CFM Rules - Engine Builder Magazine

CFM Rules

CFM and Carburetors:

Carburetors are rated by CFM (cubic
feet per minute) capacity. 4V carburetors are rated at 1.5 inches (Hg)
of pressure drop (manifold vacuum) and 2V carburetors at 3 inches (Hg).
Rule: For maximum performance, select a carburetor that is rated higher
than the engine CFM requirement. Use 110% to 130% higher on
single-plane manifolds. Example: If the engine needs 590 CFM, select a
carburetor rated in the range of 650 to 770 CFM for a single-plane
manifold. A 750 would be right. An 850 probably would cause
driveability problems at lower RPM. A 1050 probably would cause actual
loss of HP below 4500 RPM. For dual-plane manifolds use 120% to 150%

CFM and Manifolds:

Manifolds must be sized to match the
application. Because manifolds are made for specific engines, select
manifolds based on the RPM range.

CFM and Camshafts:

With the proper carburetor and manifold
it is possible to select a cam that loses 5% to 15% of the potential
HP. These losses come from the wrong lift and duration which try to
create air flow that does not match the air flow characteristics of the
carburetor, manifold, head and exhaust so volumetric efficiency is
reduced. An increase in camshaft lobe duration of 10 degrees will move
the HP peak up 500 RPM but watch out; it may lose too much HP at lower

CFM and Cylinder Heads:

Cylinder heads are usually the
limiting component in the whole air flow chain. That is why installing
only a large carburetor or a long cam in a stock engine does not work.
When it is not possible to replace the cylinder heads because of cost,
a better matching carburetor, manifold, cam and exhaust can increase HP
of most stock engines by 10 to 15 points. To break 100% Volumetric
Efficiency, however, better cylinder heads or OEM “HO” level engines
are usually needed.

CFM and Exhaust:

An engine must exhaust burned gases before
it can intake the next fresh charge. Cast iron, log style manifolds
hamper the exhaust process. Tube style exhaust systems are preferred.
But headers are often too big; especially for Performer and Performer
RPM levels. Improving an engine’s Volumetric Efficiency depends on high
exhaust gas velocity to scavenge the cylinder. This will not happen if
the exhaust valve dumps into a big header pipe. On the newer computer
controlled vehicles it is also important to ensure that all emissions
control devices, and especially the O2 sensor, still work as intended.

CFM and Engine Control:

Spark timing must be matched to
Volumetric Efficiency because VE indicates the quantity of charge in
each cylinder on each stroke of the engine. Different engine families
require distinctly different spark advance profiles. And even engines
of equal CID but different CR require their own unique spark advance
profiles. Rule: Expect 0.1% to 0.5% loss in Torque for each 1 degree
error in spark timing advanced or retarded from best timing. Also,
detonation will occur with spark advanced only 3 degrees to 5 degrees
over best timing and detonation will cause 1% to 10% torque loss,
immediately, and engine damage if allowed to persist.

– Tech Tip courtesy of Edelbrock 

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