I just finished up a 1990 twin turbo for my wife. The car runs great! However I have a greddy EBC on it and even at 100% boost the car never reaches above 12 psi. I'm not sure what could cause this. Before the mods on my z at full boost I could get 15 psi with no problems. Her car is stock accept for a z1 ecu tune, 615cc injectors, Greddy EBC, HKS Intake and Autometer 52mm boost gauge. I have also checked for boost leaks and have not found any. The only thing I did find was a collapsed vacuum hose beside the passenger side turbo. It was trapped between the metal boost hose and a mounting bracket. I repaired that and still nothing. What could it be? Thanks