Here are some of my DOs and DON'Ts when modifying your cars.
Study your car properly before modifying, especially when it comes to performance. Study its strength and weaknesses. Look for areas where you can improve your car's performance and handling.
Have a goal when modifying? What do you want to achieve? Will it be an everyday car that can also be used in the tracks? Or perhaps an all out drag car? Your goal is what will help you decide on the steps to undertake when modifying your car.
Know the purpose of each and every part you put in. Example, a body kit is not meant for "porma" purposes, but it is meant to add more downforce and to improve the aerodynamics of your car.
The most basic performance upgrade is to improve your intake and exhaust system. A cone type or open type intake system like K&N typhoon series or AEM cold-air intake gives you an additional 10bhp. A full exhaust system with less cats, less bends and a free flowing muffler may add another 10bhp. Angle your muffler only if your mid pipes requires you to do so, and if it does, this gives a better exhaust flow.
When modifying also your exhaust system, do change your exhaust manifold into headers. For a turbocharged vehicle, tubular exhaust manifolds helps improve spool ups. Headers will definitely bring out the full potential of your exhaust and intake system. Thus, a good intake system, headers, and a properly done exhaust system may give you about 25 to 30BHP. Power gains can easily be felt.
Lower your car properly by the use of lowering springs. These springs are meant to improve body roll when cornering. It also gives the car better traction improving your cornering speed. Lowering your car properly not only helps in making it look better, but it does help in bring the power better to the ground.
Always tune your car when modifying.This is one thing many misses out on. When putting in stuff like a full exhaust system, cams, intake system, or colder step spark plugs, tuning is a must to improve your AFR. Tuning extracts all the hidden power and potentials of your engine with the new bolt ons you had placed in. A proper tune may even add up to 50bhp depending on your mods. Head out to a shop with a chassis dynamometer to eliminate guess work. Tuning is VERY ESSENTIAL. Never overlook this one, especially for those with fuel injected vehicles.
When adding forward power, never ever forget stopping power too. As much as your vehicle goes forward faster, it has to stop faster as well. A good brake pad kit, or a caliper system with rotors will do, depending on the mods. Higher HP cars needs better stopping power. If you are running oversized rotors with 2-piston calipres at least, try having the rotors slotted or cross drilled to improve cooling.
Get only the right body kits when needed. I notice some would get T-Wings on their 1300cc cars with less than 90hp. Wings are for downforce at high speeds. Dont tell me these 90hp cars runs up to 230kph or more. Chins or lower lips are used to channel lower and much colder air to the radiator or filter, and not for porma purposes. So you have to know what each part is for and think whether you need it or not.
Don't be a poser or a wannabe! Example, don't try to make your car look fast if it is not really fast. Example, do not put Evo bodykits and pretend your Lancer is an Evo. If your purpose is to have others think that your car is an Evo wherein it is really not, then there must be something wrong with you already. If I had a garden fresh Lancer, I would rather just keep it clean, simple, and nice. Just get it nice rims and lower it. No need to be a poser.
Do not cut your springs when lowering. Not only is it considered as "ricey", but it ads to the detriment of your suspensions.
Do not angle your muffler if you are just running your car's stock exhaust system. Again, another poser move. People around want to project this image that their car is fast by angling their mufflers without knowing why high performance cars angles them.
Unnecessary body kit. Not only does it make your car look too "over", but it adds up to the weight of the car.
Don't put in any part wherein you will not use it. Waste of money. Like do not put in a B16 engine on your Civic if you will not run it fast or race it on the tracks.
Last but not least and I shall repeat it again, don't be a poser car! Just keep things real!