| How to Choose a Carburetor for
Your Engine
The size of the engine has a lot to do
with how much air can be drawn into the cylinders. Rpm range and
volumetric efficiency of the engine also play a big role on
carburetor selection.
Carburetor manufacturers rate their
carburetors flow capacity in cfm (cubic feet per minute). The cfm
rating is the amount of air that can flow through the carburetor at
wide open throttle when 1.5 inches of vacuum is applied. When a race
engine is under WOT it must have at least 1 inch of vacuum
differential between the air inside the intake manifold and the air
outside the carburetor (atmospheric pressure). This differential
causes air to continuously flow through the carburetor at a minimum
velocity.
You would be surprised how many
inexperienced racers bolt on the biggest carburetor they can find
thinking it will help scoot their car down the track. They think if
the carburetor was too big it would be smoking black out the exhaust
pipes. In fact, bolting on a carburetor that is too large for an
engine will do just the opposite. That's right! It actually leans
out the engine because the velocity of air slows down significantly
anytime you increase the barrel size of a carburetor. This results
in lower vacuum in the venturi which results in less fuel being
pulled into the air stream. The end result is an engine that bogs
when launching the car off the starting line. What some people will
try and do is increase the jet sizes in the carburetor to try can
compensate for the bog. However, by jetting up the carburetor you
just created another problem because at higher rpm's the engine will
now run too rich because at higher engine speeds the carburetor will
now have greater air velocity and will now be able to draw in
adequate amounts of fuel. It's the lower rpm's that suffer when it
comes to over carbureting an engine.
On the other hand you can choose a
carburetor that is too small for an engine. In this case air
velocity will be extremely high at all engine rpm's. While high rpm
horsepower may suffer, throttle response and low end torque will be
enhanced in most cases.
Vacuum Secondary Carburetors
Most all 4 barrel carburetors are
divided. 2 barrels that operate as the primaries and 2 barrels that
operate as the secondaries at the rear of the carb. Vacuum
secondaries are controlled by a diaphragm that works off air flow
demand from the engine as it accelerates in rpm. At low engine
speeds the secondaries are kept closed by a spring inside the
diaphragm. As the engine accelerates, a port inside the venturi of
the carburetor sends a signal to the vacuum diaphragm and at some
point the secondaries begin to open based on engine demand. Since
spring pressure inside the diaphragm regulates how soon or late the
secondaries open it is imperative that the correct spring pressure
is chosen for the particular engine combination. If the secondaries
open too soon, the velocity loss in the carburetor will cause a lean
condition and the engine will bog down. If they open too late then
the air fuel is restricted and horsepower and torque will suffer.
Vacuum secondary carburetors work great for the street but in most
cases they are not practical for an all out race engine. Most race
engines will benefit from a carburetor with progressive linkage,
also known as a double pumper. More on this in another article.
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