Engine

What is the "knob" on the very top of the engine?

If you're referring to the black cap at the top center of the intake plumbing (see the photo of the plenum at http://www.928gt.com for reference), that covers the access to the flapper valve that changes the
lengths of the pipes to keep them properly resonant above ca. 4000 rpm. This is part of the magic that produces the nice flat torque curve characteristic of S4s and later models.