Engines are very complex machines, and nearly every part can be replaced with a higher performance part. The catch is that certian parts can only take certian amonts of power. For example: if I modified my engine to be 400hp, but my camshaft could only take 350hp, I would have one VERY broken car.

The most common engine modifications are intake, intake manifold, and exhaust manifold (headers). These are the easiest to install, and create just enough power gain for the rest of the engine to handle.

More advanced modifications include camshafts, fuel injectors, cam rollers, replaced crankshaft, cam gears, and modified forged pistons.

This is an example of a modified STi engine.