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What benefit is a vacuum/boost gauge over a boost only gauge?

Any gas engine will naturally produce measureable vacuum any time that it is running. This is the result of the down-sweep of the pistons while the intake valves are open. This is how it draws in the air/fuel mixture, and when the throttle or closed, or only partly open, you will have a higher vacuum reading due to the restriction your throttle blade produces.

When under heavier throttle (higher percentage of throttle opening), you have lessened the amount of restriction, and air is able to flow more freely, hence a lower vacuum reading will occur.

Many times through the past, companies (including ourselves decades ago), including the OEM's would market or sell an economy gauge which was simply a vacuum gauge. The higher the vacuum reading, the better mileage you would have, due to throttle opening.

Now what is boost? If you are running a supercharger (for example), it is always turning, and always moving air, though it is not always creating "boost". Boost is when the positive pressure of the super charger becomes greater than the negative pressure (vacuum) that the engine creates. Boost typically is only going to happen when the throttle opening is at or near full throttle, with heavy load, and can become more with higher RPM. Vacuum becomes zero, and pressure occurs, which is the result of the supercharger packing air/fuel into the cylinders.

How is this helpful? You can drive by vacuum for the most in economy, and also get to see when your boost transitions in (in relation to throttle opening and load) which can be helpful for tuning purposes. A vacuum gauge can also be an indicator of possible poor running conditions such as loose valve guides, leaky valves, etc. by means of a shaky vacuum pointer. A steady pointer that fluidly (smoothly) responds to throttle and load is preferred. A shaky or vibrating pointer is an indicator of a potential engine problem.