Performance tuning is the tuning of an engine for
motorsports. Many such automobiles may never compete but are built for show or leisure driving. In this context, the power output (e.g. In
horsepower),
torque, and
responsiveness of the engine are of premium importance, but reliability and
fuel efficiency are also relevant. In races, the engine must be strong enough to withstand the additional stress placed upon it and the automobile must carry sufficient fuel, so it is often far stronger and has higher performance than the mass-produced design on which it may be based. The
transmission,
driveshaft and other load-transmitting
powertrain components may need to be modified to withstand the load from the increased power. There are many techniques that can be used to increase the power and/or efficiency of an engine. This can be achieved by modifying the
air-fuel mixture drawn into the engine, modifying the static or dynamic
compression ratio of the engine, modifying the fuel used (e.g. higher
octane, different fuel types or chemistries), injection of
water or methanol, modifying the timing and dwell of ignition events, and compressing the intake air.
Air fuel ratio meters are used to accurately measure the amount of fuel in the mixture. Fuel weight will affect the performance of the car, so fuel economy (thus efficiency) is a competitive advantage. Ways to increase power include: • Increasing the
engine displacement by one or both of two methods: "
boring" - increasing the diameter of the
cylinders and
pistons, or by "stroking" - using a
crankshaft with a greater throw. • Replacing a stock
throttle body with either a larger throttle body (Since it increases airflow due to its larger bore size), an
electronic throttle body that opens quickly so that it can access airflow sooner (Which improves
throttle response), or a combination of both. • Using larger or multiple carburetors to create a more controllable air/fuel mixture to burn and to get it into the engine more smoothly.
Fuel injection is more often used in modern engines, and may be modified in a similar manner. • Increasing the size of the
poppet valves in the engine, thus decreasing the restriction in the path of the fuel–air mixture entering the cylinder and the exhaust gases leaving it. Using
multiple valves per cylinder results in the same effect, though it is often more difficult to fit several small valves than to have larger, single valves due to the
valve gear required. It can also be difficult to find space for one large valve in the inlet and a large valve on the outlet side, and sometimes a large exhaust valve and two smaller inlet valves are fitted. • Using larger bored, smoother, less-contorted
inlet manifold and
exhaust manifolds helps maintain the velocity of gases. The ports in the
cylinder head can be enlarged and smoothed to match. This is termed
cylinder head porting. Manifolds with sharp turns force the air–fuel mix to separate at high velocities because fuel is denser than air. • The larger bore may extend through the
exhaust system using large-diameter piping and low
back pressure mufflers, and through the
intake system with larger diameter
airboxes and high-flow, high-efficiency
air filters. Muffler modifications will change the sound of the engine, usually making it louder. • Increasing the
valve opening height (lift) by changing the profiles of the cams on the
camshaft or the
lever (lift) ratio of the valve rockers in
overhead valve (OHV) engines, or
cam followers in
overhead cam (OHC) engines. • Optimizing the valve timing to improve burning efficiency; this usually increases power at one range of operating
RPM at the expense of reducing it at others. This can usually be achieved by fitting a differently profiled camshaft. • Raising the
compression ratio by reducing the size of the combustion chamber, which makes more efficient use of the cylinder pressure developed and leading to more rapid burning of fuel by using larger compression height pistons or thinner
head gaskets or by using a
milling machine to "shave" the
cylinder head. High compression ratios can cause
engine knock unless
high-octane fuels are used. •
Forced Induction; adding a
turbocharger or a
supercharger. The air/fuel mix entering the cylinders is increased by compressing the air. Further gains may be realized by cooling the compressed intake air (compressing air makes it hotter) with an
air-to-air or
air-to-water intercooler. • Using a fuel with higher energy content and by adding an
oxidizer such as
nitrous oxide. • Using a fuel with better knock suppression characteristics (race fuel, E85, methanol, alcohol) to increase timing advance. • Reducing losses to friction by machining moving parts to lower tolerances than would be acceptable for production, or by replacing parts. This is done In overhead valve engines by replacing the production rocker arms with replacements incorporating roller bearings in the roller contacting the valve stem. • Reducing the rotating mass comprised by the
crankshaft,
connecting rods,
pistons, and
flywheel to improve throttle response due to lower rotational inertia and reduce the vehicle's weight by using parts made from alloy instead of steel. • Changing the tuning characteristics electronically, by changing the
firmware of the
EMS. This
chip tuning often works because modern engines are designed to produce more power than required, which is then reduced by the EMS to make the engine operate smoothly over a wider
RPM range, with low emissions. This is called de-tuning and produces long-lasting engines and the ability to increase power output later for facelift models. Recently emissions have played a large part in de-tuning, and engines will often be de-tuned to produce a particular carbon output for tax reasons. • Lowering the underbonnet temperature to lower the engine intake temperature, thus increasing the power. This is often done by installing
thermal insulation – normally a heatshield,
thermal barrier coating or other type of
exhaust heat management – on or around the exhaust manifold. This ensures more heat is diverted from the under-bonnet area. • Changing the location of the air intake, moving it away from the exhaust and radiator systems to decrease intake temperatures. The intake can be relocated to areas that have
higher air pressure due to aerodynamic effects, resulting in effects similar to
forced induction. The choice of modification depends on the degree of performance enhancement desired, budget, and the characteristics of the engine to be modified. Intake, exhaust, and chip upgrades are usually among the first modifications made because they are the cheapest and make reasonably general improvements. A change of camshaft, for instance, requires a compromise between smoothness at low engine speeds and improvements at high engine speeds. ==Definitions==