I just found this on
http://www.magnaflow.com/02problems-a-restrictions.htm This is an example of why the stock exhaust system on the Tundra is so restrictive. Once look at that Y pipe and you can see what I'm talking about. "PROBLEM: Pipe diameter reduction is a common problem with OEM style bent tubing. In a sharp bend, this reduction may be as much as 20%. A 2" diameter pipe may see a flow reduction equivelant to a 1-3/4" diameter pipe. In effect, this reduces the entire system's flow capacity to the smaller dimension and increases backpressure."