From the streets of Yokohama, powering some of Japan’s finest offerings to the automotive society, the SR series is well known among enthusiasts and tuners for being one of the most refined 4 cylinder engines even today.
It started out as a replacement for the CA-series of 4 cylinder engines which powered the pre 90's Bluebird and Stanza. Nissan produced SR engines as 1.6,1.8 and 2.0 liter engines powering the bluebird, sunny , pulsar GTi-R(left) and Silvia(S13/S14/S15)(right) to name a few.(NOTE:The Silvia given below is an S15)
The SR20DET engine was a 2.0 liter 16 valve DOHC 4-Cylinder engine featuring a Garrett T25g turbo powering the bluebird and the Garrett T28 turbo powering the Silvia(S13/S14/S15).The engine from the S15 produced 247 hp at 6400rpm and 275 Nm of torque.
Now coming to its variants there were two, one featuring a red valvetrain cover having a redline of 7500 rpm and a black one having a redline of 7200 rpm.(below)
Nissan also produced a naturally aspirated version labeled as SR20DE which itself had two variants, one having a red top cover having a high port fuel injection and a better performance index and a black top cover having a low port fuel injection having a better ecological index.
The SR20DE/SR20DET engines were discontinued in 2002 and were fully replaced by SR20VE series which powered the Bluebird and Primera to name a few.
The SR20VE engines featured VVL(variable valve timing) and a higher compression ratio with most parts updated. The SR20VE series was discontinued in 2003, except the SR20VET which ran till 2007.The SR20VET was the most powerful SR engine , producing 280 hp at 6400 rpm and 315 Nm of torque, powering the 1st gen X-trail. In comparison to the SR20DE, the only thing it lacked were Hydraulic valve adjusters which means it required regulating the valves when necessary. Picture below shows the SR20VET engine (JDM spec).
The SR series was replaced by the QR series of engines in 2007, which was introduced 2 years before the SR series was discontinued completely.
(The content above was derived from Wikipedia and other motor forums and tries to consolidate major information as shortly as possible. Being my first article i would not mind some advice!)