Originally Posted by MrMoPar

A turbo needs exhaust flow, not backpressure. A turbo always acts as a restriction in the exhaust flow. It becomes critical with increased boost pressure. For example, (numbers for example only), if turbo is pushing 20 psi into the intake, and you have built up 10 psi back pressure in the exhaust, that pressure effects the flow into the cylinder as the incoming pressure must push against the outgoing pressure to overcome it, so the waste gasses are slower leaving the cylinder, so you don't get as complete of an intake charge into the cylinder so your available oxygen to support combustion goes down. You leave potential horsepower on the table. The higher the rpm, the less time to fill the cylinder with a fresh charge. Cam profiles also have a big impact as well.