All backpressure is bad.

For both turbo and N/A cars, backpressure is unecessary and bad. Less backpressure=more hp and tq. Here's the best answer to the age old question that I've found.

The commonly accepted backpressure theory is wrong, but backpressure does exist. In reality, you want as little backpressure as possible. Having as little backpressure as possible will give you more hp and tq in all rpms, but bigger piping can INTRODUCE backpressure into your exhaust system because of increased turbulence from not enough flow. Just like piping that is too small will restrict flow and not allow enough air through. It's not about the backpressure, it's about the flow.
Running no exhaust at all isn't very helpful either because of something called the "scavenging effect," where exhaust that is already flowing through the tubing sort of helps "pull" exhaust that is just leaving the engine. Dumping exhaust straight to the atmosphere from the block doesnt allow for that... thats why most dragsters have exhaust manifolds and short lengths of exhaust tubing. The exhaust can be tuned so that the inertia from the exhaust pulses pulls the next beat out of the engine.
The best example I have for disproving the 'back pressure needed for TQ' arguement is F1. F1 cars are most likely the most expensive, most researched cars on earth. F1 cars have a wide range of rpms that the engine revs through (since courses can include long straightaways as well as hairpin turns). F1 cars have near-perfect exhausts which create only about 1-2psi (normal cars have approx 18-20psi) backpressure (0 is ideal, but impossible unless you are in a vacuum). So why would the top racing cars in the world only have 1-2 psi if back pressure was needed for low end torque?

 

well I can tell you that on an na you do need back pressure. On an na with no bp you risk long term valve damage. You can use most of the motor tech for building v8's but not all of it will work on a motor that could potentially have a 11000 rpm red line! The VG30 makes power on a longer rpm duration. Running no bp would severly hurt your low end power. Now you are right to a certain point, a little less bp will improve your power, but if you go too far........you lose hp. On the realistic side of this, you would have to have a custom header with 3" runners and a 4" straight pipe all the way back no can or converter, or open headers to get anything close to an f1 car. One other thing f1 cars are running sick compression compared to your car, and you dont rebuild you motor after every race! Ive seen a lot of RnD work in japan with a lot of different cars to find the best pipe size and it seems that a 70-75mm pipe made the best power and torque. Thats only about a half an inch larger that the stock pipe on your car. You also have to take ecu tuning into account!