Practical Engine Airflow. John Baechtel

the lowest wide-open-throttle (WOT) brake specific fuel consumption (BSFC) numbers. Below the torque peak, torque trails the VE curve due to reduced combustion efficiency caused by inadequate intake flow velocity (ramming), air/fuel separation issues, and poor mixture quality. Above the torque peak, torque and VE decline due to insufficient time for cylinder filling caused by increasing engine speed (RPM).

      Fortunately methods are available to address VE inefficiencies on either side of the torque peak and to inflate the overall torque curve. This refers to the “area under the curve” and seeks to expand the torque curve in all directions. Successful efforts to increase torque automatically improve horsepower. More important, a broader torque curve often produces greater acceleration even with a slight reduction in peak torque because it applies more torque over a broader range.

      If the ideal mix of engine components targets an engine speed range most beneficial to the application, superior vehicle performance is ensured across the board. Complementing these performance gains with correctly matched gearing and tire combinations ultimately leads to faster cars and better racing through the effective production and utilization of torque. This works effectively even for engines operating well above the torque peak because the upper end of the torque curve expands accordingly, thus contributing more horsepower to the car’s performance.

Volumetric Efficiency

      The cornerstone of power building is volumetric efficiency. The more air an engine can process, the greater its power potential. VE is determined according to the engine’s static air capacity, or displacement. A displacement of 400 ci represents 100-percent air capacity for an engine of that size. At any given engine speed, a percentage of that volume is being processed into torque depending on a host of variables that conspire to limit airflow. Without these pesky restrictions, atmospheric pressure can easily fill the cylinders completely (100 percent) every two crankshaft revolutions.

      In practice this is difficult to achieve because airflow is restricted by a throttling device (carburetor, throttle body, or other), imperfect intake manifolds, intake ports, valves, and all the attending flow restrictions and pressure dynamics present in a running engine. Hence VE in a production engine rarely exceeds 80 to 85 percent. As previously mentioned, VE is reduced below the torque peak due mainly to insufficient airflow and poor mixture quality. And when operating at low RPM the piston pushes some of the charge back out after BDC so no ramming occurs.

      Above the torque peak, VE is limited by inadequate time to fill the cylinder due to increasing RPM (typically two effects apply past peak VE, one for flow stagnation and one for the loss of intake/exhaust wave tuning). One of the engine builder’s primary goals is to exceed the static air capacity of the engine and optimize combustion efficiency once fuel is introduced to the process. Savvy engine builders skillfully manipulate the component composition to accomplish this, broadening the torque curve and positioning it to best suit the intended application.

       Hard-core racing applications such as NHRA Pro Stock still rely on highly specialized tunnel ram intake manifolds topped with twin Holley Dominator 4-barrels. Despite the precision drivability of electronic fuel injection (EFI), this combination actually makes identical or better power when finely tuned within its particular operating range.

       Air filters are an important induction component because they are the first airflow restriction encountered by incoming air. Over time, racers and hot rodders paid greater attention to air filters and their utility for directing air into the carburetors or throttle bodies. This integral filter top is designed to provide greater flow capacity and is said to help straighten and smooth the airflow.

       Highly efficient for their day, mid-1960s Corvette fuel injection systems were thinly disguised tunnel rams with sealed tops and side-entry throttle bodies. It was quite a tidy setup with a bit of ram tuning. Air capacity was more than adequate on this small-displacement 375-hp 327-ci small-block. (Photo Courtesy GM Media Archive)

      All engines generate their torque signature based on displacement, engine speed, VE, flow-path dynamics, and not surprisingly, specific architecture (I-4, I-6, V-6, V-8, V-10, V-12, etc.), each of which applies different attributes to cylinder filling, mean net torque, and overall engine smoothness. Every combination generates a torque peak, or “sweet spot,” where its operational dynamics achieve maximum VE.

      With competition engines, this often exceeds 100-percent VE, sometimes by a considerable margin. At 100 percent a cylinder contains a volume equal to the same space at atmospheric pressure above the inlet.

      The volume within the cylinder also contains a fuel mixture that reduces the amount of air (and oxygen content) by the specified air/fuel ratio. So the two volumes are similar but different. More correctly you might argue that 100-percent VE means that the cylinder has achieved pressure equilibrium with the atmosphere. When a cylinder exceeds 100-percent it effectively becomes naturally supercharged. This occurs by the ramming effect of proper inlet sizing and valve timing to mildly pressurize the cylinder at IVC.

      By specifying components to meet VE requirements builders target intake ports, dimensional qualities of intake manifolds and exhaust headers, carburetor size, rod-to-stroke ratios, valve timing, and static compression ratio. The specific component matrix is adjusted to suit the application’s operational requirements.

      Oval track and road racing engines typically call for a component mix that produces a broad torque curve over a wide range of RPM. This affords the engine builder an opportunity to tune the intake and exhaust systems separately to effectively broaden the power band. Conversely, drag racing applications seek a higher and narrower power band in which intake and exhaust tuning are more closely aligned. Identifying and targeting the required power band is one of the engine builder’s first steps.

      Although VE and engine speed are closely aligned, it is critical to target VE modifications to the desired engine speed. If a drag racing engine leaves the starting line at 7,000 rpm and cycles to 9,000 rpm through the gears, its VE at 5,000 rpm is largely irrelevant. And, of course, an engine delivering power between 4,500 and 7,200 rpm needs a broader tuning efficiency because of its parts combination. Hence airflow management within the targeted engine speed range becomes a central challenge in matching or exceeding an engine’s potential VE capacity.

      Trapped Mass and VE

      When all is said and done, all you’ve got to work with is the effectively trapped mass you are able to contain and compress within the combustion space (clearance volume) at the point of ignition. That is the effective VE. Larry Meaux uses the following formula in his broadly popular PipeMax Header Design software program. It incorporates parameters that you may fail to consider:

      Trapped VE = measured CFM – ring-blowby CFM – CFM lost during overlap

Theoretical CFM

      In a properly configured engine, the ring blowby and CFM (cubic feet per minute) lost to overlap should be minimal, but must be considered for accurate modeling. And thus they are integral to modeling programs such as Meaux’s PipeMax, Hale’s Engine Pro, and most other high-quality simulation programs. For the purpose of airflow improvement, you are largely limited to static or steady-state flow measurements of individual flow path components or some combination thereof (where, for example, you might flow a head port with the manifold and carburetor attached).

      Many street-strip types of high-performance engines still do not achieve 100-percent VE largely because they are a mixture of economic and manufacturing compromises. Cylinder heads are designed to have broad application on various engine sizes, and the camshafts are typically catalog grinds with the same prerequisites. The