Automakers are usually more eager to improve gas mileage and keep maintenance costs low than to maximize performance. That means most cars are born with the equivalent of a stuffy nose. Here are two ways to increase horsepower and bring out your ride's inner hot rod.

[edit] Steps

Upgrade to a free-flow aftermarket air filter.
- Buy a free-flow air filter that fits your car.
- Open hood. Locate the air filter housing. (Look for the long pipe-like thing connecting the engine to a square, plastic box.)
- Remove housing. Unscrew clamps or screws and open filter housing.
- Remove filter. Discard the old air filter (Note: remember how it is fitted).
- Clean the inside of the box with a damp cloth.
- Insert new, free-flow filter.
- Close housing box and drop hood.
Enhance your engine's computer with a power module.
- Disconnect negative battery cable.
- Locate the car's computer. Look in the driver's manual, a Chilton or a Haynes guide to see how to access the car's computer.
- Unplug wiring harness from computer.
- Plug in an after-market power module. Insert it between the wiring harness and the computer.
- Reconnect negative battery cable.
- Follow the module's instructions for starting car.
- Fill 'er up with gas and race with up to 30 extra ponies.
Enhance your car's computer with a programmer.
- These devices modify the programming on your vehicle's computer, allowing more horsepower, torque, and sometimes better fuel economy.
- Once you purchase a programmer for your vehicle, read the instruction manual to install it. Programmers differ on how they install and work, so reading the manual is the only safe option.
Upgrade to a free flowing exhaust
- First, find out the exhaust piping diameter of your exhaust. Do not just measure the diameter of your exhaust tip. Measure the pipe that comes out of your catalytic converter, as its usually the smallest.
- Search online or in a store for a performance exhaust system for your car. Search also for videos or sound clips of different exhaust systems as some of them may make your car sound unappealing to you.
- You'll want to find a "cat back" system. These systems run from your catalytic converter to the end of the exhaust piping.
- Almost always you'll want to get a system that is 1/4 to 1/2 inches LARGER than the one your car came with from the factory. Some people can go larger, but it requires serious modification to the engine. Making the exhaust TOO free flowing can have a negative impact on low end torque, so you'll probably want 1/4 inch for a 4 cylinder, 3/8 for a six and 1/2 for a v8.
- Once you have the system, you can either cut the old one out yourself, or have a shop do it.
- If you cut the old one out, cut it just after the catalytic converter and leave an inch or two to clamp or weld onto. Detach the rest of the exhaust system from the hangars.
- Position the new system in place (assuming you've assembled it) and weld or clamp it to the spot you just cut. Reinstall the hangers that you removed.
- Start the car and feel a good 10-15 hp increase.

[edit] Tips

When testing your car after upgrading, take it to a safe sanctioned race track, if possible. Race tracks are not only safer, but give you a record of how fast your car really is, and how much your performance has improved when making comparison runs.
Join an online car forum (http://www.n-y-m-e.co.uk is a good one) tailored specifically for your make or model of vehicle. Learn from others about what works and what doesn’t so you don’t waste your time and money or damage your vehicle.

[edit] Warnings

Check with your local licensing agency about emissions regulations. Certain upgrades to your car’s engine and exhaust can prevent you from passing emissions and possibly violate federal emissions laws.
Certain states, such as California, require each after-market performance part to pass a certification for use on a street vehicle (CARB, or California Air Resources Board). If your upgrades don’t have this sticker, your vehicle can be impounded!
Be aware the many after-market air filters will actually allow greater particulate matter to pass into the engine and oiled filters, especially if improperly maintained, will also spray oil into the intake. The oil from the filter can damage or destroy the mass air flow (MAF) sensor on cars so equipped. In forced induction engines the marginal benefit of increased air flow with an after-market filter is swamped by the fact the turbo will supply the requested air regardless of filter and greatly increased reliability from not having oil and sand in the intake and on the MAF.
Many modern cars have snorkels on the airbox intake placed at a high pressure point in the front of the vehicle to intake cooler outside air. If one chooses to install an after-market cold air intake (CAI) attention should be payed to the current setup. Furthermore, be aware that low-mounted CAI installs have a greater chance of sucking up water in rainy weather causing hydrolock and major engine damage.