When most people hear the word Volvo, it no doubt conjures up images of soccer moms driving their kids around in a estate car. But Volvos are also popular cars for tuning and modifying, particularly in recent years as they have become more and more sporty. The S40 is a great example. This is a medium-sized sedan that takes extremely well to the addition of performance parts.
Install larger wheels and tires. This is one of the easiest areas to upgrade as well as one that can make the most difference visually. Larger wheels fill out the wheel arches better then the relatively small original wheels. Wider, stickier tires will improve your Volvo's handling considerably as well.
Tune the suspension for better handling. The Volvo S40 is already a pretty sporty car in stock form, but can take advantage of a performance-oriented suspension. You can go with lowering springs and shocks or for the maximum in handling upgrades install a coilover system. This will allow you to dial in the ride height as well as how stiffly or softly sprang your Volvo is. There are also urethane bushings that can be installed that move less than the stock rubber bushings for a more precise feel. A strut brace for the front is also recommended.
Upgrade the engine. More power is a good thing when it comes to an over-engineered car like the Volvo S40. Here, the usual suspects are a free-flow exhaust, cold air intake and a reprogrammed engine control unit.
Upgrade the brakes. Now that your S40 is faster, better brakes are a good idea as well, since you want to make sure your Volvo remains balanced. There are a couple of options here, one of which is simply installing cross-drilled performance rotors that will help dissipate heat and better brake pads that will reduce your stopping distances. On the more extreme end is a big brake kit which will drastically improve your Volvo's ability to stop.
Modify the interior and exterior. Volvo did a good job on the S40's interior, so upgrades here can be kept to a minimum. There are custom door lock bezels for the S40 as well as upgraded floor mats that will make your interior more luxurious and welcoming. After that move on to the exterior, where you can install a small rear spoiler and a set of mudflaps to keep mud off your car.