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...a TACH tells you how FAST the engine in BREATHING.
...a VACUUM gauge tells you how DEEPLY (or not) the engine is BREATHING.
...in other words, a tachometer is a "rate" measurement device (rev-per-minute) while a vacuum gauge is a "differential" air pressure measurement device, ie: air (29.92 inches-Hg) vs. vacuum (no air)...basically, "how deeply" the engine is trying to intake/breath air.
...at wide-open-throttle (WOT), the vacuum gauge will read nearly ZERO inches of vacuum (very little difference in air pressures), but at idle will be somewhere near 20-22"Hg (67-75% of full vacuum).
...the amount of "throttle" restriction, engine RPM, and engine load determine the vacuum gauge numbers.
Last edited by gone-ot; 08-25-2009 at 04:34 PM..
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