Spec #'s GM-1, GM-3, GM-4, or GM-5/GM-8
GM Spec #'s 1, 3, and 4 are all interchangeable and are direct replacements for each other. GM-5's and GM-8's are also interchangeable and direct replacements for each other. You cannot install a GM-1, 3, or 4 in the place of a GM-5 or GM-8, or vice versa, without modifications. There are no advantages, as far as performance, from one spec # to the next. All of these turbos use the exact same compressor wheel and turbine wheel. The only differences in the turbos are the outward designs and wastegate actuator locations.