The TC is a far better motor than the Evo just like the Evo was far better than the Shovelhead. The inprovement are many but here are just a few:
Much stronger case.
Higher CR with cooling jets to tame the added piston heat
More cooling fin area.
Much better heads, (reversion step bath tub chamber)
A biggey is the valve trane geomertiy, the use of two cams is how thats better.
Much better oiling system. on & on & on....
since I own 2 Flh's, one with an Evo and the other a TC I will give you my opinion. the TC 88 is much smoother, higher revving much more modern motor. the Evo on the other hand seems to have more bottom end grunt,is more old fashioned,noisier mechanically and has more of the endearing qualities that harleys of old had like vibration.
All Big-Twins have always had seperate engine and driveline component cases. Flywheel(crank)case, primary and tranny. They all hold thier own specific lubricants except for very eariy models with total loss oiling systems where as the engine oil drips into the primary via a tube to lube the chain and sprockets then out a small weep hole to the ground, and of course any belt driven primary and dry clutch system which requires no lubrication.
What you're looking at is the engine to trans. interface. The Twin-Cam was designed that way so as not to rely on the primary case as a stressed member. The engine and trans. do not share the same oil. The seperate housings are bolted together for reasons of rigidity.
Yes! Only the Twin-Cam has the crank-case to trans interface. The Evolution and all it's predeceasors did not. However, some Evo owners opted to install what is called a Carlini or Screamin' Eagle Race Brace on the right side that effectively interconnected the crank case and transmission to suplement the strength or lack thereof, of the stressed primary on the opposite side. Some hopped up Evos were notorious for cracking the inner primaries due to the nature of the design and lack of sufficeint strength to handle the torque loads imparted.
That's a pretty broad question! Power wise, about a 10% increase. Superior oiling system, stronger, less porous castings, straighter pushrod angle for better valvetrain geometry and less sideloading of the rockers due to the two cams rather than one since the pushrods are almost parelell in relation to the valve stems. Larger pinion(crank)diameter, much larger pinion bearings. Bathtub shaped combustion chambers as opposed to the semi-hemispherical design of the Evo promotes more efficient combustion event and flame travel. Larger bore/shorter stroke making for a higher revving engine. These are just some of the improvements.
Here are some of the drawbacks... Somewhat restrictive heads if you want to run anything but a light action set of cams(trainer cams). IMHO A less than perfect cam drive system, chain drive, which may be less desirable than gear drive in ultra high performance situations. '99 and early '00 had some cam bolt, support and bearing failure problems addressed by the issue of extended warranty.
Again, this is merely scratching the surface. Do some more research and gather the facts.