Quadrajet
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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. | 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== | ||
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| + | '''Power Piston Springs''' | ||
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| + | The power piston (PP) spring is seldom mentioned when Q-Jet tuning is being discussed, but it need to be addressed just like if it were a Holley. The same principals apply when tuning for low vacuum cams, etc. The power piston spring allows the power piston to behave like a Holley's power valve. That's to say the power enrichening system is vacuum controlled. | ||
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| + | At high vacuum, the vacuum exceeds the PP spring rate and the primary rods are pulled down into the main jets, leaning the fuel/air mixture. Conversely, when vacuum drops like when under a load or the accelerator pedal is whacked WFO, the PP spring rate exceeds the vacuum, which lifts the rods up to their smaller diameter "rich" portion. | ||
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| + | If a “medium” PP spring = a Holley 6.5 in/Hg, a "soft" PP spring (allows enrichening to come in at a lower vacuum) would be like a 3.5 Holley PV; a “stiff” PP spring = a Holley PV of, say, 8.5 in/Hg. Longer duration cams will “like” a softer PP spring. | ||
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| + | Below is an image showing a variety of different springs, another shot shows three different types of power piston. The springs vary in length, coil diameter and wire diameter (0.012"- 0.020" in my collection). | ||
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| + | Edelbrock has different PP springs in a "Race Calibration Kit" that contains a selection of main jets and rods, secondary rods, hangars, PP springs, high flow needle and seat and an accelerator pump. Gessler and Ruggles both carry a selection of PP springs as well as many other tuning and repair parts. | ||
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| + | GM part # 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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==References== | ==References== | ||
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