SILVIO FIORITO is the proud owner of a Subaru WRX and, like any engineer, he is happy when his hard work speeds up his car from 45 seconds to 5 seconds. A good engineer can always improve on what comes from the manufacturer.

He’s not talking about the car itself, but the PC he installed to link the navigation, music system and video playback systems. You can go get a cup of coffee while your desktop starts, but a car is different.

“If my girlfriend gets into my car, she shouldn’t notice,” said Mr. Fiorito, 29. “If they don’t even realize that I have a car computer in my car, it means it’s working properly.” So he tore apart his version of Windows, tweaked the start-up routine and ripped out every unnecessary part.

“You don’t need a printer in your car computer,” he explained and paused a second, perhaps imagining what he might do with a printer in the back seat, and then added a qualification to the next sentence:

“If you don’t have a network, you don’t need the Windows fire wall.”

Engineers like Mr. Fiorito, of Reston, Va., are just some of the car lovers who want an able computer in their vehicle to complement the turbocharged engine and the fat brake pads. They’re also wiring these computers into the stereo, the cellphone, the backup cameras, the Onboard Diagnostics Port (OBD-II), the navigation system and almost any electrical part. Then they tinker with the PC to make it easy to control everything with a few taps on a button or a touch screen.