What does the '985' in the R985-22 aircraft engine designation indicate?

The designation 'R985-22' contains critical information about the engine, and in this case, the '985' specifically indicates the total piston displacement of the engine in cubic inches. This is a standard practice in aircraft engine nomenclature, where the numbers following the prefix often represent the engine's displacement, providing insights into its size and capacity.

Each engine designation follows a structured format: the 'R' denotes the type of engine (in this instance, a Radial engine), and the subsequent numbers refer to the engine's displacement. Thus, the '985' indicates that the engine has a piston displacement of 985 cubic inches, which is an essential piece of information for understanding engine performance characteristics.

The other options, while they represent important aspects of aircraft engines and nomenclature, do not apply in this context. For example, one might think the model number represents the engine’s identification; however, the entire designation (including the letter and numbers) serves that purpose. Similarly, the year of manufacture is not typically represented within the designation, nor is the horsepower rating; those would need to be referenced through other identifiers or manufacturer documentation.