Carburetion is my acknowledged area of expertise. I ran a carburetor
rebuilding shop for many years.
Carburetion, like camshafts, is seldom well understood by the layman. A
carburetor has two functions. Those are to act as a valve for the big air
pump known as an IC engine, the other to mix gasoline with that air at a
predetermined rate. The stoichemetric (chemically ideal) ratio for an IC
engine is 14.7 to one.
It is fairly simple to use fuel injection and a computer to attain this
ratio throughout the full rpm range. Carburetors, however, need to
compromise at certain rpms in order to function throughout the full range.
This is where the rub comes. If one sizes a carburetor for full throttle
only, there is little limit to its size. Merely open the floodgates and
the engine will pull in as much air and fuel as it needs. At other
throttle and rpm settings, however, serious consequences occur with too
big a carburetor.
The first thing lost is velocity of the air stack. This brings in the
equation Bernouli's principle; that is, air pressure is inverse to speed.
Since a carburetor requires a venturi in order to create a vacuum and pull
fuel in, this creates a natural block to the air column. If that venturi
is too big, the engine will be severly hampered, resulting in erratic fuel
ratios.
Ideally, the CFM of a carburetor should take into account the uses of the
engine and rpm range. There are numerous methods used for this
calculation, and a simple Google search will find various formulas for
this calculation.
Marcus A. Pryor
For a new age of reason
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