I have heard it said that because of the torque produced by the turbo, the tall gears allow the turbo to use the torque more efficiently, even causing the turbo to produce more torque than it could with low gears. Talk about a win-win. :cheers:
Turbos love low gears, it violates all gearhead normal thinking.
My Reliant runs a 2.60 final drive ratio.
Doubling HP from 1 bar -> 2 bar isn't true, there are a number of issues that crop up. A 75% increase isn't unreasonable though. The higher the boost climbs, the percentage of power increase will drop.
Just look at the Chrysler 2.2 engines.
TBI 2.2 = 110hp
Turbo II = 174 hp
Take it from someone who runs 35 psi on gas and was hitting the dyno tuning dozens of cars on a regular basis.