Competition Fuel Systems


The Brains of a Carburetor

For fuel metering, the items in the picture are major components of the “brains” of a Holley carburetor. The needle and seat assemblies introduce the right amount of fuel into the bowls to maintain the proper fuel levels through the entire RPM range, idle to peak RPM. The venturi boosters develop a vacuum “signal” that pulls fuel from the bowls through the metering blocks, and into the airflow that is being drawn into the engine. The air bleeds control how much air is introduced into fuel while it is still in the metering blocks, forming a fuel/air “emulsion” that helps the fuel vaporize when it is mixed into the airflow by the boosters. The fuel/air ratio that the carb creates is controlled by the interaction of all of these carb components. One of the most important characteristics of the boosters is their “signal curve.” This curve describes how the booster vacuum “signal” changes as the airflow through the venturis goes up or down.


A booster’s signal curve is described by a mathematical equation. The signal curve of any booster can be analyzed mathematically to see how close it comes to the “perfect” curve described by the equation. Some of the stock Holley boosters have a near perfect signal curve, but only in a small range of venturi sizes. These boosters can be modified to have better signal curves over a wider airflow range. This is an important aspect in getting an engine to run well in a broad RPM range.

Most carb modifiers, even though they have heard the term “booster signal curve,” do not know what it is, so they ignore it. A few have figured out how to measure it, but don’t know how to analyze their data mathematically to see how good or bad the curve is. Even fewer have figured out how to do the mathematical analysis. Very few have figured out how to modify boosters to make their signal curves follow the equation. Most modifiers assume that any booster in any venturi size has a manageable signal curve. Not so, not even close.

In 1985 I figured out how to measure and analyze booster signal curves, and started a long learning process on how to modify boosters to get better signal curves, and how to calibrate carbs for booster curves that range from near perfect to far from perfect. The “far from perfect” signal curves are usually encountered when racing class rules give you no choice of booster type, and allow few if any modifications. This learning process led to an understanding of how to modify boosters not only for better signal curves, but also for better carb airflow and fuel vaporization. It’s easy to modify boosters for increased airflow. It’s much harder to do that when you want to have a good signal curve and good fuel vaporization at the same time. At C.F.S., booster modifications are always done with all three of these factors in mind. The benefit of this approach is accurate control of air/fuel ratios over the entire RPM range of the racing engine. The result is more power, a broader power curve and safer engine operation.   © Norm Schenck


520-241-2787          Competition Fuel Systems, Inc.   Vassar, Michigan