Practical Engine Airflow. John Baechtel

and temperature, give or take some heat contributed by the hot cylinder.

      This cylinder filling scenario achieves a VE of more than 100 percent because the clearance volume (chamber) is also filled with 100-percent undiluted charge. The simplest description is an 11:1 compression ratio where the clearance volume is 10 percent of the swept volume. The ideal result is a VE of 110 percent.

      Unfortunately, some conditions resist our efforts to fill the cylinder adequately, and these are the problems we address as engine builders. A few of the engine features that affect the intake pumping process include:

      • Intake port flow capacity

      • Carburetor and intake manifold runner steady-flow characteristics

      • Degree of influence of restrictions

      • Maximum piston velocity and camshaft timing

      • Charge contamination during the overlap period

      Intake port flow capacity must be measured on a flow bench at a high pressure drop and at the mid to high valve lifts that you intend to run, which might be established by modeling or consultation with your cam designer.

      Carburetor and intake manifold runner steady-flow characteristics are best determined by the average plenum (manifold) pressure during intake pumping, although that’s difficult to accomplish on the front end. Many builders often extrapolate by flowing the port with the manifold and carburetor attached to determine a more realistic approximation of the flow characteristics. They’re simply determining the existing steady-state flow capacity and characteristics of the available flow path.

      Finally, the maximum piston velocity and crank angle can be calculated via RPM and rod length–to-stroke ratio so it can be related to camshaft timing. This is done for you in most modeling programs.

      With the goal of providing 100-percent VE at BDC plus the clearance volume VE, Hale’s work describes VE as a strong predictor of engine speed at both peak torque and peak horsepower. The pumping process is not so much a function of the flow path cross section, but rather a result of the overall intake steady-flow capacity.

      Flow-bench measurements are rightly viewed as trend indicators. A flow bench operating at 28 inches of water cannot emulate the same characteristics of a rapidly descending piston, which can easily induce a pressure drop more than double that of the flow-bench capacity. Under more dynamic conditions rapid piston motion results in a very strong tug or “yank” on the intake charge that the flow bench cannot replicate.

      Intake Ramming

      This cycle accounts for the considerable momentum (inertia) that the intake charge has accumulated during the initial pumping process. As the piston reverses direction at BDC, the intake valve is closing. But the momentum generated by a properly configured flow path continues to ram mixture past the valve and into the cylinder, even as the cylinder volume begins to decrease and cylinder pressure begins to increase.

       Capturing more air at speed via velocity ramming is the primary intent of most air scoops. The racy look is an attractive co-benefit. And, like air cleaners, scoops have evolved into some pretty bizarre shapes in an attempt to slow and direct the air to build even inlet pressure.

       A storm is brewing between the air scoop and the header tips on this supercharged dragster engine. Air enters the engine at atmospheric pressure along with the ramming effect at speed. The supercharger compresses it and feeds it through the intake manifold and cylinder head ports to the cylinders where the magic happens. Supercharged drag engines generate a lot of exhaust volume and zoomie headers are the most effective way of passing it through without restriction.

      Depending on the flow path characteristics and the strength of the intake column flow volume, it continues filling the cylinder well past BDC. At some point, equilibrium is achieved as rising cylinder pressure finally overcomes the intake charge momentum. Proper valve timing is essential here because the intake-ramming event ends when the intake valve closes (IVC), approximately 60 degrees after BDC.

      Intake ramming is an essential component of true performance engines. You might call it VE augmentation. Recall that VE is the ratio of the actual trapped mass in the cylinder at the end of the intake valve event (and the ramming process) to the actual mass of the swept volume of the cylinder at ambient temperature and pressure. The ramming event accounts for VE percentages that often exceed 110 in racing engines. In very high-end engines the additional filling contribution of the ramming event can approach 18 percent with effective wave tuning offering another 2 percent of additional filling capacity. (See page 58 for more information about wave tuning.)

      Again, the previous cycle can affect the current cycle. For example, extra-large ports and valves can initially help the intake pumping process, but they kill the subsequent ramming process because they can’t build enough velocity to accomplish and sustain it. Port energy simply remains too weak except at very high engine speeds, which may be well out of your particular engine’s operational range. It’s why oval-port Chevy big-blocks often outperform big-blocks with larger square-port heads.

      Influences on Intake Ramming

       Engine features that play an important role in the intake ramming process include:

       • Precise camshaft timing of IVC and RPM to prevent intake system reversion

       • Overall intake runner length

       • Intake runner cross section

       • Total intake runner volume

       • Intake system pressure waves (arriving at the intake valve before valve closing)

       • Overall plenum volume

       • Intake port flow capacity (as measured at lower pressure drops and low to moderate valve lifts)

       Street supercharging has become a mainstream means of providing additional airflow to performance engines. Matt and Debbie Hay’s award-winning 1988 Pro-Street Thunderbird sets the bar high with its radical fuel-injected twin-blower setup feeding a 351-ci small-block Ford V-8.

       Engines with individual runner induction systems are tuned for a very specific operating range such as that found at Indianapolis. When Chevrolet returned to open-wheel Indy racing in 2002, it brought a 3.5-liter naturally aspirated methanol-burning engine that won 14 of 15 events and captured all three major titles: the driver’s championship, manufacturer’s championship, and team championship. (Photo Courtesy GM Media Archive)

      Wave tuning can further aid the cylinder filling process. These finite amplitude pressure waves should be timed to arrive at the valve before BDC and again just before IVC to lend their energy to ramming even more intake charge past the valve to increase VE.

      Pressure wave timing is largely a function of inlet tract length. Rapidly moving positive and negative pulses traveling back and forth between the valve and the inlet entry carry energy that can be harnessed to push more charge into the cylinder if timed correctly. Opening and closing valves generate a reflected pulse as does a large area change such as found at the inlet entry. When a pulse encounters an area change or closed valve, it reflects back in the opposite direction and changes its energy value, or tense.

      For example, a negative pulse encountering a closed valve reflects a positive pressure pulse because of the stacking effect of the air against a closed valve. When that pulse travels back to the inlet entry, it loses energy because of the area change and reflects a weaker negative pulse traveling back to the valve.

      The pulses swap phase, if you