Another method of improving the engine performance by increasing the mean effective pressure consists in improving the so-called volumetric efficiency i.e., the efficiency with which the cylinders are charged with the fuel-and-air mixture. The quantity of mixture drawn into the cylinder during the suction stroke determines the mean effective pressure and therefore the power output.

This quantity should theoretically be equal to the working volume of the cylinder = piston area x stroke. In reality the quantity of mixture drawn into the cylinder is less. The ratio of the actual to the theoretical quantity is known as the volumetric efficiency. It depends on the size and shape of the inlet and exhaust ducts and ports, the shape of the combustion chamber, and the method whereby the fuel is introduced into the cylinder.

There are several standard ways to improve volumetric efficiency. A common approach for manufacturers is to use larger valves or multiple valves. Larger valves increase flow but weigh more. Multi-valve engines combine two or more smaller valves with areas greater than a single, large valve while having less weight. Carefully streamlining the ports increases flow capability. This is referred to as Porting and is done with the aid of an air flow bench for testing.

The ducts should be so designed in such way that it will offer the least possible resistance to the gas flowing through them at high velocity. They should therefore be as straight as possible and with the fewest possible changes in diameter. Fig.1 shows a section through a four-cylinder engine. The inlet pipe has been designed to ensure favorable gas-flow conditions. The water-heating jacket at the bend under the carburetor serves to prevent condensation of the vaporized gasoline in the mixture. The pipe functions as an oscillation tube and is, for this reason, relatively long. Within the cylinder head itself the duct is very short, to avoid excessive heating of the mixture drawn into the cylinder.

The diameter adopted for the inlet and exhaust ducts depends on the cross-sectional areas of the valves associated with them. For a high-output engine the inlet valve should be as large as possible, to keep flow resistance low at high engine speeds (minimum throttling effect at the valve). In a well-designed normal engine operating at its maximum output, a gas velocity of about 300ft./sec (90m/sec.) should occur in the opening between the valve head and its seat when the valve is fully open (Fig.2). The exhaust valve may have a 15% lower discharge capacity than the inlet valve.

In many engines the inlet duct is made to taper toward the inlet valve. This helps to keep the flow velocity and therefore the resistance low in the carburetor and inlet manifold, while proving a suitably high velocity at the actual inlet valve. The exhaust duct also is given a divergent shape toward its outlet end for similar reasons (Fig.3). Sudden changes in direction and cross section of the exhaust duct must likewise be avoided.

The combustion mixture is normally produced in the carburetor, which is connected to the inlet manifold that delivers this mixture to the inlet duct of each cylinder. Quite often the manifold is integral with the cylinder-head casting. The manifold and ducts system inevitably comprises bends which have an adverse effect on gas-flow conditions. Engines designed for very high outputs have an independent intake and mixture-producing system for each individual cylinder. Twin and multiple carburetors are used in such engines (Fig.4).

Volumetric efficiencies above 100% can be reached by using forced induction such as supercharging or turbocharging. With proper tuning, volumetric efficiencies above 100% can also be reached by naturally-aspirated engines.