By Skiming a VR6 head you do not alter the compression ratio as the head is completely flat and the compreesion chamber is part of the piston. If the head is skimmed please have the vavle seats re cut as otherwise you will only encounter problems further down the line. A lot of engine rebuild firms dont have much, if any experience with VR6 engines. So if that is the case with who you are using then would recomend finding someone who has. If the head is flat then a reface is sufficent and the head would not need skimming Hope that helps