This thesis investigates the impact that boundary layer control systems have on aerodynamicmeasurements in an automotive wind tunnel with a slotted-wall test section. The study was carriedout in the Volvo Cars PVT wind tunnel at the Torslanda site in Gothenburg. Through experimentationa relationship was established between the suction rate of the boundary layer control systems, staticpressure, and measured drag.Reviewing the design and upgrade of the Volvo Cars PVT wind tunnel along with designs of othermodern automobile wind tunnels and the relevant literature in the field led to the conclusion that thesuction rate of the boundary layer control systems used in the tunnel was too high and excess suctionprovided little benefit. By reducing the suction rate of these systems by up to 40 − 50% in somecases, a remarkable reduction of five counts in measured drag was observed (∆CD = −0.005), witha small increase in displacement thickness. This change is related to the flattening of the axial staticpressure curve and is related to the static pressure gradient; however, the large change in CD cannot beexplained by just looking at the change in the centerline axial static pressure in the empty test section.The shape of the boundary layer profiles remained largely unchanged, with negligible influence on theshape factor seen. With further refinement of the parameters of the boundary layer suction systems,the change in CD could be even greater. This, in combination with the turning off of the tangentialblowers and the change of the wind speed determination method from nozzle to plenum led to a largereduction in CD. The total change with all these changes was twenty counts (∆CD = −0.020), withthe change in method reducing the measured drag by ten counts (∆CD = −0.010) and disablingtangential blowers reducing the measured drag by five counts (∆CD = −0.005).The insights gained in this thesis are valuable both for Volvo Cars and the automobile industry asa whole. Studies related to slotted-wall test sections in automobile wind tunnels are few in numberand the effects of boundary layer control systems on the flow in the test section are not as well studiedas their 3/4 open-jet counterpart. This study helps fill that knowledge gap a little more and provides astarting point for further investigation on the topic. The ideas for future investigations are outlined inthe conclusion of this thesis.