Looks like I did have it backwards, at least to a degree. The D.VIIF had the BMW IIIa, with many of the features described above. Wiki says:
"The engine was successful, but the real breakthrough came in 1917, when Friz integrated a basically simple throttle butterfly into the twin-barrel "high-altitude carburettor", enabling the engine to develop its full power high above the ground. Burning a special high octane fuel of gasoline blended with benzole, the carburettor adjusted the richness of the fuel-air mixture according to the aircraft's altitude. It enabled the engine, now dubbed BMW IIIa, to develop a constant 200 horsepower (150 kW) up to an altitude of 2000 meters – a decisive advantage over competitors' engines."
There was also the Maybach Mb.IVa:
"It was tested on Wendelstein (mountain) at an altitude of 1800 m and rated there at 245 hp. This would theoretically correspond to rating of about 300 hp at sea level; however, the engine was not designed to withstand such power - it needed to be carefully throttled down at low altitude, so it would not exceed the safe level of 245 hp. It had three carburettor settings, to be changed during the flight depending on the altitude."
So not throttle gates?
And in the references for the Maybach article, possibly the most German book title ever:
Kyrill von Gersdorff; Kurt Grasmann; Karl Prestel; Helmut Schubert (1985). Flugmotoren und Strahltriebwerke : Entwicklungsgeschichte der deutschen Luftfahrtantriebe von den Anfängen bis zu den internationalen Gemeinschaftsentwicklungen (in German) (2. erg. und erw. Aufl. ed.). Koblenz: Bernard & Graefe. p. 26. ISBN 3-7637-5283-8.
Looks like the Jupiter VI is what S Shortround6 was thinking of with the throttle gates. The earlier German engines had carburettor settings or other carburettor manipulation.Lumsden "British Piston Aero-Engines and Their Aircraft", p96
Unless you build one that has, say, 16:1 compression ratio. You can't use full throttle (or even half) at lower altitudes, but at higher altitude where the air pressure is half or less of sea level, it would act like an 8:1 ratio. ?You can’t. All naturally aspirated engines produce more power the denser the air is. increasing altitude always lowers the intake manifold pressure, in a way similar to closing the throttle at a given altitude pressure. its like saying, how do i get a N/A engine to produce peak power at a fifty percent throttle position.
Saying that, one could design an engine so that its intake pressure is limited to say, 7 psi, or so. And by some means, mechanical, or electronically, limit the intake manifold pressure to a max 7 psi. That would alow the engine to produce constant power up to the altitude that is equivalent to the maximum inta manifold pressure. The engine would still be as heavy as an engine designed to use full atmospheric pressure at sea level, and the unavailable power would be as useless to a pilot as runway behind him or sky above him.