Editing Quadrajet
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==Overview== | ==Overview== | ||
− | The '''Quadrajet''' is a 4 barrel carburetor made by the Rochester Products Division of GM that was first used on the Chevrolet 396 Mark IV engine in 1965 then widely used on General Motors motor vehicles until 1990. Its last application was on the Oldsmobile 307 V8 engine, which was last used in the Cadillac Brougham and full size station wagons made by Chevrolet, Pontiac, Oldsmobile, and Buick. Not widely known, the Q-jet was used briefly by Ford on the 1971 429 Cobra Jet and was also used by | + | The '''Quadrajet''' is a 4 barrel [[carburetor]] made by the [[Rochester Products Division]] of GM that was first used on the Chevrolet 396 Mark IV engine in 1965 then widely used on [[General Motors]] motor vehicles until 1990. Its last application was on the [[Oldsmobile V8 engine#307|Oldsmobile 307]] V8 engine, which was last used in the [[Cadillac Brougham]] and full size [[station wagons]] made by [[Chevrolet]], [[Pontiac]], [[Oldsmobile]], and [[Buick]]. Not widely known, the Q-jet was used briefly by Ford on the 1971 429 Cobra Jet and was also used by Mopar on their 360 truck engines in the '80's. |
− | The Quadrajet has been described as one of the best carburetors made for passenger vehicles, offering the best compromise of fuel economy, performance, driveability, and | + | 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}} |
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+ | [[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== | ||
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− | + | 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. | |
− | In 1971 Pontiac had | + | Most Quadrajets made prior to 1976 were capable of 750 [[cubic feet per minute]] (CFM) maximum air flow measured at a 1.5"/Hg pressure drop, but some were manufactured as 800 CFM. The increase in airflow was due to a larger primary bore and 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 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! |
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+ | [[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 "power piston" | + | |
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− | There were also two | + | |
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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. | ||
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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. | ||
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==Drawbacks== | ==Drawbacks== | ||
Significant negative features of the Quadrajet were: | Significant negative features of the Quadrajet were: | ||
− | + | # Its leaky fuel bowl. As in nearly all carburetors, the Quadrajet bowl had pressed-in plugs used to seal holes left after drilling fuel passages during the manufacturing of the carburetor. These plugs in the Q-jet sometimes leaked fuel causing: (a) a cold engine being hard to start, (b) erratic idling, (c) poor fuel mileage, and (d) excessive emissions. Many Quadrajets have their fuel bowl plugs sealed with [[epoxy]] when rebuilt to prevent leaks. This may be good for a short time but epoxy will not stand up to the extreme conditions encounterd for long. Fortunately, leaking well plugs is primarily a problem only on the first few years of production when pressed in cup plugs were used instead of the later (and better) spun-in aluminum plugs that were swaged in place. | |
− | + | # The very small float bowl/fuel chamber can result in fuel starvation in extreme high-performance situations, but can usually be traced to a fuel delivery problem to the carburetor, such as a worn fuel pump, pump push rod, or camshaft eccentric. In normal driving and mild performance use it provides excellent control of fuel level. | |
− | + | # The fuel inlet/fuel filter housing threads tend to be very fragile. It is common for the fuel inlet threads in the main casting to strip. There are several "fixes" available in the aftermarket: New oversize self-tapping [[fuel filter]] housings; new fuel filter housings that seal with [[O-rings]]; and [[Heli-Coil]] rethreading kits. | |
− | + | # Almost all Quadrajets today have some amount of warpage of the castings. The root cause of this warpage is over-tightening the front two carburetor mounting bolts, often in combination with a base gasket that doesn't have hard [[nylon]] inserts for the bolt holes. | |
− | + | # The steel primary throttle shaft will tend to wear the aluminum casting material. This results in an air leak and in extreme cases could cause the primary throttle blades to not close properly. This results in poor idle quality. The [[aftermarket]] has responded; several vendors are supplying bushing repair kits for the Q-jet throttle body. | |
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− | == | + | ==Variants== |
