For high performance six-cylinder and V8 engines, Technocarb used to offer a variety of 4bbl venturi carburetor kits. Venturi carburetors do not have a restrictive air-gas valve impeding flow. Instead, air flow through the carburetor's venturis creates a vacuum inside the carburetor that causes fuel to flow from the converter. A control valve in the fuel supply line to the carburetor maintains the correct fuel mixture. Because the vacuum signal from the venturis is generally too weak at idle for the control valve to work properly, there is a separate idle fuel mixture adjustment that controls the low air-flow fuel mixture.
The venturi carburetor is available in both open-loop and closed loop configurations. The open-loop Technocarb 4bbl carburetor kit is similar in price to a Model 425 carburetor kit and a Basic Component Package. Because the closed loop kit includes the cruising and idle mixture controllers, the O2 sensor, the wiring harness, and the Scan Tool, the closed loop kit was more expensive than the open loop kit. However, the cost of fuel over the life of the vehicle is many times more expensive than the additional cost of the electronics in the close loop kit. Even though the Technocarb 4bbl carburetor system appeared to be significantly more expensive than an Impco Model 425 system, it was much cheaper in the long run. For performance applications, it would take multiple Impco mixers to come close to the performance of the Technocarb 4bbl carburetor.
Beside the above standard configurations, it was possible to mix venturi sizes in a Technocarb carburetor. This was especially useful in applications where a hot cam affects venturi vacuum at idle. In this case, Technocarb has often put a smaller pair of venturis in the primary side of the carburetor. This is a bit like turning this squarebore carburetor into a spreadbore carburetor. You could have either sent your carburetor back to Technocarb for this work or bought a new pair of venturis and do it yourself. Technocarb changed out venturis at a very nominal charge to their customers. For the time being, Technocarb 4bbl carbs are out of production.