written by: Duane Carr with Technical Information supplied by
In order to run a Holley 4150/4160 with power valves on a GMC type Super Charger, the power valve circuit/s needs to be modified to get its vacuum signal from below the rotors of the blower. This is commonly referred to as a "Boost Referenced Power Valve". On a 4150 Model you would be looking at doing both the Primary and the Secondary side. On a 4160 model just the Primary side has a Power Valve so that would be the only side done.
This modification will allow you to connect a vacuum hose from what WAS the ported vacuum source on the side of the metering block to the vacuum source in the Intake Manifold where you would hook up a vacuum/boost gauge.
Now, lets get started. First completely disassemble the carburetor/s. On the ends of the main body where the Metering Blocks bolt up there needs to be a slot cut to redirect the route for the Power Valve to receive a Vacuum Signal. The slot would go from the Power Valve Cavity (PVC) "A" (picture 1) to hole "B" (picture 1), which is located just to the left of the PVC. The Slot "C" (picture 1) that needs to be cut is outlined in Red and needs to be .125" wide x 100" deep. If a milling machine is not available to cut the slot then a 1/16 or 3/32 hole can be drilled at a slight angle from hole "B" (picture 1), to the PVC "A" (picture 1) making sure it is at 10 to 15 degree angle. The idea here is to have just enough angle so that the hole stays under the surface to provide a path for the new vacuum source for the Power Valve. It is also possible to use a dremel or die grinder and cut off wheel but extreme caution should be used if that way is chosen. Also in picture 1 you can see the original vacuum passage that supplied the Power Valve "H" arrow. It will be getting plugged from the bottom in the final step.
Whether you choose to Mill/Cut the slot or Drill a hole, the end result is that the PVC is now connected to the bottom of a backwards "L" in the metering block "D" (picture 2) then to what was the ported vacuum port.
The new port that will be used "F" (picture 3) most commonly has a small tube (3/16) pressed into it and was used for the Vacuum Advance source for the distributor. Some Metering Blocks are already tapped for a 1/8" NPT thread that uses a nipple as the hose connector. In some cases you might have to finish drilling the passage and connect slot D to Port F. Use a #39 .099" drill for hole "E" (picture 2) with the drill in it. If there is no tube in port F then a 1/8 NPT tapped hole will need to be machined to allow a nipple to be installed to connect the vacuum hose to.
It is this port that will now supply the signal to the Power Valve. It will be connected with a T hose fitting to the same hose as the Blower Boost Gauge uses.
Next, both the holes in the bottom of the Main Body circled in red with arrows "G" (picture 4) that supplied vacuum to the Ported port in the metering block and "H" (picture 4) in the Power Valve Cavity need to be plugged. This can be done with silicone, epoxy or drilled and tapped for 6-32 set screws, my choice being the 6-32 screws.
So, what we have done is blocked the original passages so the Power Valve can now receive its vacuum signal from below the rotors of the blower through the vacuum hose we connected to the boost gauge with the T fitting mentioned above.
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