Actually, I have not completed my T70 change yet. (Still wiring up and Maft Pro)
On my 60-1 bolt on T-4, which dyno'd at 420rwhp at 18psi, and I was running around at 21psi all the time, and often to 25psi with 30% Toulene in the fuel, the Bosch valves worked fine.
Granted, the T70 will be flowing more air at lower pressures, and I'm not using a surge resistant housing, but the Bosch valves are designed to be open, or leak whenever there is a vacume in the intake manifold.
The ones that I have seen "leak" complaints about are always installed backwards. Jeff from the old Suprasport had photos of one installed backwards too, so that did not help.
They have to be setup so the port of the "underside" of the valve, directly opposite the vac nipple goes to your turbo intake.
The port off the side of the valve, goes to your upper Intercooler pipe that is connected to the throttle body.
The valve is designed so under pressure, it is held closed. The more pressure you add, the tighter it's held closed. When you snap the throttle closed, the intake goes immediately to vacume, and that pulls the valve open, bypassing compressed air back into your turbo. At higher pressures, it makes quite a whoosh noise, very little flutter. Just crusing around town, I did notice some flutter (opening and closing of the valve quickly) if I shifted at low boost in a relaxed driving style. No compressor surge however.
Another benefit of this setup is the shortcut to the throttle body off boost. When you first open the throttle, the turbo is not spinning very fast, and during this "lag" time, any air going into the engine has to go all the way from the filter, through the turbo, the IC pipes and FMIC and then into the motor. This long path adds to the lag time. The Bosch type valve is open under this condition, and since it goes straight from the intake filter area, to near the throttle body, it is a short cut, and improves throttle response at low engine speeds. (And that is why I use two of them
) More flow at boost, and it improved my low speed throttle response, so no need for a FFIM in my opinion. (Other than to clean up the engine bay, and keep the intake air away from the hot exhaust/turbo, but not so much to improve throttle response...)
One last thing. For many years, I used the Lexus/550 setup, and on any metered air system, if you do not recirc the air measured, it makes the car run poorly. Especially when you shift and "blow off" it goes way rich, and backfires. I do not need that kind of attention from the cops.
Good luck
