Vacuum Secondaries on Holley

Believe it or not I replaced my 2V Autolite carb with a 4v Holley to increase economy and obtain performance. My thinking was that the 2v carb was probably rated around 350CFM and the 4V replacement is rated at 450CFM. Therefore, if I can run the boat in 2V mode the effective rating may be 225CFM. But, when I want to go fast I will have 450CFM available (in theory). (The only time I want to go fast is when some young kid in a new shiney boat tries to outrun me.)

I am considering installing a selinoid to unlock the secondaries with a flip of a dash mounted switch or a prox sensor near the 4V carb to detect exactly when the secondaries begin to open (so I can throttle back and keep the secondaries closed) but I began to think a dash mounted vacuum gauge may do the trick (and be a lot easier to install).

Question: Can a vacuum gauge be used to precisely detect when secondaries begin to open?

A vacuum guage could definately be used but it might be hard to tell exactly when the secondaries crack (begin to) open (unless your slamming the throttle). A vacuum drop of a specific value would be what your looking for, and the more you open your primaries (from idle) the more of a progressive vacuum drop you'll see (best idle is usually the highest vacuum reading you can possibly get). You would have to make some runs on the water to see when the secondaries open-get a visual on the secondary plunger movement and compare with the vacuum guage. You should be able to pick up a manifold vacuum (drop) that corelates with the secondaries opening. A slightly higher vacuum reading would represent the secondaries being closed. Try a manifold vacuum guage used for tune ups and let us know how it works, if it works well you could put a guage in your dash-but it probably would have a delay due to the length of vacuum line required to mount it. Do you have provisions to hook up a vacuum guage? Maybe somebody else out there has some idea's. Have I confused you yet?

The vacuum guage should work fine without any real delay. You would see a slite delay on start up but once it is drawing vacuum on the whole system it should respond instantly. I would suggest a gauge that has the range that you are looking for instead of a universal one. That way the calibarions will be finer and it will be easier to see a drop in vacuum in smaller incruments. For instance say the max vac you would see is 15 in-lb I would look for a gauge 0-18 or 0-20 range verses one that went up to 0-40 or higher.

The proximity switch is a novel abroach and should work equally as well if not better becuase you would know exactly when the secondaries moved regardless of the vacuum reading. With the vacum reading there will be a slight range of a in-lb or two before the secondaries open.