Due to smaller post-atomic particle size of R134 versus R12 ,heat transfer
coefficient is less.[R134 wiil carry away less of the passenger compartment
heat as compared to R12]

To maintain the same efficiency,a Parallel Flow Condensor [30% more
efficient than O.E.M.] is necessary.

Even with a P.F.C. a conversion will never equal the vent temperatures
possible with R12 in a O.E. R12 system.

R12...30 Degrees "F" at the vents [without a frozen evaporator]
R134... Maybe 45 Degrees "F" at the vents.