You can use a vacuum gauge as long as you have two of them and they are balanced to read the same. The goal is to have both throttle bodies with the SAME vacuum. A differential manometer does that by pulling vacuum on both sides of the tube. The side with the higher vacuum will have a higher column of fluid. I use a Motion Pro SyncPro and it isn't differential. All four columns are individual gauges.
Here's how to build a manometer for dirt cheap. I used auto. tranny fluid because it's red and easy to see, and doesn't bounce around so easily as water. If using water, put some food color in the water to aide seeing the water level. Slow and a bit of a pain to get the tubing filled with the fluid, though, but once it's built, it's done.
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