Yes backpressure is needed. unless you car has a hell of a lot of power (which I doubt it) the back pressure will help the bottom end. So you are saying that you have no cat or nothing? just exhaust manifold, downpipe, and then a pipe to the back? Do you notice your car reving to about 2K before it actually starts rolling at the stop lights, and sounding like you are trying to fly while you're only going 20 mph? That's your car telling you it's dying trying to get up and go.