Does a turbocharged motor really need no backpressure?
When exhaust gases pass through the turbocharger, they become swirled. This swirling motion needs to be broken up as soon as possible (18-24 inches post turbo). One way to do this is to use a larger pipe post turbo, then gradually taper it down to a more reasonable size. This is probably the reason why turbocharged cars get larger downpipes. Actual exhaust pipe diameter will still depend on the application. And to further quote Hugh MacInnes, “Even on an all-out racing engine it may not be desireable to use as large an exhaust pipe as possible.” For 4 cylinders planning greater than 15 psi, I’ve been told 3″ pipe is a must.
