Practical Engine Airflow. John Baechtel

       INTRODUCTION

      This is a book about airflow through internal combustion (IC) engines, more specifically high-performance and racing engines. All the fundamentals of IC engine operation apply, but the chief concern is with the movement of air through an engine in the most efficient manner to produce optimal cylinder filling (volumetric efficiency) and maximum output in the form of tire-shredding power, or torque. It’s not an engineering text, although engineering content and appropriate lingo are included where necessary along with input and commentary from recognized experts in the field. It’s not a math book, although some mathematical equations are provided so you can learn how to calculate the values of various functions.

      It is a book for performance enthusiasts eager to gain a fundamental knowledge of engine airflow and how it affects the operation and output of high-performance engines. My intent is to help build your general knowledge of core principles to help you select components that best suit the requirements of your specific application whether it is a hot street machine, bracket racer, road racer, or whatever.

      Without discounting the necessary core physics of IC engines and the fundamentals of gas exchange that characterize engine performance, I begin with the assumption that performance-oriented readers already possess a basic understanding of four-cycle engine operation and the well-established reality that engine airflow is the key path to maximum power.

       Airflow through an engine is the key path to power. As illustrated in this cutaway of an earlier 32-valve Corvette LT1 V-8, tuned-length flow paths usher air from the throttle body inlet to the cylinder where it is burned with fuel and discharged via a tuned exhaust system. More air mixed with more fuel equals more power. (Photo Courtesy GM Media Archive)

       A small-block hot rod with a single 4-barrel carb, or a small-displacement blower, and a good exhaust system is a great recipe for hot street performance. Beyond that, the bar continues to rise based on the fundamental requirement to put more and more air through the engine to increase output.

       Tri-power carburetion featuring a trio of 2-barrel carburetors was a popular early induction choice for moving more air. Most muscle car versions used Holley 2-barrel carbs, but traditional hot rods are typically fed by 350-cfm Rochester carbs with progressive linkage (shown).

       CHAPTER 1

       AIRFLOW BASICS

      Everyone has heard the traditional analogy that an engine is nothing more than a basic air pump, a very sophisticated air pump. In effect, a running engine provides continuously recurring spaces, or power volumes (cylinders), into which air flows due to atmospheric pressure or, in some cases, pressurizing sources such as superchargers and turbochargers. These spaces are essentially empty voids (vacuum) created by descending piston motion. They have negative pressure relative to atmospheric pressure and the atmosphere automatically seeks to fill them through the intake flow paths as each volume is created. The engine is not specifically pumping air, but rather mechanically providing an ongoing series of pressure differentials that encourage air movement through the engine’s inlet flow paths. Air movement, or transfer, is similar to the pumping action; hence, it is referred to as intake pumping.

       The intake valve in the cylinder head feeds fresh air to each cylinder for every new combustion event. The size, shape, and configuration of the intake port play a major role in how much air you can feed the engine to increase power.

Airflow Path

      Every time a piston descends on an intake stroke it creates a cylinder filling and fueling opportunity. This occurs on every other revolution of the crankshaft for each cylinder in the engine. The dynamics of this are extraordinarily complicated on a thermodynamic level and yet simple enough that even when things are pretty far out of whack, the engine still runs and drives comfortably in everyday vehicles. The descending piston creates a void, or space, that atmospheric pressure immediately attempts to fill when the intake valve opens because it is greater than the pressure in the empty cylinder.

      The sucking sound you hear at the carburetor is the air rushing in to fill the void. It follows a torturous path through a venturi where it gains speed and mass because fuel is being added. Then it exits the carburetor throttle bores at high speed into a larger staging area, or plenum. The dramatic change in area causes the air to lose velocity quickly, and the local pressure changes. This change presents the first opportunity for the atomized fuel to drop out of suspension.

       This factory cutaway of a 1950s Chevrolet 348-ci W-engine shows the inlet path from the carburetor to the cylinder on the driver’s side and a portion of the exhaust path on the passenger’s side. Not much has changed since then. The flow path starts at the air filter and can be traced all the way through the engine to the end of the exhaust pipe (not shown). (Photo Courtesy GM Media Archive)

      The next cylinder in the firing order submits a filling request by exposing the empty cylinder via the opening intake valve. The mixture immediately seeks to fill the void in that cylinder by rushing into an intake runner where it picks up velocity and regains some pressure due to the smaller cross-sectional area of the runner. On its way to the intake valve, the mixture may experience a variety of obstacles and area changes that affect its speed and flow characteristics. Curved runners and intake ports that narrow around the pushrod area restrict flow. In a sense, runner taper (see Chapter 4) restricts the flow, but it builds pressure and velocity, which encourages the intake ramming process.

       Most street engines and a great many Sportsman racing classes still rely on single-plane 4-barrel intake manifolds to support their induction requirements. Individual intake runners connect to a common central plenum. The intakes are typically outfitted with various-capacity Holley 4-barrel carburetors to suit their high-RPM operating range.

       This cutaway view of an Edelbrock dual-plane intake shows the upper and lower plenums and the individual runners that lead from each one. For the most part, the dual-planes are street intake manifolds that build more low-end and mid-range torque than single-plane intakes because they help produce more efficient low-speed flow velocity. In some cases, the dual-plane intake can outperform single-plane intakes throughout the operating range while nearly matching them on the top end.

      After negotiating various curves in the manifold, the air may stumble at the gasket interface between the manifold and the cylinder head; this point is rarely an efficient transition unless steps are taken to ensure it. Then the air has to make a relatively sharp turn into the bowl area above the valve where it is interrupted by the valvestem and valveguide.