Sorry guys stolen from another thread, but this was my understanding too. I was told by a few experts that remapping is need to gain the benfit of a free flow system, but backpressure is useless, it's just that the car is tuned to have that effect on low down torque. If I'm right I'll see when my car gets remaped with the cat off.
""""The Backpressure Myth
It all started back in the day when the government started regulating vehicle emissions. The first catalytic converters were incredibly restrictive, and most cars were still runing carbs.
The cats were so restrictive that at low engine rpm, air would sometimes pass through the carb twice! Carbs were then re-jetted to compensate for this - to keep the air from getting a double-dose of fuel.
When you take one of these cars and remove the restrictive cat/exhaust to replace it with a free-flowing system, the stock carb will not work right anymore - you will not be getting enough fuel at lower rpms and will get a loss in torque. The solution - change the carb!
Modern catalytic converters are free-flowing compared to early pellet-filled versions. Modern fuel injection systems are not designed to compensate for super restrictive catlysts. Modern fuel injected cars do not need backpressure.
The truth - exhaust gas velocity
Here's the real deal about choosing an exhaust that is too big. I'm not a physicist, my understanding of fluid dynamics is very limited.... but here's my understanding.
Exhaust gases moving out of your exhaust can be used to pull other exhaust gases out of your cylinders. This is why headers are sized and the tubes matched the way they are. This effect is called scavenging. The faster the gases move, the more you benefit from scavenging.
The larger the tubing, the slower the gases move, the less scavenging occurs. (but, more ultimate flow with less restriction is possible)
The smaller the tubing, the faster the flow. (but obviously, the resistance can quickly overcome the benefit of scavenging if you start reducing your tubing size... a drinking straw simply can't flow enough for a motor)
Notice I didn't say anything about backpressure. Backpressure is just a measure of the restriction in the system. It can be caused by bends, narrow tubing, badly flowing mufflers, etc. Backpressure will be present in any system - but the less that is there the better. A large diameter system with a restrictive muffler (and therefore a high backpressure) will not gain any velocity. So now you will have to worst of both worlds - reduced scavenging and a restriction to free flow at higher rpms.
This is the reason that apexi made the N1 dual. The two pipes can support the flow of a larger pipe, but maintain better gas velocity and scavenging.
Some people claim to lose low end power when strapping on a big bore exhaust. This may be true, but it's because of a decrease in the scavenging effect, not because you don;t have enough backpressure.""""
I'll run and hide now.