Big carb, little carb, in-between...
Big carb, little carb, in-between carb-which one do you run? The five shown here range from a 625 Road Demon to the annular booster 950 Race Demon drag-race special.
There's an old saying in the racing world: If some is good, too much is just enough. In the world of four-barrel performance carbs, the first question is how big should you go. The bottom line in power production is airflow, and the more airflow an engine has, the more potential there is to make power. Does this mean all it takes to put really big numbers on the board is bolting on the biggest carb you can get your hands on? No. Carb capacity has to be matched to the airflow capacity of the rest of the engine's flow system, including the heads, intake, exhaust, and camshaft specs. While it's true that too small a carb on an otherwise airflow-hungry engine will soak top-end power, the other side of the equation is that too large a carb will make for a larger venturi (the opening where the fuel enters the air to create an atomized mixture), which in turn will reduce air speed (or velocity) through the carb's booster, hurting low-end response and atomization.
Carburetors are rated by an airflow number: 650, 750, 850, and so on. The number is the carb's flow capacity, rated by number of cubic feet per minute (cfm) of air that will go through the carb when wide open. This air just won't go flying through the carb by itself, however; there must be a pressure differential, with less pressure on the throttle side of the carb than on the air-cleaner side for flow in the normal direction to occur. With a huge pressure differential, a correspondingly large volume of air will flow through the carb. Conversely, if the pressure differential is zero, no air will go through.
We ran all the carb tests...
We ran all the carb tests on our solid street roller, Edelbrock-headed 440, feeding into Westech's Superflow dyno to see what they would do. Headers are tti 2 inch.
To provide meaningful comparative ratings on carb airflow, a standard pressure drop was needed to rate the cfm of carburetors. The industry adopted a standard of 1.5-inch Hg pressure drop across the carb for rating four-barrel carbs and a higher figure of 3.5-inch Hg pressure drop to rate two barrels. The standards are different for the two carb configurations based on a belief that these twin pressure drops most accurately represent the operating conditions found in passenger car engines under running conditions. In actual use, the engine provides the pressure differential to prompt airflow through the carb via the action of the pistons and the valves. A performance engine creates a certain amount of airflow based on its displacement, volumetric efficiency, and maximum rpm of operation. There are formulas for calculating the engine's airflow requirements based on these parameters, which will give a good ballpark of the carb capacity required, but is that the end of the story?
For example, let's take a rebuilt .030-inch-over 440 Chrysler big-block. Plugging into a pumping efficiency formula, we get an airflow requirement of 774 cfm based upon a displacement of 446 ci at 6,000 rpm, assuming we have a highly developed performance engine with a volumetric efficiency of 100 percent. Therefore, the mathematical airflow demand of the engine can be delivered by a 774-cfm four-barrel carb operating under a peak manifold vacuum of 1.5-inch Hg at WOT. Maintain that 1.5-inch Hg atmospheric depression in the intake and the 774-cfm carb will deliver the airflow demanded. However, a larger carb will satisfy this airflow demand, yet at the same time allow the manifold's peak vacuum level to drop to less than 1.5-inch Hg and likely result in the engine aspirating more air and producing more peak power.
Steve Brule, Westech's dyno...
Steve Brule, Westech's dyno operator, was the key to making this test work, interpreting the info from the dyno so the air/fuel ratio of each carb could be calibrated for max power.
How much larger can you go before drivability suffers? That depends on the intended use and ultimately the weight and driveline in the car. Steep rear gears and a four-speed or high-stall converter in a lighter car is the most forgiving situation favoring the larger carb choice. Conversely, a heavier, higher-geared car used mainly for the streets and seldom wound out to peak rpm will typically be happier with a more conservative carb choice