when your wastegate opens it increases exhaust flow by a percentage by introducing an extra area for exhaust to flow out...
when the downpipe has a cast collector, or a tubular "double hump" style collector. all it does really is make an easier path past the flange but just to be bottlenecked and cause turbulance as its re introduced to the exhaust...
a divorced downpipe, keeps the turbine chamber, and the wastegate chamber. completely seperated and re introduces the wastegate flow further down the exhaust tract, this reduces turblance and gives considerable power gains..
dumping to atmosphere. although both loud, and illeagal, produces the best gains by giving a substantial increase in exhaust flow when the wastegate opens.
becuase the lower "turbine" chamber (mines 2.5) stays sealed and the upper "wastegate" chamber (mines 1.5) dumps out a short pipe...
it prevents spiking alot more too.. boost stays level at set boost controller psi..
but if your downpipe is 2.5, and at wastegate pressure you give your exhaust flow a kick in the pants by opening another 1.5 inch exhaust.
it gives rather nice gains of topend flow equaling power...
any time you can reduce backpressure at the turbo, your going to gain power on the top end..
in the way of flow. all other variables the same.. ie. fuel ,timing, spark intensity, Ect.
