MSR Technical Centre
Research & Development: Exhaust System - Dump pipe
I have tried every variation and found that by using an open mouth type collector in the dump pipe, it produced the best results to achieve extra power at lower RPM. The diameter and length of the taper on the collector also plays a role as to how much torque and at what point in the RPM range it is delivered. Large collectors allow the hot exhaust gases to expand without reducing flow. If hot gas cannot expand, then flow is reduced. However, as hot gas cools down, the size of the collector needs to reduce accordingly in order to sustain flow otherwise a loss of torque will result.
The wastegate angle opens to a maximum of ~30°, so most of the excess flow from the wastegate is flowing directly into the path of the main flow coming from the exhaust turbine. As a result, this reduces gas flow, creates turbulence and increases the exhaust pressure between the exhaust valves and turbo (ie. the exhaust manifold). A splitter between the wastegate and turbo exhaust turbine is also beneficial provided it is made and fitted correctly.
The position and length of the splitter is very important in order to extract the most power from the system. If the wastegate flow is divorced from the main exhaust turbine flow for too long, it reduces the amount of remaining room available in the collector for the main flow to expand in order to flow. So it is important that the splitter be as short as possible once inside the collector, but also fit in the back of the turbo neatly providing an optimum seal. TD and VF turbochargers use different length splitters so it is important that the correct one is used.
Take a look at the images below (click to enlarge). It offers a detailed view of the open mouth collector type dump pipe that I found to be most efficient. As a testimonial to my findings, you will also find a screenshot from a video of prodrive engine testing the same open mouth collector type dump pipe by putting it through its paces. This was back in 1996.
