In the striving to build more efficient internal combustion engines more focus is placed on simulations as this would reduce costs and speed up the development process. The only cost effective way of simulating an entire engine today is with one dimensional simulation software. Flow losses in complex geometries such as valves and ports are described using flow coefficients. It is generally assumed that pressure ratio over the valve has a negligible influence on the flow coefficient. However during the exhaust valve opening the pressure difference between cylinder and port is large which questions the accuracy of this assumption. In this work the pressure ratio dependence of the exhaust valve flow coefficient has been investigated experimentally in a steady-flow test bench. Two cylinder heads from a Heavy-duty engine with different valve shapes and valve seat angles, 30 and 45 degree, were investigated. The pressure ratio over the exhaust port was varied from 1.1:1 to 5:1. The tests were performed with both valves open and with only one of the two valves open to better study the losses over the valve seat. The valve flow coefficients were found to be dependent on the pressure ratio. The trends differ based on the geometry of the valve and valve seat. The flow coefficient for the single valve case with the 45 degree valve seat angle decreases significantly when the pressure ratio was increased. The flow coefficient for the 30 degree valve seat angle were in general less affected by pressure ratio and even increased slightly with pressure ratio for certain valve lifts. The two valve cases perform similarly to the corresponding single valve case at lower valve lifts. At higher valve lifts the shared exhaust port outlet is the limiting factor which equalizes the trends for the two cases.