The Crown Victoria Police Interceptor, also known as the CVPI or the P71, remains the last rear-wheel V-8 in production in North America. New cars are only available to fleet buyers in law enforcement and transportation, but a vibrant trade in used specimens persists in the face of high energy costs. This car has developed a sizeable following, seeing as the competition has largely disappeared. Used police interceptors in good condition are relatively affordable and make for a good starting point for those interested in spending time, effort and money on modifications. Because of its potential, the Crown Victoria provides great reward for those who invest in modifications, seeing big gains in performance and responsiveness.
Decide if you want to go soft or deep on the modifications--soft means replacing those easy-to-reach and change parts that don't have as much impact on the car, those that cost less but also give less. Deep means really getting in there and modifying the essentials. The former typically refers to intake, exhaust, tires and computer; the latter addresses the power train.
Begin with swapping the air intake with a cold air intake from K&N. This will net you an extra 10 to 15 horsepower on most cars.
Install a Flowmaster cat back exhaust system--this takes the stock 2-inch exhaust and upgrades it by at least 1/4 inch, providing the car with more air and therefore more power. Other options do exist, but the basic Flowmaster performance cat back should suffice, easily adding an extra 10 horsepower.
Change the rear gears. This is an excellent way to increase torque and off-the-line response, but doesn't add horsepower. For sheer speed, this modification is a must. Go with Ford-brand 3.73s; this is what most Crown Victoria aficionados recommend.
Invest in so-called shorty headers once your Flowmaster exhaust is fully broken in. These performance headers create more power by allowing the engine to breathe easier and do not heat up as much as do the stock headers, resulting in less power loss. Headers are hard to install, so this may be the most expensive mod on your list until you get to the engine proper. Expect up to 20 horsepower more once this is done.
Look into modifying your transmission--the ones the police interceptors come with leave much to be desired. A five-speed transplant would be best, but costly and even risky. A simpler route would be to install a shift kit, such as the ones made by TransGO.
Install a so-called underdrive pulley, which is a crank that better uses energy to drive vehicle accessories and pumps, hence "underdrive." It doesn't consume as much power as do the cheaper varieties from the factory, theoretically leaving more power for the engine to use. The amount of horsepower added remains debatable.
Don't bother changing your tires and rims at first--police interceptors come with speed-rated tires that can take moderate increases in performance. Wait until the process is well under way, but remember tires and rims are largely a cosmetic mod in this case. There's no need to invest in a chip--police interceptors don't have a governor limiting speed, and chips do not make your car go faster.
When modifying your exhaust system, resist the temptation to remove the catalytic converters and mufflers. Not only is doing so illegal, it will generate noise and vibration that can adversely affect your car over time, often resulting in exhaust leaks and substantial power loss. If a Crown Victoria Police Interceptor costs you only a few hundred dollars and has more than 200,000 miles, chances are this car was a taxi. While not a bad thing in itself, you must be aware of this, as it implies abuse and certainly a loss of performance compared with non-taxi Interceptors.