Originally Posted by diver dave
I've seen these, but never fully understood them. They must have a clutch for both input shafts, and then the usual F-N-R clutch after that? And, I never seen a "small one".
Yea, each engine has a clutch, then the gearbox has FNR gearset.
These came about when they developed the 6-71 detroit in the late 30's. That engine was about 185hp then and some military apps it was not enough hp. So they ganged them up. Two engines per shaft, and also four engines per shaft in a quad "H" configuration.
It was not done for any efficiency reasons, but for wartime logistics. They had gotten an efficient production line on the very reliable and compact 6-71, but in some apps they needed more hp. There was not a larger hp engine at the time in high production numbers until you got up to the locomotive engines. And in wartime those were needed for locos, subs and the larger landing craft.
After the war, they developed the vee-block 12V71, 16V71, 149 series, etc that took the place of the twins and quads. But by then the war was over. And those twin and quad packs were everywhere in the war surplus market, and could be had cheap. Last one I saw running was only a few years ago. Still running fine!