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Re: Internal or External wastegate
The spring psi is the lowest boost that the wastegate can run. The bigger the valve or flapper for a wastegate, the more likely it will be blown open by backpressure. So without any help, a smaller wastegate will be able to handle higher boost.
Most external wastegates have an extra port on them though that can be used to push boost pressure to the top side of the diaphragm to help hold the valve closed in high boost/high backpressure situations. I ran a 35mm Tial with my 67mm turbo and it was fine, but I didn't run extremely low boost and it picked up right from the collector.
There are tricks, not really the best for power, that can be done to help control boost on an internally gated turbo. You could run an O2 housing, that has a big open chamber that causes the exhauast flows from the internal gate and exhaust wheel to collide and mix together right as it leaves the turbo. You see this with a lot of OEM turbos on other kinds of cars, Mitsu kept the two exhaust channels seperate on our O2 housings stock. PTE tries to force the same thing to happen with most of their Mitsu internal gate turbos by cutting a channel in the divider wall. When the two channels of exhaust flow are allowed to collide right as they exit the turbo, you end up with a lot of turbulence and backpressure that helps to control boost and hurts your power.
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