I hear a lot of people wanting to increase the
horsepower in their
vehicle, and I hear a lot of misconceptions about it too. The following should help you out a bit.
AIR FILTER
A performance air filter can get more air into the vehicle, and a high-flowing model is a good start. K&N is a popular brand. It is simple to install and you can expect a 5-10 HP gain (usually closer to 5), of course varying with your automobile. They usually cost about $70.
EXHAUST
The exhaust system in most vehicles just doesn't allow enough air flow. A good thing to do is install a cat-back exhaust system. This will probably cost anywhere from $170 for a cheapie on up. This can add 10-20 HP to an engine.
EXHAUST HEADERS
Aftermarket headers can be placed on in place of stock headers. Headers start usually around $230 and go up. They are a very good investment and can add up to 20 HP. When selecting a set of headers go with the ones with a ceramic coating. Paint won't hold up.
IGNITION SYSTEMS
Ignition systems can add power and increase gas mileage. I'm not sure of actual HP gains using a performance ignition but usually it is noticable. A good ignition usually starts at around $400 and goes up.
COMPUTER CHIP
A computer chip upgrade for a vehicle can make a significant difference. However, you will probably want to modify other things and do the chip last. This way the chip will be programmed to utilize the mods you have. It wouldn't be good to add a chip and then make a lot of modifications unless you order a chip from a company that will let you send the chip back to be reprogrammed. These work by modifying the air/fuel ratio, idle speed, and spark timing of the vehicle. While you will probably be forced to run high-octane fuel afterwards, it is often a great investment. Chips start at around $170 and often can add 20 HP or more.
UNDERDRIVE PULLEYS
These work by reducing accessory drag. There are a few problems with these, one being that sometimes the alternator won't be able to charge the battery at idle. These can provide up to 15 HP , but I generally don't care for them.
SUPERCHARGER/TURBOCHARGER
This is often the best mod that you could make. These will usually provide around a 40-50% increase in power. They can add 100 HP or more to almost any engine. They both work by forcing air and fuel into your engine, although they work differently. The vehicle will need to be modified somewhat before putting these on. They normally cost about $3500 and go up from there. If you know a mechanic you can probably get a cheaper one.
NITROUS OXIDE
A cheaper alternative to the supercharger is a Nitrous Oxide kit. It delivers high amounts of oxygen to the engine. A NOS kit will cost you around $500 and up. Traction must usually be worked on before adding a kit because the instant horsepower is hard on traction. Most kits provide about a 60-200 HP boost, but I've seen models that will provide around 350HP. Wow! I've seen cars take off 2 seconds in the quarter with a nitrous kit.
While many, many other things can be done, I feel these things are done pretty commonly and they will all help with boosting your HP. Figure horsepower gains to be on the lower end of what I have stated. The higher end is what you may see on a V8 350 or bigger engine.