Air filters come in all sorts of different shapes and sizes and are there to prevent potentially damaging particles entering the engine. Smaller holes in the filter protect the engine better, but also restrict air flow. Clever design can improve the passage of air without compromising the engine, so at the very least you should replace the standard air filter with a higher performance version. Many filter upgrades are disposable so you'll need to stock up on a few for when you next service your car, some are reusable but will need cleaning periodically – make sure you follow the instructions to get the most out of your purchase.
Air Intake Systems
Changing the amount of air intake is going to yield the most dramatic results in increasing horsepower.
The way to increase natural air flow is by improving the pipe construction transfering the air to the engine. Fluid dynamics is an fascinating science, but also fairly inaccessible to most people. To sum up in a sentence, gas flows better in specially designed pipes. Think of the transition when pouring a bottle of water from the glug glug action to the nice fast flowing stream, this is what you need to achieve with the air going into the engine. Even with your nice new air filter fitted, you'll still be limited by the inlet pipe design. In a Porsche you can be certain they've fitted the best possible turbulence free solution, but in bargain racers you can probably improve on the stock pipes. The major air filter companies such as K&N and Pipercross do a variety of 'induction kits' and if you dig around you can probably find one which is suitable for your car. These will consist of an air filter, and all the necessary pipes and fittings to get the air into the engine as fast and smoothly as possible.
Unlike natural induction of airflow, a turbocharger is a device which forces air into the engine. It is, in fact, a pump, usually operated by exhaust fumes. This however means that the turbo pump will not act effectively before the engine revs increase to a point of creating enough pressure through the exhaust to activate it. At a certain moment, the turbo will burst into action. This can be improved by Anti-Lag systems and Left foot braking. A turbo must be matched to the engine type. Turbochargers also require additional cooling, hence the need to intercoolers.
The secret of increasing power via the turbo is to ensure the correct fuel to air ratios are maintained while increasing boost pressure – if you're pumping in more air you need more fuel too. At first, it's usually safest to increase the pressure by only a few PSI. The software on most modern vehicles will be able to adapt to this level of increase and deliver increased fuel to compensate. The methods involved with increasing boost pressure are varied, and you'll almost certainly need a professional to do this for you, but the technique is outlined below.
The basic boost pressure is maintained by the waste gate which is designed to open at set pressures. Adjusting the waste gate to open at higher pressures is probably the easiest method of increasing boost. If you're looking for more power, you'll need a specialist to tweak the injection settings.
Higher flow injectors (or carburetor), and a higher flow/pressure fuel pump are generally not needed unless other engine modifications have significantly increased the potential air flow, but should follow major power modifications such as turbos, superchargers, higher compression pistons, bigger cams, etc.
Remapping or "updating" the ECU system that monitors the plugs and engine economy, can also help in improving performace. better plugs, wires, and spark coils will increase the intensity of the spark in the cylinder chamber, burning more of the fuel and generating more power.
<Ed: Please list your preferred brand, a link, and an explanation why they work well in your application>
See Also: Lubrication
It's also important to clear out the waste generated by combustion in order to enable the engine to intake air to-fuel mixture faster. This also helps in powering up turbochargers. This is done by upgrading the exhaust. The first station is the exhaust manifold: Exhaust exits each cylinder via small pipes, and depending on the configuration of your engine these combine into one, two or more larger versions. The resulting tangle of metal pipes can make it difficult for the gas to flow smoothly and freely, and a replacing this with performance exhaust manifold will aid the flow.
An exhaust pipe needs to be matched to an equally decent manifold for it to yield the largest gains in performance. This final part of the exhaust system is designed to reduce sound and pollution (if a catalytic converter is fitted) and have a variety of filters and chambers to help achieve this. 'Cats' remove poisonous chemicals such as nitrous oxide and sulphur dioxide, mufflers deaden the noise but both sap precious power. If your conscience allows it, removing both of these elements can provide a few additional horsepower. A good 'straight through' stainless steel pipe combined with an optimized manifold can be an easy and worthwhile investment.
Having upgraded the exhaust itself, the muffler, the piping between the catalytic converter and muffler, the piping to the catalytic converter, the exhaust manifold, and the catalytic converter itself, are usually the stages best followed.
The car's drivetrain, is the transsmission of power from the combustion within the engine's cylinders and unto the road. This includes: The Engine, transmission, Driveshaft, differential and driven wheels. Different cars are driven by different sets of wheels. Most modern road cars, are "Front-wheel drive" (FWD or 2WD), meaning that power is transmitted to the front wheels. Some old cars, or modern cars that are considered sporty (like most BMW's) are rear-wheel drive (RWD), where the power is going to the rear-wheels. A smaller portion is All-wheel-drive (AWD, 4WD), where all four wheels are permenantly or partially driven by the engine. Another important aspect of the drivetrain is the differential and the location of the engine. Most cars have front-mounted engines, but some have middle or rear mounted ones. This is important because, while driven wheels effect road holding, engine mounting effects car balance, together forming the car's road handling characteristics. A front engine means that transmitting power to the front wheels is easier, and that more weight is layed on them (typically 60%). Additionally, engine location generates momentum. If located in the rear, that part will actually be the one doing most of the rotation, or on the front. A mid-engine car generates a central axle around of which the chassis can be more easily and accuratly rotated. In fact, momentun usually has a more crucial effect, becuase there are other mechanical components that make up for the car's weight distribution, in order to attempt and achieve something closer to a 50:50 distribution in sports cars.
This is the common sort of drive in a road car. It's cheaper to make and to purchase, though not usually fit for racing without being slightly modified.
This is the common type of racing vehicles, and the ones most adventagous in advanced motoring. Typically, a front or mid-mounted engine is better. A. Front-Engine:
A. Front Engine
B. Mid Engine:
C. Rear Engine: