It's not always true but for both of those cars the 6th and 7th digits of the VIN designates engine size in liters - e.g. 28, 30, or 32. A 1988 300E would always be a 3.0 liter as a 2.6 liter was called a 260E. A 93 "300E" would be a 3.2 as a 2.8 was actually called a "300E 2.8", at least on the title. Of course some cars have had the "2.8" trunklid emblem removed.