Emmisions with Headers

[perpetual gyro]

good stuff....

You need to have the lowest backpressure possible to produce the maximum power by keeping pumping losses low. Too big of an exhaust pipe causes power loss, especially low-end torque. This is because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging (or sucking) effect from tuned headers, which we will discuss in more detail later in a future installment. To simplify things, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the 4-stroke cycle where both the intake and exhaust valves are open, especially with longer duration performance camshafts.

If the exhaust pipe is too large, the flow will be sluggish with low velocity and the scavenging will not be good. Remember that a good exhaust has low backpressure and high velocity. The only possible exceptions to this rule are for turbocharged or nitrous motors. It is almost impossible to put too big of an exhaust past a turbocharger as a turbo depends a lot on the pressure differential across its turbine to get power recovery efficiency. A turbo engine can have an exhaust gas volume about 1.5-2 times more than an equivalent displacement naturally aspirated motor. NOS motors also have a pretty high exhaust volume and require a bigger exhaust if they are to be optimized for NOS operation.