Seneca 1's that I've flown weren't turbocharged.
Seneca 2 & 3's were. They used a fixed wastegate system + an overboost warning light & an overboost relief valve.
As I recall the warning lights came on at 39.8" & the valve opened at 42". Don't quote me though!
The Seneca 2 had 200HP at 2700/40", the Seneca 3 220HP at 2800/40" but that's time limited and 200HP at 2700/40" is max continous.
The wastegates had to be calibrated with maintenance test flights to ~12,000 to measure the MAP obtained at full throttlet then adjusted on the ground if necessary.
[This message has been edited by Tinstaafl (edited 30 April 2001).]