The term tuning applied to automobile engines refers to a number of measures all aimed at getting better performance, particularly in the case of standard production engines. Because of mass-production requirements, some of these measures requiring individual operations on the engines cannot be applied in factory.

Other measures include partly or wholly removing certain limitations on the engine speed and power output which have deliberately been embodied in the engine by the manufacturer with a view to obtaining longer service life. The enhanced performance is attended with working conditions of the components, so that engine life is shortened. However, it is possible partly to offset this by suitable modifications to the cooling system and lubrication.

Engine tuning is the adjustment, modification or design of internal combustion engines to yield optimal performance, either in terms of power output or economy. It has a long history, almost as long as the development of the car, originating with the development of early racing cars, and later, with the post-war hot-rod movement. Tuning can describe a wide variety of adjustments and modifications, from the routine adjustment of the carburetor and ignition system to significant engine overhauls. At the other end of the scale, performance tuning of an engine can involve revisiting some of the design decisions taken at quite an early stage in the development of the engine.

In older engines, setting the idling speed, mixture, carburetor balance, spark plug and distributor point gaps and ignition timing were both regular tasks on all engines and the final but essential steps in setting up a racing engine. In modern engines some or all of these tasks are automated, although they still require periodic calibration.

Increasing the cubic capacity: Within limits, an increase in the cubic capacity of the cylinders is a fairly simple technical matter. There are two ways of doing this:

Increasing the cylinder bore. This process known as overboring is practicable only if pistons of appropriately larger diameter are obtainable. If the cylinder wall is thick enough to permit overboring beyond the maximum piston diameter available from the engine manufacturer, it will be necessary to obtain or make pistons specially. Many automobile manufacturers can, however, supply suitably larger pistons, which are normally used in a higher-powered type of engine than the one whose capacity is to be increased by overboring.

Increasing the piston stroke. This is not always a technically easy modification. It will in any case be necessary to fit a new crankshaft with a larger crunk radius. As a rule such a crankshaft is not readily obtainable. However, the Volkswagen 1200 cc engine, for e.g., can be fairly easily modified to near 1300cc by fitting it with the crankshaft normally employed in the 1500 cc engine. For some types of engine it is indeed possible to obtain from components dealers the appropriate crankshaft for increasing the piston stroke. If such a crankshaft is not obtainable, however, it is still possible to achieve the desired result by obtaining a crankshaft forging and eccentrically grinding the crankpins, bearing in mind that the increase in stroke is equal to twice the eccentricity e (Figs 1a and 1b).

Increasing the compression. An increase in the compression ratio of an engine results in better fuel utilization and a higher mean effective pressure. A drawback is that the firing pressure is considerably increased as well, so that all the main moving parts of engine are more severely loaded. Besides, detonation phenomena associated with self-ignition of the combustion mixture, impose limits on the attainable increase in compression ratio. The most reliable method of reducing the volume of the combustion chamber is to employ pistons of special design of which the head protrudes into the combustion chamber at top dead center. Many engine manufacturers and components dealers can supply pistons with specially shaped heads (Fig.2b) for various makes of engines.

Another possibility of reducing the combustion chamber includes removal of metal from the cylinder head at its contact surface with the cylinder block. However, care must be taken not to remove too much metal, as this could result in piercing of the water jacket, undue loss of structural rigidity, and excessive increase in compression (Fig.1). Alternatively, the individual cylinder or the whole cylinder block can be reduced in height, for which purpose the cylinder head must be suitably grooved to receive the top of the cylinder (Fig.2). Finally another way to reduce combustion chamber volume is by means of buildup welding on the inside.

With all measures so far described it is essential to determine the actual volume of the combustion chamber with the aid of kerosene and a graduated pipette (Fig.1). The piston should be at top dead center and the inlet and exhaust valves closed when this is done.

Improving the gas flow. The fuel-and-air mixture for the cylinders of a multi-cylinder engine is usually supplied by only one carburetor. A relatively narrow long and often bent or curved inlet pipe connects the carburetor to each cylinder. This shape is unfavorable, from the point of view of gas flow, as it causes throttling effects which adversely affect the volumetric efficiency. Quite often the shape of the inlet pipe is utilized by the engine manufacturer as a means of throttling down the gas flow and thus limiting the engine performance. Changing the inlet pipes and employing more than one carburetor often constitute the easiest means of tuning. It does not involve dismantling the engine.

Multi-carburetor assemblies are available for many engines from components dealers, so that in most cases the owner of the vehicle is spared the effort and cost of developing such a system for his engine. The greatest gain in performance is achieved when each cylinder is provided with its own individual carburetor. However, if the inlet duct is cast integrally with the cylinder head, it is not possible to do more than merely enlarge the opening to which the carburetor is connected. For this purpose, the cylinder head must be removed. The inlet opening can be enlarged as much as the wall thickness of the water passages will permit. Fig.2 shows the increased inlet cross section in the cylinder head of a Fiat engine. The connection to the carburetor is formed by means of an adapter.

In a Volkswagen engine the inlet ducts of the two rows of cylinders are located far apart, with the carburetor midway between them. The gas-flow passages comprise bends and are quite long (Figs 3a and 3b) besides being narrow. To prevent condensation of gas vapor, the inlet pipe is heated (Fig 3b). This heating lowers the volumetric efficiency of the engine, however, because the density of the combustion mixture is thereby reduced, so that actual quantity drawn into the cylinders is reduced.

Measures to improve the gas flow to the cylinders of a Volkswagen engine are therefore particularly rewarding in that they result in a marked increase in performance. Each pair of cylinders shares one inlet opening in the cylinder head, giving access to a forked inlet duct. For this reason it is not possible to fit more than two carburetors to such an engine (Fig.4).

The pipe connections to the inlet and to the exhaust side of the engine should be modified to suit the enlarged gas-flow passages. It is essential to achieve a clean, smooth transition at such connections (Figs 1a and 1b).