ipernity: Throttle and venturi of a Mercedes-Benz diesel engine - by Michiel 2005
"From 1936 to 1979 Mercedes-Benz used a pneumatic governor on its diesel engines for passenger cars. The pneumatic governor used a throttle and a venturi in the air intake to govern the speed of the diesel engine."
If you look at the pic if the site you will see the Venturi in the Intake Manifold. However, if you look at the pic in the site you see the Venturi and the Butterfly Valve.
The Governor actually runs off of the Venturi Vacuum. However, it is hard to believe that with the Butterfly Vlave partly blocking the air way there is no vacuum in the Intake Maiflod beyond the Venturi Vacuum.
Also you can have a partial vacuum and still have Air Flow. But, I have not ever read anything one way or another on how the vacuum effects compression in Diesel Engines.