LS1 Header Flow Test Pipe is 11″ length and 2.00″ O.D. This item is used for flow testing exhaust ports on General Motors V8 LS1 cylinder heads. The flow pipe has numbers stamped on the flange 1-4. Number the cylinder heads exhaust ports from left to right (see photos) with a black Sharpie marker. Match the number on the header flange with the numbers on the head to perform test. By flow testing your exhaust port with and without a pipe you will know how it effects the flow positively or negatively. Flow pipe improves flow roughly 5 percent over an exhaust port with no pipe. Mounting hardware included. Please e-mail us if you have any questions.