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Originally Posted by kmarccoco
i understand that the power valves = about 8 jet sizes when activated
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This is a fairly popular misconception. Holley/Demon style power valves are not
metering devices at all; they are basically "on/off" devices that are referenced to engine load by way of intake manifold vacuum. The actual metering of fuel through the power valve enrichment circuit is controlled by the size of the passages – generally called “power valve channel restrictions” -- behind the power valve which lead into the main wells (vertical passages that connect the main jets to the main circuit discharge nozzles). To determine the correct increase or decrease in jet size necessary to compensate for use/removal of a power valve in your carburetor, a bit of simple math is necessary…along with a jet orifice size chart if you are using O.E.M. Holley jets or equivalent. First, measure the size of the power valve channel restriction (PVCR) passages in your carburetor metering block(s). For the sake of this example, I’ll use a stock Holley #4781 850cfm 4brl. The power valve channel restriction size in the primary-side metering block of this carburetor is .070”. Next, check the size of the main jet currently in use and determine it’s orifice diameter (in this case, a #80 jet, which has an orifice diameter of .089”). Now, compute the flow area for each of these two passages using radius X radius X 3.1416
(figures are rounded slightly for simplicity)
For the power valve channel restrictions:
.070” diameter / 2 = .035” radius
.035 X .035 = 0.0012
0.0012 X 3.1416 = 0.0038 sq in
For the main jet:
.089” / 2 = .0445” radius
.0445 X .0445 = 0.0020
0.0020 X 3.1416 = 0.0063 sq in
Now, considering that at full load conditions the power valve is open and the venturi has both the main jet and the PVCR available from which to draw fuel, it stands to reason that if we are going to replace the power valves with plugs and thereby eliminate the function of the PVCRs, then we must increase the flow area of the main jet by a like amount if we want to maintain the same WOT air/fuel ratio we had with the power valve in place. So, adding the area of both together:
0.0038 sq in
+
0.0063 sq in
0.0101 sq in
…which is the area that we now need to convert to our new jet size. To do this, simply work backwards through the formula: Area (0.0101) divided by pi (3.1416) = radius squared (0.0032). Finding the square root of 0.0032 gives us a radius value of 0.0057”, which we simply double to find the diameter (0.1134) of the jet we need. Holley jet size numbers ARE NOT directly tied to their orifice diameter, so be sure to check the chart. Here we find that a Holley #93 jet has an orifice diameter of 0.1125”, and a #94 jet has an orifice diameter of 0.1155”, so you’re going to be right in that range; a 13 – 14 size number increase. Had we used a #4776 600cfm 4brl (using .043” PVCR diameter) or a #9375 1050cfm Dominator 4brl (using a .112” PVCR) the actual change would have been a lot smaller/greater respectively.
Note that you aren’t “locked in” to using a particular size PVCR; this is one of the areas that we carb tuners use to tailor a carburetor’s fuel delivery characteristics, particularly part throttle. Not saying that you can’t screw up here…you can create big-time problems if you get the available flow area too far away from what the venturi and booster actually want…just that this is one more area you can tune a carburetor for a specific engine application.