Spectre Air Filter Inlet Adapter / Velocity Stack 3in.Spectre's air filter inlet adapters and velocity stacks are designed to allow you to attach a larger 6 inch diameter flange air filter onto a smaller diameter air intake tube. The velocity stacks are available with different neck sizes, to match 2. 5 inch to 4. 5 inch tubes. In addition, Spectre velocity stacks also have a smooth funnel shaped inlet that reduces restriction and turbulence of incoming air.