what do you mean no power gain?
RB25 turbo at the same boost the RB20 turbo was set to will produce a fair bit more power.
remember to use the actuator from the RB20 turbo on the RB25 turbo
that way you get 10psi stock instead of the 7psi the RB25 actuator holds.
many years ago, I put a series 1 R33 turbo on an RB20.
with 10.5psi and no tune it made 152rwkw
Also, R33 and R34 turbos are not the same.
R34 turbo has a bigger rear and will be laggier on teh RB20.
Stick to R33 turbo.
just go to an exhaust place, tell them to remove the intercooler piping that goes from the turbo tot he cooler, and tap a nipple to the pipe as close to the turbo as they can.
that is where the vacuum line from the actuator will go.