If run at the correct pressures (which are not the same as R12 likes), R134 is MORE efficient, not less. I'm running R134 in the converted factory A/C in my '72 911T, and it works great.
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2018 718 Cayman 2.0
Priors - '72 911T coupe, '84 911 Carrera coupe, '84 944, '73 914 2.0
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