Quadrajet

The Quadrajet has been described as one of the best carburetors made for passenger vehicles, offering the best compromise of [[Fuel economy in automobiles|fuel economy]], performance, driveability, and [[automobile emissions control]].{{Fact|date=July 2007}}

The Quadrajet has been described as one of the best carburetors made for passenger vehicles, offering the best compromise of [[Fuel economy in automobiles|fuel economy]], performance, driveability, and [[automobile emissions control]].{{Fact|date=July 2007}}

[[File:Q-JET 750 TO 800 CFM VENTURI COMPARISON.jpg|thumb|100px|Comparison of 750 cfm and 800 cfm Q-jet primary venturi/throttle bores.]]

[[File:Q-JET 750 TO 800 CFM VENTURI COMPARISON.jpg|thumb|100px|Comparison of 750 cfm and 800 cfm Q-jet primary venturi/throttle bores.]]

==Basic design==

==Basic design==

The Quadrajet is a "spread bore" carburetor; the primary [[venturi]]s are much smaller than the secondary venturis, and the center to center spacing of the throttle bores are different, primary to secondary. By comparison, a "square bore" carburetor may have primary and secondary venturis of similar if not exactly the same size and the center to center spacing of the primaries and secondaries are the same, thus the name "squarebore".   

The Quadrajet is a "spread bore" carburetor; the primary [[venturi]]s are much smaller than the secondary venturis, and the center to center spacing of the throttle bores are different, primary to secondary. By comparison, a "square bore" carburetor may have primary and secondary venturis of similar if not exactly the same size and the center to center spacing of the primaries and secondaries are the same, thus the name "squarebore".   

In 1971 Pontiac had designed a special Q-jet 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 it was a "one year wonder" that fetches outrageous prices from restorers today. Many more 800 cfm Q-jets were installed from 1976-up. One way to locate a large casting carb is to source one from an '80-up light truck. Even the Chevy/GMC 4.3L V6 trucks used the large casting 800 cfm Q-jet!

In 1971 Pontiac had designed a special Q-jet 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 it was a "one year wonder" that fetches outrageous prices from restorers today. Many more 800 cfm Q-jets were installed from 1976-up. One way to locate a large casting carb is to source one from an '80-up light truck. Even the Chevy/GMC 4.3L V6 trucks used the large casting 800 cfm Q-jet!

[[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|300px|Early APT, ca. 1975]]

[[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|300px|Early APT, ca. 1975]]

==Power enrichment==

==Power enrichment==

Most Quadrajets use a vacuum operated piston (the "power piston" or "PP") to move the primary metering rods to control the air/fuel ratio, allowing the mixture to be lean under low load/high vacuum conditions and rich during high load/low vacuum conditions.  A less-common, early version uses a linkage driven off the primary throttle shaft to mechanically move the power piston.  There were also two Q-jet designs used mainly in 1975 only, that used an auxiliary enrichment system in addition to the primary PP system. There are two types of auxiliary systems: one has an aneroid "bellows" that responds to barometric pressure changes; it used no vacuum or power piston to operate it. The other uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one jet to feed both sides of the primaries. These were abandoned soon after they were released; it was found the primary PP system could be tailored to meet the requirements without the added complexity of the auxiliary system. Other changes came on-line in that same era, info on this can be seen at [[Quadrajet#Variants|Variants]].  

Most Quadrajets use a vacuum operated piston (the "power piston" or "PP") to move the primary metering rods to control the air/fuel ratio, allowing the mixture to be lean under low load/high vacuum conditions and rich during high load/low vacuum conditions.  A less-common, early version uses a linkage driven off the primary throttle shaft to mechanically move the power piston.  There were also two Q-jet designs used mainly in 1975 only, that used an auxiliary enrichment system in addition to the primary PP system. There are two types of auxiliary systems: one has an aneroid "bellows" that responds to barometric pressure changes; it used no vacuum or power piston to operate it. The other uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one jet to feed both sides of the primaries. These were abandoned soon after they were released; it was found the primary PP system could be tailored to meet the requirements without the added complexity of the auxiliary system. Other changes came on-line in that same era, info on this can be seen at [[Quadrajet#Variants|Variants]].  

The "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 to the ECM, 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.

The "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 to the ECM, 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 secondary fuel metering rods are lifted by a cam-actuated hangar connected to the air valve shaft.  As the airflow increases through the secondary bores, the air valves are pushed down (open), rotating a cam that lifts a hangar that holds 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, thus delivering more fuel.  Therefore, the position of the air valve controls 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 air/fuel if 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 secondary fuel metering rods are lifted by a cam-actuated hangar connected to the air valve shaft.  As the airflow increases through the secondary bores, the air valves are pushed down (open), rotating a cam that lifts a hangar that holds 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, thus delivering more fuel.  Therefore, the position of the air valve controls 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 air/fuel if it is needed.