Exhaust Manifold - Why A Cross Plane V8 Needs An H or X Exhaust Pipe

Why A Cross Plane V8 Needs An H or X Exhaust Pipe

Crossplane V8 engines have a left and right bank each containing 4 cylinders. When the engine is running pistons are firing according to the engine firing order. If a bank has two consecutive piston firings it will create a high pressure area in the exhaust pipe, because two exhaust pulses are moving through it close in time. As the two pulses move in the exhaust pipe they should encounter either an X or H pipe. When they encounter the pipe, part of the pulse diverts into the X-H pipe which lowers the total pressure by a small amount. The reason for this decrease in pressure is that the liquid, air or exhaust gas will travel along a pipe and when it comes at a crossing the liquid/air/exhaust will take the path of least resistance and some will bleed off, thus lowering the pressure slightly. Without a X-H pipe the flow of exhaust would be jerky or inconsistent, and the engine would not run at its highest efficiency. The double exhaust pulse would cause part of the next exhaust pulse in that bank to not exit that cylinder completely and cause either a detonation (because of a high air-fuel ratio (AFR)), or a misfire due to a low AFR, depending on how much of the double pulse was left and what the mixture of that pulse was.