Let me take a stab at this one....
The SC300 is powered by a 3.0 liter Inline-6 motor with 225hps(?). It comes available with a 4-speed automatic transmission with ECT-i control or an opinional 5-speed manual transmission. The 5-speed is rare in the States and is a coveted model. It's also about as fast as the OBD-I SC400. I believe they discontinued production of the 5-speed SC300 in 1996.
The 92-97 SC400 is powered by a 4.0 liter V-8 engine wih 250hps and 260ft/lbs of torque. They come with only a 4-speed auto tranny. Same as the SC300's 4-speed. The 96 and 97 models got a horsepower and torque upgrade of 10 and 10 each to make it 260 and 270.
In 1998, the SC400 motor was equipped with VVT-i (variable valve timing) and got a power boost to 290hps and 300ft/lbs or torque. Also the tranny is upgraded to a much quicker 5-speed auto wih a new torque convertor, and enhanced ECT-i. A slightly quicker SC400, but much harder to introduce forced induction on because it is highly computerized (OBD-II).
In 1997 all SC models got a dramatic appearance upgrade including flush foglights, front grille, front lip, side skirts, raised spoiler, chrome trimmed taillights, gated shifter, snow mode ETC, three spoke steering wheel, and much more...
That's a quick run down of some rudimentary changes. For a more detailed outline of specific model year changes on the SC400, refer to my website: http://www.intellexual.net/sc400frames.html
[Edited on 26-4-2002 by lex400sc]
[Edited on 26-4-2002 by lex400sc]
