Can anyone reference any article that states that exhaust backpressure is helpful to low end torque on a non-turbo engine? Primary tubing and collector sizes can be too large and hurt low end performance. However secondary exhaust pipe flow is mainly mass flow, with the lower the restriction the better. Now possibly too large an exhaust pipe may result in cooling of exhaust gases which increases resistant and backpressure, or that too low a flow rate through the exhaust pipe may actually increase flow resistance and therefore backpressure, which is not optimum. But I question that any increased exhaust backpressure from the secondary exhaust pipe is benefical. One of the main concerns (with the general public, not us) of a large exhaust pipe is the increased low frequency rumble exhaust noise.
Concerning exhaust pipe diameters and flow, the flow rate is related to the cross sectional area of the pipe, or pi x diameter x diameter. So small changes in diameter have larger changes in flow capacity. Going from a 2" to a 2 1/2" tube increases the capacity by 56%.
Diameter....Relative Flowrate Capacity
2" ...............1.0
2 1/4"..........1.26
2 1/2"..........1.56
2 3/4"..........1.89
3"................2.25
