Indeed, the LR MPA must be used to find out convoys and dispatch the acquired data to the subs. The Jumo diesels (204, 205, 207, 208 ) did indeed use only 6 cylinders, but also 12 pistons - each cylinder was shared by two opposing pistons.
The Jumo 208 was being developed from 1939, it featured different cylinder dimensions (bore increased from 105 to 130 mm), hence the volume was increased from 16.6L to 25.5 L. I don't know the weight. Power was 1500 PS for take off, 1100 PS was Steig Kampfleistung at 10000 m through use of turbocharger. Engine remained in prototype stage.
From 1940, another diesel was tested, the Jumo 223. A 24 cylinder engine - with 48 pistons!? Due to small cylinders (80mm bore, stroke 2 x 120 mm), the permissible RPM skyrocketed at 4400 rpm, the power was at 2500 PS for take off. The engine weight went also considerably up, at 1400 kg (with turbo?). Remained at prototype stage. Cylinder layout was similar to the Napier Deltic, but with 4 cylinder blocks, rather than 3. A 'diamond' layout, we might say.