Boost control, not via wastegate.

[craigory]

I have a 2.3L compound Twincharged project.
Holset h1c turbo with 1.35AR turbine housing blowing into an Eaton m62 supercharger geared at 2:1 crank speed. W2A ic between the turbo and charger and fmic after charger. Throttle body is after fmic.

Controlling how boost is delivered is problematic with a mechanical valve (very on/off). Going with a servo controlled butterfly bypass valve from toyota Previa. PWM out from ecu converted to a voltage signal to servo motor controller.

Can i ditch my external wastegate on the turbo and utilise the closed loop pwm output to control my bypass on the compressor side of the turbo..? As in bleed off excess boost.

Over spinning the turbo would be the first thing that comes to mind but as its blowing into the charger that should stop it from over spinning?
The Eaton gulps 62 cubic inches per revolution geared at 2:1 crank so 124 cubic inches per engine revolution.
Turbo thinks its bolted to a larger cc engine and doesn't see the bleed off happening post supercharger.

My bypass valve vents back to turbo inlet (not charger inlet) So the turbo is always spinning. Boost comes on very quickly.
The turbo on its own without charger could barely push 6 psi at red line with thst large turbine housing and wastegate held shut.

Anyone care to guess or suggest what'll happen with the suggested arrangement?

Thanks

[gruntguru]

If you need a lot of bypass, there is a possibility the turbo operating point will head off the RHS of the compressor map. (Causing high shaft speed and high discharge temp). Throttling the turbo discharge is a safe alternative.

[craigory]

I'm thinking the same thing now too.
Without some actual flow figs I'm just guessing and could end up with a busted turbo.