A car's engine makes power, but where exactly is the place in the engine does the power get made? Most will argue that the output of the engine is transmitted through the crank and that is where the power is made. Others will say that the pistons move the crank so that is where the power is made. And while those arguments do hold plenty of weight, for me an engine's power is made in the head.
Consider that a gasoline engine is basically an air pump: I t sucks air thought the intake side of the head and blows it out the exhaust side. For a brief moment that air is mixed with gasoline or a similar combustible gas, ignited with a spark from a sparkplug and the resulting explosion sends the piston down the cylinder which in turn spins the crank. The more efficiently an engine sucks in air, ignites the air/fuel mixture, and then passes the exhaust out of the engine, the more power it makes. As the head is the portion of the engine devoted to moving air and containing the explosion that is each cylinder firing, it is the single most important component for creating power.
Commonly, the tyro At Home Mechanic will seek to add power to their car's engine by bolting on some simple improvements like a Cold Air Intake (CAI) or a Tuned Exhaust Manifold (commonly called a "header"). These are external devices that free up the flow of air into the head and they can easily increase power and response by a few percentage points. But if you want to make a lot of extra power in your engine, it will be necessary to go beyond these simple bolt-ons and do some work to the internal workings of the head.
The head is a complexly designed piece in today's motors and most manufacturers do a good job of designing their heads to facilitate flow. But even in the most expensive cars, there is still room for improvement over the factory's design. The primary improvement to be made is to improve the flow of air through the head's ports with a process called Porting and Polishing. This is process where the ports are enlarged and smoothed to let more air move smoothly through the head. Done by a professional to insure that every port is ported and polished exactly the same amount to preserve engine balance, this can add a realistic 10% extra efficiency in your engine. And efficiency adds power.
Contained in the head are the valves (the actual gateways to allow air and fuel into the engine and the exhaust out), which can be reground and polished to improve flow past the valves. And changing the cam, which is the part that controls the opening and closing of the valves, will complete the upgrade to the head and its components to make more power in your engine.
So the key to making real power in any internal combustion engine is to improve flow through the head.