The design of the PORSCHE wind tunnels - two facilities, one in full- and the other in quarter-scale - was determined by the demand for simulating both passenger car models and racing vehicles. One peculiarity, the very low ride height of the latter requires a reduction of the oncoming boundary layer that develops along the test- section floor. The number of difficult practical engineering problems in using and operating full-scale moving belts (*Bearman et al. ), led to the development of two suction systems using porous plates in the test section floor. These have been installed in the full-scale and in the 1:4 - scale windtunnels. For verification or optimization of the originally estimated suction rates required to meet realistic road conditions, a number of experiments on the road and in a moving-belt facility were conducted and the results compared to values from the suction facilities.Very good agreement between the reference results and the boundary layer control (BLC) results with floor suction have been achieved, and the criteria for adjusting the suction rates, which were worked out in earlier investigations, have been fully confirmed. The results show also the magnitude of the errors, especially in the vertical forces, when insufficient floor simulation is used during the aerodynamic testing of race-cars of the Group-C kind. The suction devices have proven to be essential and reliable components of the PORSCHE aerodynamic test facilities and are used on a regular basis during race-car testing.