Quadrajet
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==Design== | ==Design== | ||
| − | The Quadrajet is a "spread bore" carburetor; the primary [[venturi]]s are much smaller than the secondary | + | The Quadrajet is a "spread bore" carburetor; the primary [[venturi]]s are much smaller than the secondary venturous. By comparison, a "square bore" carburetor has primary and secondary venturis of similar if not exactly the same size. Most Quadrajets were capable of 750 [[cubic feet per minute]] (CFM) maximum, but some were manufactured as 800 CFM for use on high performance engines. 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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Quadrajet carburetors were also built under contract by [[Carter]]. This seems to have happened at times when Rochester's facility could not keep up with demand. Carter-built Quadrajets will have the name "Carter" cast into them, but are functionally identical to the Rochester-built equivalent. The "newest" Q-Jets were built for, and sold by [[Edelbrock]]. There were several versions made, for both stock replacement and "performance" applications. One version was specifically intended as a replacement for Carter Thermoquad carburetors. The Edelbrock Q-Jets have been discontinued, although at this time Edelbrock still supplies some replacement parts. | Quadrajet carburetors were also built under contract by [[Carter]]. This seems to have happened at times when Rochester's facility could not keep up with demand. Carter-built Quadrajets will have the name "Carter" cast into them, but are functionally identical to the Rochester-built equivalent. The "newest" Q-Jets were built for, and sold by [[Edelbrock]]. There were several versions made, for both stock replacement and "performance" applications. One version was specifically intended as a replacement for Carter Thermoquad carburetors. The Edelbrock Q-Jets have been discontinued, although at this time Edelbrock still supplies some replacement parts. | ||
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| + | You need to pay attention to a few important things when choosing a carb to rebuild/use on your engine: | ||
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| + | • Fuel inlet (straight or 90º), so it doesn't interfere w/any of the accessories or water neck, etc. | ||
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| + | • Throttle arm- whether or not it can accommodate the trans you're using. This is important if using a TH350 that needs a "below-the-shaft" hook up point to pull the detent cable. | ||
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| + | • Choke type. Most prefer an electric. A hot air type choke can be converted to electric easily. | ||
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| + | Beyond that, you basically just work w/what you have. The Q-jet is very versatile and will work fine on any number of engines. | ||
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| + | 1976-up carbs are often preferred due to less wear than the older carbs, often an electric choke will be used, the float and needle and seat assembly set up is better. Also because there were improvements made throughout the run of Q-jets, newer is better. The '76-up carbs will also have APT (adjustable part throttle, a way to fine tune). | ||
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| + | Interestingly, one of the best/easiest way to find a good carb to use is to look for one off of a truck or van. Up until '86, these truck carbs were still non feedback (not an "electronic" carb) in some applications and they were nearly all 800 cfm units- even those found on 4.3L V6 engines! These engines can also have the large coil-in-cap, non-feedback HEI distributors. | ||
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| + | Cars from '81-up are going to have feedback carbs. No good for anything other than computer use (CCC), or possibly as an all-out race carb where it is set up to run w/o primary metering rods. | ||
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| + | So, all Q-jets are the same, right? | ||
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| + | Not hardly! These are a few differences off the top of my head. In no way are these '''all''' the differences: | ||
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| + | Generally, there were the ‘small’ and ‘large’ casting carb bodies. These were 750 and 800 cfm, nominal- other cfm requirements were met by tailoring the air valve and/or the secondary throttle stop. Then there was the rare (one year-‘71) Pontiac HO carb that used a smaller booster to get even more flow than the “normal” large casting (800 cfm) carb. | ||
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| + | There were straight or 90° fuel inlets, w/short and long filter housings. There were at least 4 different choke arrangements. There were 5 different idle mixture screws. 4 different float pivot pins, 5 different float bowl inserts. There were both allen head and straight blade screw heads used to secure the aiur horn. | ||
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| + | There were two different needle and seat arrangements regarding the type, and at least 3 different floats made from 2 different materials (nitrophyl and brass. | ||
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| + | Later carbs (~’76-up) have APT (aka “adjustable part throttle). Some have idle air bypass, others don’t. Most had only two primary rods, some had three- one hooked to an aneroid. Sort of like 2 power pistons. | ||
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| + | Various carbs had different bowl vents, some had a hot air compensator. Many had secondary fuel booster outlets (passive accelerator pump) that were located above the air valve, others below (below better for high performance). Some secondary air valves were slotted, others not. | ||
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| + | Accelerator pump pistons varied by length and spring rates (at least 6 different), inlet seats were different sizes (at least 6 different sizes available, 0.095” to 0.149” [0.149” is aftermarket only]), power piston spring rates varied (10 different OEM springs, others available through the aftermarket), obviously jets (at least 13 different OEM sizes, from #64 to #78), primary metering rods (15 different ‘65-’67 rods and 18 different ‘68-up rods. Sizes range from 0.033” to ~0.060”), secondary metering rods (93 to select from), secondary hangars (20 different p/n’s) varied between carb applications. There are [b]millions[/b] of different combos possible for the ‘68-up carb alone! And this doesn't even consider all the different linkages, choke pull-offs. the two different CFM ratings, etc. ad infinitum. | ||
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| + | Throttle linkages varied greatly- some were made for manual trans apps, these can be used w/a TH400 if the kickdown switch is relocated (or is at the throttle pedal), some were only for a TH400 apps w/carb-mounted kickdown switch, some were used w/the TH350 (has the ability to "pull" a detent cable), some for the Powerglide trans. Some have cruise control studs, some not. Early carbs used throttle linkage, later used a cable. | ||
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| + | There were at least 20 different front choke pull offs, at least 10 different rear choke pull offs, carbs with front only or both front AND rear choke pull offs. Vacuum port size, number and locations were all over the place. | ||
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| + | There are two different length primary rods, there are stepped and tapered rods. | ||
==References== | ==References== | ||
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