Manual Boost Valve for 300ZX

Hi,
I have a 1991 300ZX Twin Turbo with a Bailey motor sport replacement ECU, & is otherwise stock. After a settling in period the car seems a little sharper & smoother with the new ECU. I would like any assistance that anyone can offer in fitting a manually operated boost control valve to occasionally take the boost up to 15 psi. I have already fitted an “English” boost gauge. I have spliced into the two rubber pipes that run from the back of the engine on the inlet manifold, to the radiator wall in which I could have placed boost jets in instead. I have teed these two pipes into one and attached to one end of a valve in the car. The other ends of the line returns into the engine bay and splits back into two joining up to the pipes going towards the radiator as they would have if not been cut. If ive got the theory right the indicated manifold pressure going to the waste gates can now be lower than the actual manifold pressure when required. This should fool the waste gates into opening later, when I decide, and the turbos turning faster producing more power. Can anyone please advice if the plan is sound and any ideas on why the valve just makes a hissing noise when restricting the airflow and no power gains?

Many thanks



Michael Skepper