[evolutionmovement]

Another one of those persistent myths! Back pressure is NOT helpful. Ever. The changes you're seeing are from changing the scavenging effects in your exhaust. Different diameter pipes as well as volume changes throughout the pipe's length change the velocity of the exhaust pulses as well as move the rpm range of the scavenging effect of their reflected waves. A larger pipe will slow velocity, but move more volume. Changes in diameter (from a bend, for example) or volume (muffler, converter, etc.) reflect pressure waves back to their source (exhaust valve). These reflected waves, when the timing of them coincides with the exhaust pulse of a new cycle, can help pull the new pulse out by creating a low pressure zone for it to move into. At a different rpm range than the optimal one for scavenging, the reflected pulses will hinder it by creating a high-pressure zone at the open port.

The longer the distance before a change in volume initiates a reflection, the longer it takes the pulses to return (lower rpm scavenging). Larger pipe diameter will have the same effect by slowing the pulse and can cause back pressure if there's too little energy or volume in the exhaust (which is why 4" pipe doesn't help a 1.6 liter engine, but might work well on a big block or a diesel). This is where exhaust tuning comes into play (these same principles apply to the intake side. The reason for variable length intake manifolds is to increase the rpm range(s) where reflections in the intake help to push air into the cylinder) and depends on the intended use of the car (high or low rpm optimization). Back pressure helps at no speed, it just reduces pumping efficiency at all rpm.