Known the world over under various names such as Velocity Stacks, Air Horns, Trumpets or Ram Pipes they allow you to "tune" you engine in a relatively simple way and this particular version is a 15mm "tall" top quality machined alloy bolt-on trumpet for 40mm diameter carburetors.
So, these trumpets measure 15 mm in total length.
Diameter at the flared end is some 57mm.
They feature 65 mm stud centers and will fit, amongst others, the following carburetors:
This product is priced and sold as a single unit.