Knowing the mods you will want before you buy a used Audi A4 will help you decide what can fit into your budget as well as which year and engine size A4 to look for. The 1.8 engine has more mods for it, but is less reliable. The 2.8 does not have as many, but is known for its higher reliability. As for year, the pre-2000 models are drive-by-cable and thus easier to tune and better for modding, but bad timing belts need frequent replacement on them compared to later models. There are a few common mods that you might be interested in, including the ECU, intake, exhaust, and water injection.
1. Chip. The engine control unit manages fuel flow, and thus upgrading it can give you more horsepower (15 or so) and better performance for a less than huge outlay of cash. This will cost about $500 for the better brands, such as GIAC. Stick with the higher-priced name brands, otherwise you might just be throwing your money away for very little extra boost. This is probably the first mod you will want to do.
2. Intake. Replacing the intake will not get you much more added horsepower beyond a handful, and so upgrading the intake is more for the sound. The highest quality one is Carbonio, however, at $400 it might be a bit much just for sound. Some owners have built their own, however, caution must be taken here as water leaks can result if you don’t do it right, which can cause misfiring and other problems.
3. Exhaust. This will actually add a bit more horsepower to your A4, if you get a high-flowing one. But again, the primary purpose is to get a better sound out of the car. Make sure you get a good name brand, or if you want high quality for cheaper find a good shop that will do a custom exhaust for you. To get an even louder sound, you can add a test pipe, but your A4 will not be able to pass emissions tests if your state requires it because this will involve getting rid of your catalytic converter.
4. Water/meth. This mod injects a mixture of methanol alcohol and water to cool the intake charge. This will give you a boost and help eliminate engine knocking, thus effectively increasing the octane rating of the fuel mixture. Bad knocking can lead to engine damage, so reducing it is definitely in your interest. Often this is coupled with a supercharger for extra boost.
Of used cars, Audi A4s can be a great choice as long as you are aware of the possible expenses involved. Parts are not expensive, but labor often is. Know the mods you are interested in beforehand and the approximate cost of each before you buy your A4, and keep in mind that a used Audi with some light modding can result in a cool car with good power.