Recently we’ve been working with tuning a Holley 600 to work with a Ford 289, and realized that some of this information would be very good to have posted where others could get to it.  Particularly in reference to power valves.  We all know what a power valve is, and anyone who has taken apart a Holley knows how to replace it, but it seems less is known about the actual workings of the power valve.  First off, the basics.  The higher the number on the power valve, the sooner it opens; the lower the number, the later it opens.  There is a wide range of power valves, from 2.5 to 10.5, if memory serves, and the basic application is as follows.  If you have a low vacuum, measured with a vacuum gauge at the base of the carb  – or elsewhere there is full vacuum, with the point being that you don’t want to measure it at the metering block, as this is metered vacuum and does not give full vacuum measures – then you need a lower number power valve.  Knowing this, if you have a vacuum reading at idle of say, -8 psi, then you would want a power valve rated at about 3.5.  What this means is that at 3.5 psi vacuum, the power valve will be fully open, but it will have started to open at about 7.  Key here is knowing that your greatest vacuum is pulled at idle and cruise conditions, and when you hit the throttle, the vacuum drops off.  So if at idle the engine has 8 psi vacuum, when the throttle is opened, the vacuum will drop off, as it does so, the power valve opens.  So the vacuum looks something like 8, 7.5 (power valve opening), 7, 6.5, 6, 5, 4, 3.5 (valve fully open), 2, 1, 0 (butterflies open fully, allowing all the air and fuel necessary to the engine and providing no restrictions, hence, no vacuum).

The point here is that if you have a low vacuum engine, you need a lower number power valve, not vice versa, as commonly believed.  Once the power valve is figured out (and this takes some experimenting sometimes, especially if you don’t know what cam is in the engine) you can fool with the main jets to get the right mixture out of the engine.  Generally factory specs on jets for the engine are pretty much what one will need to follow, but if you have just bought a performance.aftermarket carb, chances are you’ll have to do some messing around to determine just what the engine likes.