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% higher.
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 RPM.
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.