Quadrajet
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==Design== | ==Design== | ||
| − | The Quadrajet is a "spread bore" carburetor; the primary [[venturi]]s are much smaller than the secondary venturis. By comparison, a "square bore" carburetor has primary and secondary venturis of similar if not exactly the same size. | + | The Quadrajet is a "spread bore" carburetor; the primary [[venturi]]s are much smaller than the secondary venturis. By comparison, a "square bore" carburetor has primary and secondary venturis of similar if not exactly the same size. |
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| + | Quadrajets made prior to 1976 were capable of 750 [[cubic feet per minute]] (CFM) maximum, but some were manufactured as 800 CFM. The increase in airflow was due to a larger primary venturi. This was first done in 1971 by the Pontiac and Buick division of GM for use on their 455 cid/high performance engines. In 1971 Pontiac had a special Q-jet designed that had the most flow of any production Q-jet, but because the modification decreased the vacuum signal on the primary side too much it was discontinued so was a "one year wonder" that fetches outrageous prices to restorers today. Many more 800 cfm Q-jets were installed from 1976-up. One sure way to locate a large casting carb is to source one from a light truck. Even the Chevy/GMC 4.3L V6 trucks used the large casting Q-jet! | ||
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| + | Most Quadrajets use a vacuum operated piston to move the primary metering rods to control the (air fuel ratio), allowing the mixture to be lean under low load conditions and rich during high load conditions. A less-common version uses a linkage driven off the primary throttle shaft to mechanically move the power piston. "E" (Electronic Control Module controlled) series of Quadrajets use a computer controlled Mixture Control Solenoid that responds to electronic signals from the throttle position sensor, ideal for precise fuel metering and allowing additional fuel under load. The solenoid-controlled metering rods allow the fuel mixture to be very close to optimum, then the solenoid is [[Pulse-width modulation|pulse width modulated]] at about 6 Hz to fine-tune the air fuel ratio under closed loop conditions. The electronic versions have a throttle position sensor that is mounted inside the carburetor body, actuated by the accelerator pump lever. | ||
Quadrajet carburetors have mechanical secondary throttle plates operated by a progressive linkage (primaries open before secondaries) but use "on-demand" air valve plates above the secondary throttle plates. The air valves are connected by a cam and linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts the secondary metering rods. The secondary rods are tapered in a similar fashion to the primary metering rods, effectively increasing the size of the fuel metering holes as the rods are lifted and delivering more fuel. Therefore, the position of the air valve will control both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. The end result is that the Quadrajet acts like a "vacuum secondary" carburetor and only delivers more fuel as it is needed. | Quadrajet carburetors have mechanical secondary throttle plates operated by a progressive linkage (primaries open before secondaries) but use "on-demand" air valve plates above the secondary throttle plates. The air valves are connected by a cam and linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts the secondary metering rods. The secondary rods are tapered in a similar fashion to the primary metering rods, effectively increasing the size of the fuel metering holes as the rods are lifted and delivering more fuel. Therefore, the position of the air valve will control both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. The end result is that the Quadrajet acts like a "vacuum secondary" carburetor and only delivers more fuel as it is needed. | ||
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