Q: How do I know what size power valve to use?
Holley Performance responds: The power-enrichmentsystem supplies additional fuel to the main system during heavy-load orfull-power situations. Holley-style carburetors utilize avacuum-operated power-enrichment system and a selection of power valvesis available to "time" this system's operation to your specific needs.
Each Holley power valve is stamped with a number designating the vacuumopening point. For example, a "65" indicates that the power valve willopen when engine vacuum drops to 6.5 in-Hg or lower. An accurate vacuumgauge should be used when determining the correct power valve to use.
A competition or race engine with a long-duration high-overlap camshaftwill have low manifold vacuum at idle speeds. If the vehicle has amanual transmission, take the vacuum reading with the engine warmed upand at idle. If the vehicle is equipped with an automatic transmission,take the vacuum reading with the engine warmed up and idling in gear. Ineither case, the power valve selected should have a vacuum opening pointabout 2 in-Hg below the intake-manifold-vacuum reading taken.
A stock engine, or one that is only mildly built for street use, willhave high manifold vacuum at idle speeds. To determine the correct powervalve, drive the vehicle at various steady speeds and take vacuumreadings. The power valve selected should have an opening point about 2in-Hg below the lowest steady-speed engine vacuum observed.
Most popular Holley performance carburetors incorporate apower-valve-blow-out protection system. A special check valve is locatedin the throttle body expressly for this purpose. This check valve isdesigned to be normally open but will quickly seat to close off theinternal vacuum passage when a backfire occurs. Once closed, the checkvalve interrupts the pressure wave caused by the backfire, thusprotecting the power valve.
Q: How much clearance should I have from the top of the venttube to the lid of the air filter?
Edelbrock responds: A minimum of 3/4 to 1 inch isrecommended for proper airflow.
Q: How do an air door, an air-valve secondary, and vacuumsecondaries differ?
Edelbrock responds: An air door is typically acounterweighted set of throttle blades that operate separately from theactual throttle blades of the carburetor but is located inside thecarburetor body. It is only accessible by removing the top of thecarburetor and is not easily adjustable. The purpose of an air door isto allow a smooth transition from the primary circuit into the secondarycircuit. Rather than giving 100 percent of the secondary's "flow" all atonce to the engine, the counterweighted air door opens gradually as thedraw from the engine increases with rpm, therefore slowly allowing moreair and fuel into the engine.
An air valve operates pretty much the same way as an air door, but it isnot located inside the carb, and it is adjustable. This is aspring-loaded air valve, or "flap" that sits in the top portion of theair horn of the carburetor, and you can adjust the amount of tension onthe valve by means of a screw. Making the tension tighter brings thesecondary's full potential in later in the rpm range, and loosening thetension brings it in earlier.
Vacuum secondaries typically only apply to Holley-style carburetors.This is a diaphragm with a spring inside that sits off to the side ofthe carburetor but is still part of the carburetor and operates thesecondaries using the engine's manifold vacuum. It is adjustable bymeans of a spring that can be removed and replaced inside the diaphragm.
Q: Why is it necessary for a blower carb to be boostreferenced?
Holley Performance responds: A carburetor does notsense the vacuum of the engine when there is a supercharger between thecarb and engine. This lack of vacuum causes the power valve to remainopen. Referencing the power valve to the lower intake allows the powervalve to operate properly, giving precise fuel metering as you wouldachieve without the supercharger. If the power valve is not referencedto the lower intake, it can be sucked shut from the high vacuum signalfrom the blower. This will cause the engine to run 6 to 8 jet sizes toolean, possibly causing engine damage.
Q: How is jet size determined and why do two jets with the samehole sometimes flow differently? Can I just drill jets to a biggersize?
Holley Performance responds: There is a basicmisconception that size alone determines jet flow characteristics. Thisis not the case because the shape of the jet entry and exit, as well asthe finish, affects flow. Holley jets are finished-drilled in anenvironmentally controlled room so tolerances are as close as possibleeach time a set of jets are run. Holley takes into consideration the jetentry, exit, and finish as a constant variable. Samples are taken atvarious times during each jet run to be sure the machines are holdingtolerances and flow. These hole sizes are measured in thousandths andthe jet part number generally matches the hole size--give or take onethousandth--on jet sizes up to 70. For example, a PN 122-44 jet has ahole size of 0.044 inch. Above a 70 jet, the hole sizes aresignificantly larger than the number.
Holley has allowable tolerances for each standard jet size, whichexplains why some jets with the same number can seem to flowdifferently. For example, the tolerance range on a 65 jet ranges from351.5 to 362 cc per minute while a 66 ranges from 368.5 to 379.5 cc's.Those tolerance ranges explain why a 66 jet may not always give a richermixture than a 65--if the 65 is on the "high" side of the tolerance andthe 66 is on the "low" side. The two jets may flow very close to thesame.
Holley offers close-limit jets that range in size from 60 to 74. The jetnumbering follows that of a standard jet with a third number added. Forexample, a jet stamped with PN 122-632 indicates it is a 63 jet. Thethird number, 2, is added to indicate whether the jet flows on the leanside (631), the middle (632), or the rich side (633) of the flow band.There is about a 1.5 percent difference in flow between each of thethree jets in a flow band, for a flow range difference of 4.5 percent.Holley close-limit jets are only offered in the middle flow range.
Drilling out jets is never recommended because this always alters theentry and exit features to some degree and may introduce a swirl patterneven if the jet is held in a pin vise and carefully drilled. You cannotbe sure of the flow characteristics of a jet that you modify bydrilling--unless you can get that jet back on a flow machine to compareit with a standard jet.