Bret,
To the best of my knowledge, all the "50 ton" units of this design had the CAT V-12 D-397 with a roots blower as built. 1200 rpm 450/400 hp.
I think the USG 1303/1403 now GLRR 130/140 has the same engine, but rated at 450/390. The extra 10 hp loss is due to the radiator fan gearbox and traction motor blowers(?) that the 50 tons don't have.
CAT started turbocharging engines in 1955, and the Sept 1956 gross hp rating for this engine was 400/450/500 for normally aspirated/roots blown/turbocharged versions. GE claimed to offer the replacement U6B as 600/540 using the turbo version of the D-397. By the time the first one was actually built in 1959 (#60 std. gauge for the Santa Maria Valley) it was rated as 700/640 hp. Several orders were built (supposed to be 28), including several ng units for the Manila RR and UFCo Costa Rica. These are similiar to the USG units, but returned to a vertical radiator and have a front platform.
The U4B was also offered, rated at 400/340. It is thought it may have used the V-8 D-375 with a turbo, but there is a possibility that it might have been the D-397 normally aspirated. Note that the USG units do not completely fit any of these engine hp definitions.
Caterpillar dropped the D-375/397, replacing them with the D-379/398/(V-16)399, slightly bigger bore all with twin turbos and aftercoolers.
GE built the U5B using the D-379 at 700/640; and the U8B (first built for Hampton & Branchville?) using the D-398 at 900/810 hp.
After improvements resulting in horsepower increases, the U5B became the U6B (again) at 700/640; and the U8B became the U10B rated at 1050/950 hp. There was a U11B using the D-399 at 1100/1000, don't know too much about it yet. The other change included a return to the horizontal radiator like the USG units. There are claims that the last U10B had D-379, but I doubt that is right. The last locos built in the early 1990s might have had 3508 or 3512 engines.
Chiriqui Land Co. (Chiquita) apparently re-engined most (all?) the Panama 50 tonners with the D-379 (except the two that RDC got are supposed to have the 3508). Unfortunately, I cannot find out what the horsepower rating is supposed to be because CAT needs the D-379 "arrangement number" in addition to the serial number.
CAT apparently still offers a "kit" to repower D-379 equipped U6Bs with a 3508 reusing the later GT601 main generator.
GE also had a U9B/U9C mdel with a high short nose using the Cooper-Bessemer FWB-6L, the later version of the Porter's engine. It went from 660/600 in the Porter and GE 70 tons to 990/900 in the U9B/C. The early Univeral line included models using the V-8, V-12 and V-16 designs, which GE later bought and became their FDL engine line.
I just found out that Chiriqui's 40 ton GE-EMC boxcabs all had their original Winton engines replaced by White-Superior diesels. Amazingly all the boxcabs and the two Alco HH300 were still in service on the southern division in February 1979!
I haven't seen any more information on a boxcab on the Sumpter Valley other than what I sent you.
