I love this question!
Being an old "Screamin' Jimmy" guy at heart!
I have a soft spot for the venerable 2 cycle detroit engine.
They are still being built, rebuilt and used around the world today.
One of the reasons they aren't used in the trucking industry is that Detroit decided not to try to compete with the four cycle emgine when it came to emissions regulations, after all, they engine made it's debut in 1938, it was time for a change.
On a side note, the most power ful diesel engine in the world is the Wartsila-Sulzer RTA96-C turbocharged.
Some facts on the 14 cylinder version:
Total engine weight: 2300 tons (The crankshaft alone weighs 300 tons.)
Length: 89 feet
Height: 44 feet
Maximum power: 108,920 hp at 102 rpm
Maximum torque: 5,608,312 lb/ft at 102rpm
It is also, (I think), one of the most efficient diesel engines in the world with 50% thermal efficiency as opposed to a normal automotive engine which is around 25% efficient.
(All these figures are taken from various places on the web, but one place with good photos is:http://people.bath.ac.uk/ccsshb/12cyl/
You might also consider that the 6-71 can put out around 238 hp with a displacement of around 426ci. or 1.8 hp per cubic inch
Or about 6.3 Litres, 38 hp per litre.
The 12 litre 4 stroke Detroit puts out around 450 hp or 35.4 hp per litre.
I also still have a soft spot for the old Cat engines too, but, hey, I'm *ussy whipped!