Letter designations for EMD engines in the 567 and 645 range are commonly used in all venues, as they have been published widely by Electro-Motive from the beginning. Actual designations for other makers' diesel engines aren't so easy to find; they're buried in technical manuals. Luckily, we have lots of those.
One of the best is a manual from a set issued to Conrail by General Electric in April 1979, covering all GE locomotives that Conrail owned including all those from all predecessor roads. The latest locomotives covered by this manual are the B23-7 and C30-7 units. This large manual is GEK-30130A, Volume IIIA, Diesel Engine Maintenance Manual. In this book are the some of the secrets of GE diesel engine designation. It's complicated.
Let's make some simple statements first off. The U25 materials in our collection, including a locomotive overhaul manual published for the NYC covering its first two orders of U25B units and a number of sales brochures, often did mention the designation 7FDL-16A for the diesel engine employed in those units. Following this, for most U25 production and U28 production GE sales materials are silent on model designation. With the release of advertising material for the U30, the designation 7FDL-16D appeared. Following this, the model designations disappear from sales materials again for good.
The fact of the matter is that there were subletter designations for the GE engine all the way from A through F, and there were sub-designations in each range as well. Let's first just cover the basic models and ratings - and remember that this covers domestic (that is, US railroad) Universal Series locomotives only.
FDL-16A U25 2500 HP for traction / 2750 HP gross
FDL-16C U28 2800 HP for traction / 3080 HP gross
FDL-16D U30 3000 HP for traction / 3300 HP gross*
FDL-16E U33 3300 HP for traction / 3600 HP gross
FDL-16F U36 3600 HP for traction / 3940 HP gross
*Earlier manuals give 3250 HP gross for the FDL16D.
This basic sequence held true in designation, even though, for example, the U30 continued in production alongside higher rated models.. and yes, features that appeared on the E engine did get applied to D engines concurrently so that D engines began immediately to resemble E engines in fitted parts if not in rating, fuel rack travel, or governed speed.
The old hack line that GE engines were just uprated by fuel rack setting is totally untrue. Many significant alterations were made over time. These were fitted to lower-rated engines still in production, and in many cases backfits were possible to older engines. For example, the steel-head cylinder assembly introduced with the C series engine could be backfitted to A engine locomotives so long as a proper depth existed in the engine block all around for clearance; instructions are given that some interference had been noted on old blocks and that minor grinding on crank and cam bosses inside the block would allow fitting of this new assembly to old engines. In this way, old GE engines could receive new leases on life with new parts even if they weren't uprated.
That's all for now- in later posts I'll go into vastly more detail on the progression of GE FDL engines in the U-series locomotives.