During each of these phases, the air follows the path of least resistance, primarily influenced by the various shapes, sizes, area changes, obstructions, and surface textures it is exposed to along the way. In a fixed-configuration cylinder head (commercially available) you can take steps to influence the air. This is loosely referred to as porting, and it can make a considerable difference depending on the original layout of a particular cylinder head. Some heads respond better than others, primarily based on the shape and cross-sectional area of the port, configuration (raised or flat), relationship of the valve throat size to the valve size, and other things.
A combination of intake pumping, intake ramming, and wave tuning make up the cylinder filling process. Air rushes in because it is under pressure (atmospheric). The air can achieve considerable flow velocity because the intake path is very small relative to the larger source of air pressure (the atmosphere). This imparts inertia to the air. Fuel molecules rush to fill the void created by the descending piston. Depending on the stroke and the rod length, the piston reaches its maximum velocity somewhere around 75 to 76 degrees after top dead center (TDC). This point corresponds to the maximum velocity of the intake charge moving down the flow path. In a properly sized inlet path, the column of inlet air and fuel achieve enough momentum to continue filling the cylinder even though the piston has reached bottom dead center (BDC) and is beginning to rise.
At this point, the intake valve is still open, but starting to close. Resistance to flow begins to increase, but charge energy briefly overcomes it. This is the intake-ramming phenomenon that is largely controlled by piston motion and the length and cross section of the inlet flow path. It is the most important part of the cylinder filling process because it offers the potential for additional cylinder filling beyond the regular intake pumping process.
But it is only part of a broader seven-cycle process as described many years ago by Patrick Hale in his Engine Pro: The Book, a detailed tech manual that originally accompanied the Engine Pro simulation software he designed. (In 2007 Hale sold the copyright for Engine Pro, his other software programs, and the book to Don Terrell, the founder of speedtalk.com and racingsecrets.com.) The seven-cycle process (also called the horsepower chain) is now broadly recognized and largely adhered to within the performance community.
To reinforce the critical importance of engine airflow, note that top engine simulation programs such as Hale’s original Engine Pro software focus heavily on calculating engine airflow and volumetric efficiency (VE). Sophisticated, modern electronic fuel injection (EFI) systems use similar input from the mass airflow (MAF) sensor to make the proper VE and tuning calculations for optimal performance relative to engine speed and load. EFI is so efficient because it knows the condition of the air mass as it moves through the engine and can provide the proper fueling calculations for maximum efficiency. It also monitors the air leaving the engine to help it determine the proper air/fuel ratio and the efficiency of the combustion event.
Internal Combustion Fundamentals
The basic requirements of internal combustion (IC) engines are complex, particularly from the chemical and thermodynamic standpoints. From a less complicated perspective, we all understand the physical factors that characterize the process. Simply stated, the well-known breathe, squeeze, pop, and sneeze make the magic based on the available air/fuel supply and a throttling device to manage engine speed.
A basic understanding of the process requires that you recognize the following core contributors to the engine power equation:
• Airflow
• Fuel supply
• Flow paths
• Compression
• Ignition source
• Throttling device
• Containment device (cylinders)
Among these key factors airflow is the most difficult to manage. Thanks to modern performance components it is relatively easy to feed the engine enough fuel. And compression is easy to achieve with the advanced sealing characteristics of modern piston ring technology. Lighting it off is also easy with high-tech digital ignition systems while various carburetor and throttle body systems easily manage throttling concerns. Although complex thermodynamic and chemical processes govern the efficiency of all this, you don’t necessarily require too keen a grasp of the deeper science to understand airflow through the engine and the various elements that tend to resist air motion and subsequent cylinder filling.
At this point, you are not yet concerned with air/fuel mixture quality, but simply the overall definition and efficiency of the flow path from the atmosphere above the air cleaner to the atmosphere behind the tailpipe. Pressure and velocity changes that occur along the entire flow path play a pivotal role in governing engine output. There are many ways to influence and alter an engine’s air movement and the various forms of resistance that dictate its efficiency.
The carburetor is the traditional self-compensating fueling device that mixes air and fuel in the proper proportion and feeds the mix to the engine via the intake flow path, which consists of the intake runners and the intake ports.
High-performance electronic fuel-injected applications typically incorporate a large single throttle body or a four-hole unit that passes only air because the fuel injectors introduce the fuel.
Electronic fuel injectors come in various sizes to accommodate engine displacement and horsepower ratings. High-performance systems usually have the injectors in the intake runners.
The intake port is the flow path that directs the air/fuel mixture into the engine. It is the primary influence on engine performance.
The exhaust port is always smaller so that the high cylinder pressure helps evacuate the cylinder after the combustion event.
Valve size and placement relative to the bore size, particularly the throat-diameter-to-valve-diameter ratio, determine the effectiveness of the port and its ability to turn the air into the cylinder with the smoothest possible flow.
The combustion space incorporates the combustion chamber, piston top (at TDC), intake and exhaust valves, spark plug, and a fuel injector if the engine incorporates direct injection.
The carburetor (or throttle body) is also the throttling device that regulates engine speed and power output via butterfly