Engine tuning is the modification or design of internal combustion engines to yield more performance, either in terms of power output or economy. It is a popular pastime with amateur mechanics and petrolheads. It has a long history, almost as long as the development of the car in general, originating with the development of early racing cars, and later, with the post-war hot-rod movement.

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 misture into the engine, 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 size of the swept volume of the engine, raising its capacity. This can be done by increasing the diameter of the cylinders and pistons, or by using a crankshaft with a longer stroke, or both.

  • Using larger carburettors, 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, but may be modified in a similar manner.

  • Increasing the size of the valves in the engine, thus increasing the velocity that the fuel/air mixture can enter the engine, and the exhaust gases to leave. 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.

  • Increasing the valve opening height (lift), by changing the profiles of the camshaft.

  • Optimising the valve timing to improve burning efficiency - usually this increases power at one range of operating speeds 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.

  • Using larger bored, smoother, less contorted manifolds for intake and exhaust. This helps maintain the velocity of gases. The larger bore may extend right through the complete exhaust system. Similarly, the ports in the cylinder are enlarged and smoothed to match.

  • Raising the compression ratio, leading to more rapid burning of fuel.

  • Adding a supercharger or turbocharger. This forces more fuel/air into the engine by raising its pressure, rather than simply relying on the suction of the descending piston.

  • Using a fuel with higher energy content, or by adding an oxidiser such as Nitrous.

  • Changing the tuning characteristics electronically, by changing the firmware of the engine management system. 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 over a wider range of speeds, 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.