Stub Stack for HS4 1.1/2" SU carburettor to fit inside Sports K&N filters.Improve performance with the addition of a "stub stack" within your K&N filter.The more air you get into the chamber, the denser the charge, the greater the chances of increased performance and horsepower.These stub stacks made by K&N have been designed to do the job and will easily fit into most other filters.