Carburetor Tuning - Oval Track Carburetors

During the late '90s, carburetor tuning and modifying received something of a jolt when Barry Grant introduced his racing four-barrel, the Demon. The innovations included streamlined air intakes, concentric venturis, removable-booster venturis, billet-metering blocks, float bowls with sight glasses, and a billet baseplate. It's uniqueness was further exploited when the RS version (removable venturi sleeves) burst upon the racing scene. Furthermore, in an attempt to prevent ridges in the venturi bores and to obviate the potential of core shift during the casting process, the main body was formed using a method known as Concentracast.

The advantage of having smooth concentric carburetor bores was obvious, but there was a secondary prize: consistency of measurement. The selection and tuning process of the new Demon, especially the RS model, was also far-reaching. Whether prevailing conditions or, perhaps, engine size dictated a change, having the option of replaceable venturi sleeves meant that the carb could be instantly resized without being replaced.

Carburetor Selection
Small carburetors tend to provide better throttle response at lower engine speeds, and bigger carburetors are inclined to produce more power in the mid- and upper-rpm ranges. When running on a dry, slick racetrack, a large carburetor that produces less power at lower rpm may be the ideal solution for keeping the car "hooked up" on corner exits. Conversely, on a tacky racetrack, a smaller carburetor delivering lots of bottom-end power will provide strong acceleration from the corners. During the spring and fall, or when the track is sticky, some racers will qualify with a smaller carburetor, then switch to a larger model during the evening if the track becomes slick. Where restrictions apply, and the rule book only permits the use of one specific carburetor, it's advisable to have it blueprinted and flowed to obtain maximum performance.

A common misconception about carburetors is that the greater signal (vacuum) and fuel draw created by venturi boosters, the more power the engine will produce. Furthermore, since the annular-type boosters draw more fuel with less air speed than the straight-type or the drop-leg booster, why not install them in all racing carburetors? This reasoning is entirely understandable. However, although annular-style boosters function well on some applications, they can be restrictive and/or create a disproportionate fuel curve (frequently too rich at high revs) on others. In order to have a carburetor perform to its full potential, the combination of airflow and fuel metering must be of the correct proportions throughout the entire rev range.

A venturi booster with excessive draw, as indicated above, will cause the fuel curve to climb and become overly rich as engine speed increases. The result is a loss of torque and horsepower on the straightaway. Reducing the jet size, in an attempt to correct the mixture, will cause a loss of power on corner exit. The best choice of carburetor is one with scientifically shaped venturis that can flow sufficient quantities of air at higher revs, without sacrificing acceleration in the lower rpm band.

Additionally, the booster configuration and the calibration of the metering circuits must provide proportional fuel draw throughout the entire rev range of the engine. A good technician at any quality carburetion and fuel-system shop can advise as to which carburetor and venturi booster combination is best suited for a particular engine size, track length, and track condition.