The best kinds of performance modifications are those that are simple and inexpensive. While it's easy to get lost on the costly road to squeezing more horsepower out of an engine, another and often more effective route to take is to improve handling and put existing horsepower to better use. Horsepower that goes up in smoke or causes uncontrollable handling while driving in anything other than a straight line is all show and no go.
One of the easiest ways to tighten up vehicle suspension performance and handling is with a strut tower bar. Performance cars like the Mitsubishi Lancer Evolution come from the factory with a strut tower bar. Chances are good a strut tower brace is available from the aftermarket if your car didn't come so equipped. A good example of this availability is the Cusco strut tower bar for the 1982 Toyota Starlet shown here.
The Macpherson style strut suspension is pretty much standard issue on most modern cars-especially those of the economical or compact variety. The struts themselves are the key to the simplicity of the system. By combining the shock absorber and spring into one unit, the need for an upper control arm is eliminated. The tops of these struts mount into strut towers. The towers are part of the unit-body structure of the car.
The bottom side of the strut towers bolts up to a ball joint and lower control arm, which is also bolted up to the unit body of the car. The long and short of it is that the body of the car acts as a chassis. Since the suspension is connected directly to the body, a certain amount of body flex is to be expected. As the steering is connected to the body, this flex can change steering geometry as the body flexes.
The strut tower bar lives up to its name by connecting the tops of the two strut towers via a stiff bar. Think of the strut tower bar like taping up a cardboard box. The box is flexible before the top is closed and taped. Once the sides and top of the box get taped together, the box structure becomes rigid and strong. The strut tower brace boxes up the upper part of the MacPherson strut suspension, and forms a full circle of bracing to help prevent unwanted suspension flex.
The reason every car doesn't come with a strut tower brace is that most economy cars are built with economy in mind. Most folks don't pick up economy cars with performance in mind. For the few who like to push their cars to limits on a racetrack, or enjoy spirited driving on canyon roads, a strut tower bar is an easy modification that can bring noticeable improvement in handling.