There are a few conventions, first off. In the S13 motors there were red and black tops. S14 and S15 motors were all blacktops, so most people just call them "S14" and "S15" motors, although some noobs get confused by the "blacktop" thing. So, the terms "redtop" and "blacktop" imply that we're talking about S13 motors.

I'm going to have to disagree with stokdd, and say that there were never S13 Silvia's with blacktops. I'm pretty sure that S13 Silvia and all 180sx of the same years were all redtop. Blacktops started in 1994 in the 180sx (in 1994, Silvia's changed to S14 chassis). Either way, there were only very minor chages in the whole range of S13 motors. These include ECU's (62, E5, and J4 in that order), chain tensioners in 97-98, oil pickups were revised in the blacktops (maybe only 97-9, and I think that piston squirters changed as well. Also, I think blacktop pistons changed to 54C from 79E in the redtops. Blacktops also use different O2 sensors.

S14 and S15 motors got variable intake cams and a few other goodies like T28's, low port heads, different water lines, idle control. S15's picked up OBDII, 480cc injectors, revised exhaust manifold gasket, 6sp, and a bunch of other stuff I'm missing.

Anything I'm missing? Sorry for the long post, but those threads on FreshAlloy were rather thin on relevant info.