No, that's fallacious. An engine does not need backpressure. Carburetor equiped engines benefited from backpressure, but on current, computer controlled cars you want as little backpressure as possible.

http://www.uucmotorwerks.com/html_pr...torquemyth.htm
and
http://www.automotivehelper.com/topic13199.htm

What you're probably thinking of is exhaust scavenging, which is a brief moment of vacuum around the exhaust manifold area as the exhaust ports close. This increases torque slightly because it pulls air through the engine, but this is very different from backpressure.