[Black GT]

They'll make your alternator, air conditioner compressor, power steering pump and water pump operate about 25% slower than their original speeds. If you have an aftermarket stereo system that increases the electrical load you might have a problem with the alternator not being able to keep up (more so at idle speeds).

Worst case scenario is UDP failure. It might not be common but it happens: http://www.mustangforums.com/m_4435611/tm.htm