Any car that you buy today from a normal automobile manufacturer has a number of compromises built into it. These compromises are caused by several factors:

* The attempt to fit the car into a certain price range
* The need to meet emissions standards
* The desire to provide maximum life and reliability

These compromises often give you lots of room for improvement in terms of performance! There are many different ways to create more horsepower from a stock engine. Here are several examples (generally in order from least to most difficult/expensive):

* Change your computer chip -- sometimes, but certainly not always, you can change a car's performance by changing the ROM chip in the engine control unit (ECU). You usually buy these chips from aftermarket performance dealers. It is valuable to read an independent review of the chip you are contemplating, because some chips are all hype and no performance.

* Let air come in more easily - As a piston moves down in the intake stroke, air resistance can rob power from the engine. Some newer cars are using polished intake manifolds to eliminate air resistance there. Bigger air filters and reduced intake piping can also improve air flow.

* Let exhaust exit more easily - If air resistance or back-pressure makes it hard for exhaust to exit a cylinder, it robs the engine of power. If the exhaust pipe is too small or the muffler has a lot of air resistance then this can cause back-pressure. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system.

* Change the heads and cams -- Many stock engines have one intake valve and one exhaust valve. Buying a new head that has four valves per cylinder will dramatically improve airflow in and out of the engine and this can improve power. Using performance cams can also make a big difference.

* Stuff more into each cylinder - If you can cram more air (and therefore fuel) into a cylinder of a given size, you can get more power from the cylinder (in the same way that you would by increasing the size of the cylinder). Turbo chargers and super chargers pressurize the incoming air to effectively cram more air into a cylinder. Many manufacturers make aftermarket turbos and super chargers for many different cars.

* Cool the incoming air - Compressing air raises its temperature. You would like to have the coolest air possible in the cylinder because the hotter the air is the less it will expand when combustion takes place. Therefore many turbo charged and super charged cars have an intercooler. An intercooler is a special radiator through which the compressed air passes to cool it off before it enters the cylinder.

* Make everything lighter - Lightweight parts help the engine perform better. Each time a piston changes direction it uses up energy to stop the travel in one direction and start it in another. The lighter the piston, the less energy it takes. Lighter parts also allow the engine to rev faster, giving it more horsepower.

* Increase the compression ratio - Higher compression ratios produce more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burst into flame (prior to the spark plug igniting it). Higher octane gasolines prevent this sort of early combustion. That is why high-performance cars generally need high octane gasoline - their engines are using higher compression ratios to get more power.

* Increase displacement - More displacement means more power because you can burn more gas during each revolution of the engine. You can increase displacement by making the cylinders bigger.

As soon as you start to try increasing displacement, you might consider the economics of buying a new high-performance engine and trying to fit it into your car -- it may be easier and cheaper!

Here are several interesting links:

* How Horsepower Works
* Increasing horsepower
* Modifications
* Bolt-on performance