There are two issues of concern here:
Light Pattern
The low beam is a projector head lamp that functions like a slide projector, using a plano-convex lens to distribute the light, rather than a parabolic aluminized reflector used in the high beam. They are used to give a sharper cutoff and they can be field-switched between left-dip and right-dip for use in Europe. They are more efficient than conventional reflector designs, because they are able to use more (and waste less) of the light than reflector-type lamps. However, because of the sharp cut-off low beam pattern (even at a higher light output) they are not suitable as a high beam light.
Electrical System Overload
The reason that early 916’s only operated one headlight at a time is that they had a marginal 350 watt charging system design that couldn't handle two 55 watt bulbs simultaneously without needing a constant high 30 amp charging rate. Newer 500 watt systems have the capacity but the weakest link is still the regulator. The problem is compounded by weak electrical connections, small-gauge wires, and inadequate cooling of the regulator itself.
Shazaam!
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