Tuning means improving your driving experience. You can improve your engine or the look&feel.
In most cases, people are interested in increasing the power output of an engine. Many well tried and tested techniques have been devised to achieve this, but essentially all operate to increase the rate (and to a lesser extent efficiency) of combustion in a given engine. This is achieved by putting more fuel/air mixture into the engine, using a fuel with higher energy content, burning it more rapidly, and getting rid of the waste products more rapidly – this increases volumetric efficiency. The specific ways this is done include:

  • Increasing the engine displacement. This can be done by “boring” – increasing the diameter of the cylinders and pistons, or by “stroking” – using a crankshaft with a longer stroke (and pistons with shorter compression height), or both.
  • Using larger or multiple carburettors, to create more fuel/air mixture to burn, and to get it into the engine more quickly. In modern engines, fuel injection is more often used, and may be modified in a similar manner.
  • Increasing the size of the valves in the engine, thus decreasing the restriction in the path of the fuel/air mixture entering, and the exhaust gases leaving the cylinder. Using multiple valves per cylinder results in the same thing – it is often more practical to have several small valves than have larger single valves.
  • Using larger bored, smoother, less contorted intake and exhaust manifolds. This helps maintain the velocity of gases. Similarly, the ports in the cylinder are enlarged and smoothed to match.
  • Increasing the valve opening height (lift), by changing the profiles of the camshaft or the lift (lever) ratio of the valve rockers (OHV engines), or cam followers (OHC engines).
  • Optimising the valve timing to improve burning efficiency – usually this increases power at one range of operating RPM at the expense of reducing it at others. For many applications this compromise is acceptable. Again this is usually achieved by a differently profiled camshaft.
  • Adding a supercharger or turbocharger. This forces more fuel/air mass into the engine by raising its pressure, rather than simply relying on the suction of the descending piston.
  • Changing the tuning characteristics electronically, by changing the firmware of the engine management system (EMS). This often works because modern engines are designed to give a lot of raw power, which is then reduced by the engine management system to make the engine operate smoothly over a wider RPM range, with low emissions. By analogy with an operational amplifier, the EMS acts as a feedback loop around an engine with a lot of open loop gain. Many modern engines are now of this type, and are amenable to this form of tuning. Naturally many other design parameters are sacrificed in the pursuit of power.