Standard test vacuum was 1-1/2 inches of mercury for 4 barrel carburetors and 3 inches of mercury “Vacuum” for 1 barrel and 2 barrel carburetors. This was reasonable as it was about what a passenger car would develop. Of course, for the numbers to mean anything, tests would have to be run at a certain temperature and barometric pressure. Or be corrected to standard temperature and pressure. If not otherwise stated, ratings in cubic feet per minute are at standard temperature and pressure.

This system is confusing for racing applications for several reasons. Some racing engines actually develop much more or less vacuum than these ratings, so are difficult to compare. For example, a Nascar 390, 4 barrel equipped car might have 3 times the 1-1/2 inches of mercury standard or a very large 2 barrel equipped car might have only 1-1/2 inches of mercury vacuum.

Even more confusing is the fact that most flow benches in the racing industry measure dry air at low pressure drops, commonly 10 inches of water column (W.C.), one bore or venturi at a time. This result is multiplied by a factor to arrive at what that bore would flow at a higher vacuum (i.e. 1-1/2 or 3 inches of mercury), then that figure is multiplied (by four, in the case of a 4 barrel carburetor) to get the C.F.M. rating. This rating is usually much higher than it actually is.

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