The ISR doesn't have the merge collector like the Z1 header does. Both use the same primary diameter tubing.
Z1 has proven to provide a 16whp gain from this thread over stock.
https://www.myg37.com/forums/engine-...er-thread.html
I saw someone local retune his car after installing ISR shorties with 3" intakes and gained 12whp. same dyno, different day (things to consider)
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RHM 75mm TB & ported up/low mani., 3" intakes/velocity stack, Z1 Headers, Motordyne ART pipes, HBP 3.5" exhaust, Tein MonoSport, SPL F/R arms, endlinks, and tie rods , Eibach sways, Z1 rotors F/R, diff bush., subframe collar, Bell Raceworks diff brace
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