From Street Stock to Winston Cup, the three most important areas for engine performance improvements are cylinder heads, valvetrains, and intake manifolds. In particular, cylinder heads affect most aspects of power. Ports, valve size, and placement affect engine breathing or volumetric efficiency. Thermodynamic and combustion efficiencies are affected by the volume and shape of the combustion chamber. Valvetrain performance is affected by placement of valves relative to the rockers. And the water jacket design can simultaneously affect cooling performance, power, and overall engine durability.

First, it's important to understand cylinder heads that flow the most air on the flow bench may not make the most power or perform best on the track. Cylinder head flow gains are not an end unto themselves. For circle-track racing, a primary goal is to pack as much mass of air and fuel into the cylinder as possible while maintaining the highest port velocities. This will not necessarily occur with a head that flows the most air on a bench.

Process of Evolution Upon evolution of an economical airflow stand or bench, engine builders were quick to see the tool was an effective way to quantify airflow performance of one port relative to another. The flow bench is most effective when used to develop cylinder heads that are closely related to production parts. The reason is that airflow performance of a production part is so restricted that any gains in flow will most likely increase engine power.

Most flow bench testing is performed at a depression of 28 inches of water. At best, this is accurate only twice during a full inlet cycle (See Chart #1)

This chart shows an inlet port that sees negative pressures up to 2.6 psi (73 inches of water), much higher than the 28 inches of water used on an airflow bench. A true representation of cylinder head flow performance would be using a three-dimensional plot of flow at varying lifts and depressions. Flow benches become less effective as cylinder head design is less restricted and mass flow increases. For example, heads developed for CART, IRL, Pro Stock drag racing, or Outlaw sprinter engines are less likely to be successfully developed on a standard airflow bench.

Further, an intake port must flow fuel and air. Engines running methanol operate at air/fuel ratios more than twice the richness of those on gasoline. The large quantity of fuel, much heavier than air, clearly impacts the performance of an intake port. Wet flow benches that can operate at much higher flow depressions make a better tool for such engines. Some engine builders working on these engines simply use the dynamometer to test cylinder head improvements. The flow bench becomes a quality control tool, much like a go/no-go gauge, to verify a finished head compares to its prototype. The point here is that the Saturday-night racer should not become overly dependent on airflow benches. The dynamometer is a superior engine evaluation tool.

On the intake side Where in the valve event does flow take place? Study the chart, noting intake mass flow vs. crankshaft angle. In this case, one area on which to work is at low valve lifts. Since the piston has started moving downward on the intake stroke, there is already negative cylinder pressure as the intake valve cracks open, causing a jump in mass flow.

Note that the rate of mass flow increase flattens at high lifts, so increasing valve lift does not appear a good option for more flow. Also note that some reversion takes place. (This example was taken from an engine speed near peak torque.) A compromise in the valve events must occur to have good power at higher engine speeds. The reversion will disappear at power peak.