− | A major change to the Quadrajet was implemented for the 1975 model year. These newer carburetors are considered "Modified Quadrajets" or "Mod Quads". In addition to the casting revisions that | + | A major change to the Quadrajet was implemented for the 1975 model year. These newer carburetors are considered "Modified Quadrajets" or "Mod Quads". In addition to the casting revisions that result in a physically larger carburetor, the primary metering rod length is different from '74 and older Q-Jets. They were also equipped with a self contained choke mechanism that no longer relied on an intake manifold mounted choke, and a number "1" was added to the beginning of their identification numbers. |
− | 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- | + | 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 to be made 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-Jet carbs have been discontinued, although at this time Edelbrock still supplies some tuning and replacement parts. [[Quadrajet#Power enrichment|Return to top]] |
===So, all Q-jets are the same, right?=== | ===So, all Q-jets are the same, right?=== | ||
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Not hardly! These are a few differences: | Not hardly! These are a few differences: | ||
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There are two different length primary rods, there are stepped and tapered rods. The bottom line to this is 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. | There are two different length primary rods, there are stepped and tapered rods. The bottom line to this is 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. | ||
− | ==Choosing a | + | == Choosing a Carb == |
You need to pay attention to a few important things when choosing a carb to rebuild/use on your engine: | You need to pay attention to a few important things when choosing a carb to rebuild/use on your engine: | ||
− | *Fuel inlet (straight or 90º), so it doesn't interfere | + | *Fuel inlet (straight or 90º), so it doesn't interfere w/any of the accessories or water neck, etc. |
− | + | *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. | |
− | + | *Choke type. Most prefer an electric. A hot air type choke can be converted to electric easily. | |
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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 | + | |
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− | Beyond that, you basically just work | + | 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. |
− | 1976-up carbs are often preferred due to less wear than the older carbs, often an | + | 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). |
− | + | 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. | |
− | + | 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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==Tuning== | ==Tuning== | ||
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GM p/n 7029922 is a rather weak spring that was OEM for HO Pontiac engines from the early '70's. It delays enrichment until vacuum drops to 3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050”. Another GM p/n is 7037305 for a PP spring that is set for 6 in/Hg, and would work well w/a high vacuum, smooth idling and/or wide LSA-type cam. Thing is, unless the parts counter guy is cool w/you, they come in packs of 10 springs. | GM p/n 7029922 is a rather weak spring that was OEM for HO Pontiac engines from the early '70's. It delays enrichment until vacuum drops to 3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050”. Another GM p/n is 7037305 for a PP spring that is set for 6 in/Hg, and would work well w/a high vacuum, smooth idling and/or wide LSA-type cam. Thing is, unless the parts counter guy is cool w/you, they come in packs of 10 springs. | ||
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===Secondary metering hangars and rods=== | ===Secondary metering hangars and rods=== | ||
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[[File:Secondary rods 002.jpg|thumb|left|250px]] | [[File:Secondary rods 002.jpg|thumb|left|250px]] | ||
<br style="clear:both"/> | <br style="clear:both"/> | ||
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==Intake manifolds== | ==Intake manifolds== | ||
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===Tuning and repair parts=== | ===Tuning and repair parts=== | ||
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− | + | [http://www.cliffshighperformance.com/index.html Cliff's High Performance] <br />[http://quadrajetparts.com/index.php QuadrajetParts.com] <br />[http://www.carburetion.com/index.htm Carbs Unlimited] <br />[http://www.carbkitsource.com/ The Carburetor Doctor] | |
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==References== | ==References== | ||
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==See also== | ==See also== | ||
− | + | Hotrodders Knowledge Base: [http://www.hotrodders.com/kb/carburetor-articles Carburetor articles] - Contains links to various useful carburetor websites and threads. | |
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+ | ==External links== | ||
+ | [http://4x4mecca.com/upload/demouploads/Quadrajet_Service_Manual_1981.pdf Q-jet service manual] | ||
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− | + | [[Category:Automotive technology tradenames]] | |
[[Category:Engine]] | [[Category:Engine]] | ||
− | [[Category: | + | [[Category:Quadrajet]] |
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