I am not an expert on the differences others may wish to chime in. The crank stroke is greater on the later models. i.e. GEN 3 is a stroker when compared to the GEN 2. Same for the GEN 4. My engine is a standard GEN 2 crank. Head air flow GEN 2 < GEN 3 < GEN 4 RPM GEN 2 < GEN 3 < GEN 4 the upgrade allows for GEN 4 RPM due to the more robust rockers & springs which allow for higher RPM HP GEN 2 < GEN 3 < GEN 4 Throttle bodies: GEN 2 dual bodies connected to pedal by cable GEN 3 single body connected to pedal by cable GEN 4 dual bodies connected electronically thru a wire to pedal (fly by wire) The main bearings have been updated in my GEN 2 to allow for better oiling. This seems to be a fairly standard thing for all the viper engine builders to do. If your in there it is a no brainer. I am hoping to get to 520HP+ Presently not exceeding 500HP which is disappointing. I have not had the budget or time to go to the dyno and have the tune worked on. 520RWHP SHOULD be doable. The car has the GEN 4 fuel pump, larger diameter fuel lines, Bellanger headers and in theory plenty of air flow with the GEN 4 heads. I have my fingers crossed that with a little effort at the dyno the car will hit higher HP numbers. Unless there is an un-anticipated restriction in the fuel line (such as the after-market filter or pressure regulator) limiting fuel flow I am not sure what else could help.