There was an article answered on here by Stu explaining it.

Try a search.

I seem to remember that the -31 has an opening pressure on the diaphragm of around 7psi whereas the -34 is about 14psi.

Obviously if you are aiming for around 30 psi this would mean if you adjusted the
-31 upto around 10psi you would have to bleed 20psi off
What a waste of boost....

whereas with the -34 tweaked to around 18 psi you would only need to bleed off 12 psi.

Think there was more to it as well, something to do with Compressor housing pressures of a T34 being higher than the T3.........