Dude, they are stock standard Coby extractors designed for a skyline RB25DE, wicked, no mods required. Only found 1 website with specs, website nolonger working, copied data below, even attached a piccy.
1986-98 Nissan Skyline RB20 2000cc SOHC. 1 1/2" Primary 1 5/8" Secondary D6 234.82
1989-98 Nissan Skyline R32, R33 - RB20, RB25 DOHC. 1 1/2" Primary 1 3/4" Secondary 2" Outlet D12 279.45
Got extractors for $300, chromed for $100, they put in a massive coby resonator for $80 as well, my cars quieter after the mod, labour+misc bits $120, excellent investment, looks good too.