This engine seems the hardest to quantify in terms of initial changes using the available manuals although some things are fairly clear. What seems clear is that by at least 5-70 a new engine cross section drawing (GE E-16188) has appeared in the manuals; the new Diesel Engine Mechanical Service Manual GEJ-3869 contains this drawing at that date for, we believe, the first time and this manual is the first to cover the 3600 HP 16-cylinder engine for the U36 series; thus the first and most obvious change is the increase in fuel rate for the FDL-16F at 3600 HP.
Other locomotives already in production soon changed to use of the F engine, including the U23 and U30.
In terms of manufacturing alterations in the "F", one change that clearly was due to the uprating of the engine was the provision of a second oil drain hole in the underside of the piston crown cavity on the pistons fitted to these engines. GEI-81976, Instructions for Connecting Rods, Bearings and Pistons (and which is a component part of large binder GEK-30130A) instructs that earlier engines should be modified by drilling the second set of drain holes into the piston crowns. These pistons, it is important to note, are NOT yet steel-capped pistons.
A new heavy walled cylinder liner also appears in this time frame, omitting an external jacket (the old style is called "belly band liner" in GE manuals) and thus making the cylinder assembly essentially revert to a wet block construction, since the outer water boundary is again the inner boundary of the cylinder assembly. In the same parlance this new liner is called the "Annular Groove" liner, and on this style the interior was either chrome plated (requiring iron piston rings) or Tufftrided (requiring chromed piston rings.) Further, it appears that on the "F" the change to holding the cylinder head in place using the liner, instead of bolts from the head to the cylinder jacket or cylinder assembly, was made (although this may have occurred late in "E" production and during overhaul GEK-61273 instructs for the omission of these bolts on all engines... Note twice now the instruction to make alterations to in-service engines that essentially convert them to "F" engine features.)
At least one railfan oriented publication stated (quite some years back) that the U34C introduced steel-capped pistons. This may in fact be the case, especially since the U34 is listed in GEK-30130A as being equipped with the FDL-16F. However, since the date of publication of this particular table is 2-76 and reprinted 9-77 this may also reflect upgrading of all of the engines in this model of locomotive to "F" status. One cannot be sure, at least from this material.
According to the manuals, a number of changes were made to the production FDL diesel engines in 1973. This appears not to have made a difference in the letter designation of the diesel engines, however. These changes are spread throughout the various instructions and descriptions and appear to be a number of small refinements all essentially implemented at one time.
Up until mid-1973, all FDL engines had incorporated intake valves with 45 degree intake valves (the angle of the seating surface to the valve centerline.) Apparently, according to a GE technical paper in our collection, valve seat recession was experienced. The modification to remedy this was to alter the intake valve design to 15 degree seating surfaces and this occurred in mid-1973 on some engines, and by the 10-78 print date of GEK-61273 all production engines incorporated this change. This same instruction orders that no exhaust valves with a date stamp prior to 8-68 (and those with no date stamp at all) should be remanufactured, but instead scrapped due to questionable quality.
A modification to the governor drive assembly occurred at this time, and the new drive with an adaptor was then furnished any time the complete previous assembly was requested as a replacement or spare for engines in the field.
Steel crown pistons, using a steel cap bolted to an aluminum body, probably appeared in testing applications on production engines near the inception of the "F" series initially, as hinted at by the fact that the GE illustration number for the steel crown piston is fairly close to the illustration for the new diesel engine itself (which does NOT include these pistons.) It seems that by the 1977-1978 general date of the Conrail manual that is our primary source, these were production standard for all FDL engines in 8, 12 and 16 cylinders.
Also at this time all FDL engines were changed to a fuel header pressure of 39-41 psi. These used the same (large) fuel pump system as previously employed. Older locomotives could be altered to use the new pressure but had to have the fuel racks reset; the locomotive had to be on a load box if the low pressure, large pump system in more recent locomotives was to be converted.
There appear to be numerous other small changes; these are only a few at the time. We'll now turn briefly to a look at the FDL-12 series in domestic U-series locomotives.
The FDL-12 in all references had no lower model delineation than "B" - so that the first FDL-12 model in GE locomotives by these tables (which never give dates) is FDL-12B. This seems to correlate in some way to there being no sixteen cylinder engine with the letter "B" so that in the early days of the Universal series (for export) and then the U25B, the A engines were 16 cylinders and the B engines were 12 cylinders. (Interesting that the "A" variant was for the domestic U25, which appeared later - giving some further credence to the wide assertions that GE intended to enter the domestic market from the start.) This slightly complicates correlation of engine models, though. It appears that after the early production, the FDL-12 used the letter B and D for export models U20 and U22, model C and F for domestic U23 models, and model D for export models of U23 and U26.
Item: The engine model listed for the U50C is FDL-16D, at 1050 RPM maximum.
Further Item: We know from manual evidence that the U50C, and from first hand operator evidence (thank you, Noel Weaver) that some of the Penn Central U23B units used a modified engine speed schedule whereby the engine operated at half speed in notches 1 through 4 and full speed in 5 through 8, with variation in tractive effort by excitation only. This engine speed schedule or description of any such NEVER appears in this Conrail manual; we imagine this variation to be quite rare as a result of this finding.
Whatever the case, the 12-cylinder engines are covered by this same large manual and delineations are almost never made in the material so that we can be sure the same lineage of modifications occurred to the FDL-12 (and also the FDL-8) even if the engine model letters did not match up, at least for the first few years.
We hope you've enjoyed this series on the known (or rather, UNknown until now) model delineations of the GE FDL series engines. I have many engine illustrations to show and those will be coming along shortly.