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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 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 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 injectionFuel injection is a technology used in internal combustion engines to mix the fuel with air prior to combustion. As in a traditional carburetor, fuel is converted to a fine spray and mixed with air. However, where a traditional carburetor forces the incom is more often used, and may be modified in a similar manner.

• Increasing the size of the valveA poppet valve is the type of valve system used in most piston engines, used to seal the intake and exhaust ports. The valve is usually a flat disk of metal with a long rod known as the valve stem out one end. The stem is used to push down on the valve ans 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 manifoldIn automotive engineering, an intake manifold or inlet manifold is a part of an engine that supplies the fuel/ air mixture to the cylinders. An exhaust manifold or header collects the exhaust gases from multiple cylinders into one pipe. Due to the suckings. This helps maintain the velocity of gases. Similarly, the ports in the cylinder are enlarged and smoothed to match.

• The larger bore may extend right through the complete exhaust system, using larger diameter piping and low back pressure mufflerA muffler (known as a silencer in the United Kingdom and other areas) is a device for reducing the level of noise emitted by a machine, most commonly internal combustion engines. Mufflers often contain a series of baffles to cancel out the noise. They ares.

• Increasing the valve opening height (lift), by changing the profiles of the camshaftThe camshaft is an apparatus used in piston engines to operate poppet valves. It consists of a cylindrical rod running the length of the cylinder bank with a number of oblong lobes or cams protruding from it, one for each valve. The cams force the valves or the lift ( leverIn physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another object. This is also termed mechanical advantage, and is one example of the principle of moments.) ratioIn algebra, a ratio is the relationship between two quantities. It is expressed as the quotient of one magnitude divided by another, or as a relation between several variables. Examples: If a school has a twenty-to-one student-teacher ratio, that means th 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.

• Raising the compression ratio, leading to more rapid burning of fuel, by using larger compression height pistons or thinner head gasket or by milling the cylinder head.

• 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.

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

• 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.