Yes in high altitude a turbo will help, but generally you will need to up fueling to make adding a turbo worthwhile.

Just my opinion but the 617a makes the 123's handle poorly. To much weight in the front for good handling. Great cruiser but poor on the backroads. There is a 300 pound differnace in the 300 vs 240, mostly engine and trans. That being mostly on the front too. Understeer big time. Yes I know these are not performance cars but I still like to turn sometime. I have both a 240 and 300, so have a good comparision.

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1977 240D turbo