What is the difference between the SR20DET and CA18DET?

Both are Nissan turbocharged four-cylinders used in the 180SX and Silvia. The CA18DET (1.8L, ~170 hp) came first (S13 early models) and is lighter but has a weaker block and limited aftermarket. The SR20DET (2.0L, ~205 hp) replaced it from 1991 onward and became the definitive S-chassis engine with deep aftermarket support, forged-internal upgrade paths, and ability to support 400+ whp on a built motor. The SR20DET is almost always preferred.