It'* not the backpressure, but a drop in CFM. The goal is to get everything out as quickly as possible in the exhaust, but going to too big a pipe slows down the velocity of the gasses to the point to where the engine is hampered. If you go to big, it has to expand, and that kills your CFM and power.
Backpressure by definition isn't a good thing ANYWHERE. Backpressure in our context is a wave created by a physical flow restriction. Think of it on the intake side of things, you want air flow as smooth and unrestrictive as possible for maximum power. Going to big doesn't create backpressure.
Faster CFM also promotes more exhaust scavenging between cylinders, e.g. the velocity somewhat "pulls" the exhaust out of the cyl that just fired.
We're saying the same things, but with different terminology.. (i think)
'06 Vee Dub GTI 2 door Tornado Red
- 2.0 Turbocharged, Intercooled, Direct Injected, 6 Speed DSG, Underbody Aero, 17" Classix
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'93 Bonneville SE Purple Pearl H4U package - picked up June 23, 2003, Traded-in June 28, 2007...