn through the venturi by the suction (partial vacuum) of the engine. Venturi ActionThe entire function of a booster hinges on what happens when air is drawn through a venturi.
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