Has everybody above helped or confused you???? Well read the following and see what you make of it.
e36 316i had 1.6 ltr units but 98>00 e46 had a 1.9ltr and then reverted to a 1.8ltr in 01.
e36 318i's had 1.8 ltr but 98>00 e46 had a 1.9ltr then from 01 a 2.0ltr.
e39 520i started with a 2.0ltr unit which was replaced in 2000 with a 2.2ltr.
e39 523 always had a 2.5ltr - the 523 was replaced in 2000 with the 525. The reason for the earlier model being called a 523 is thus > Bmw made two 2.5 ltr engines a 170bhp one which was placed in the 323i and a 195bhp one for the 325i. As the 523 used the 170bhp unit it took the title of 523 to stay in line with the 3 series naming convention in the 2000 revamp however they decided to use the 195bhp unit so the car became a 525i - again in keeping with the 3's.
Sorry to rattle on but you asked.