Engine Number

The engine number ("engine codes" and "serial number") are found behind the joint for the cylinder block and upper oil pan section -arrow-.

Fig 1: Locating Engine Number
Courtesy of AUDI OF AMERICA, LLC
There is also a label located on the right timing chain cover with the "engine code" and "serial number".
Engine codes beginning with "C" are four-digit.
The first 3 digits of the engine code stand for displacement and the mechanical structure of the engine. They are stamped in the cylinder block, including the serial number.
The fourth digit describes the engine output and torque and depends on the engine control module.

NOTE:

The 4-digit engine code is on the type plate, vehicle data label and Engine Control Module (ECM).

Locations of the type plate and vehicle data label. Refer to TYPE PLATE AND VIN