Billet delrin version is now available and is half the weight of the aluminum version. OBR power stack is compatible with most aftermarket clamp on air filters. This is a bolt on performance item that will improve power on any engine that's stock or fully modified. We spent several months developing this stack on our in-house flow bench and dyno. We've seen more power out of this design than any other stack that we have tested. Dyno graph can be found in the images. Tapered bore increases velocity through the intake tract and gives better consistency. The clamping surface has barbs to prevent the air filters from falling off under hard running. There is no boss on this stack which allows the air filter to be pushed closer to the carb for cars that have minimal air filter clearance.