These are questions that your high school counselor can't help you with. However, have a gander at our large selection of Plug and Play products. At RallySport Direct we carry a massive selection of COBB's famous Accessport, as well as other plug and play engine tuning devices. Did you put a downpipe on your WRX and keep hitting fuel cut on peak boost? You need a tune. Does your Evo feel slower after installing a cold air intake? You need a tune. Couldn't hit full plaid in your GTR after a midpipe install? You get the idea.
The great thing about the Accessport for example, is that COBB offers several off the shelf tunes (OTS) designed specifically for your current modifications. And COBB regularly updates their site with the latest and greatest updates to keep up with your upgrades. With a plug and play engine management system, you will be able to maximize the benefits from your modifications. Your ECU is programmed to work specifically with OEM components. If you start adding a larger turbo, or add larger injectors without a tune, then you're not gonna have a good time.