I looked for more about the Saab bypass valve and found this:
High Performance Bosch turbo bypass valve
From what I understand, this is
not an
intercooler bypass valve, but a
blowoff valve. A blowoff valve relieves the high pressure ahead of the turbo when the engine suddenly goes from high to low rpm. Usually it releases the air into the engine bay, maybe in this case it bypasses the turbo and continues along the intake towards the engine, possibly bypassing the intercooler also. Either way, it does not bypass only the IC, and it is actuated by pressure difference only, so it is no longer of any interest to me.
When it gets warm enough to spend some time outside (the lastest forecast says no chance within the next 2 weeks), I will try to manually bypass the IC and see how the engine handles. Then I'll start thinking about actually constructing something.
I read that the main difference between the 90HP and 110HP versions of the HDi engine (both with the same displacement) is the lack of intercooler on the weaker one. Someone published a test (I think I posted it here somewhere, but can't find it now) in which they slapped an IC onto the 90HP HDi and saw a gain of about 10HP. The other 10HP apperently comes from remapping the ECU to sqeeze more out of the engine once the intercooler is in place. This makes me wonder if bypassing the IC will noticeably reduce performance. On the other hand, as you guys noted, the IC counts only when near full load. Maybe by the end of summer I'll finally get a ScanGuage and see if there is a difference with and without IC.