Hartge is a German BMW tuner. After making several models stronger and more agressive (such as a 5.0 litre V10 in a 3-series), they decided to put a bi-turbo engine on the Z4.
The result is quite amazing. The wheels changed to 20-inch. A full steel exhaust with dual outlets gives the H30 an agressive bark.
The Hartge Z4 doesn’t come cheap at 83.250 euro but the maximum speed of around 300 km/h makes up for it!