How to Add More Horsepower to a BMW 325IS

1. Install a cold air intake to help increase the flow of new and fresh air into the engine and its intake manifold. From the factory, your 325is has a plastic air box which houses the air filter. You can purchase and install an aftermarket intake system that has an open, cone-shaped air filter to place below the engine and behind the driver's side front bumper. This design offers a less-restrictive, more free-flowing intake that can put additional fresh air into the engine faster than the factory's air box.
2. Replace the factory exhaust system with an aftermarket cat-back exhaust system to further increase your 325is's horsepower. The exhaust system has three primary components: the exhaust headers, the mid-pipe or catalytic converter, and the exhaust and muffler at the end. While you can perform a complete replacement of all the components, a cheaper, just as effective option is to install an aftermarket system that replaces the piping and muffler from the catalytic converter and back. These aftermarket systems help push combusted air out of your engine more effectively and can also add a deep exhaust tone.
3. Maximize the horsepower by matching an aftermarket engine control unit, or ECU, to the intake and exhaust upgrades. The engine control unit constantly adjusts the engine's parameters as you drive your car. It pulls in information from the engine's sensory system to determine factors such as the necessary air-to-fuel ration. By adding an intake or exhaust to your car, you are slightly modifying the engine's factory air-to-fuel ratios, which cause the factory ECU to make constant adjustments. By installing an aftermarket ECU, you essentially upload a custom fuel program to your engine, maximizing the benefits of the other aftermarket upgrades you've added.