Duff's aren't duff, as a duff basher once told us when we turned down a 47/3 in favour of a trip back on the roarer which delivered the portion for the duff to Carstairs.
I say it again: duffs were too slab sided, bald headed, monotonous whiney locos that we just didnt like them. Like Ford Cortinas, there were masses of them and any way you dress them up, generally livery , XO or push pull, they are still a ford cortina.
Duffs in fact are only really duff because they should have been an elite 2750hp loco fleet to rival the deltic. They were actually more expensive to make than deltics and Dp2 type type 4s would have been significantly cheaper and at above 2600 hp, more reliable. Also ride quality at 100mph is pretty poor in duffs because they yaw and rock on the short wheel base, large diameter wheels and relatively high centre of gravity that the stress loaded body design entailed as against EE's and other manufacturers' underchasis construction as the load bearing members, achieving a low centre of gravity.
Down to earth with 2500hp, that is how far that engine type could ever go in traction. Old, simple and most of all twin bank. 2750 was a bridge WAY too far in terms of balance, mechanical and thermal stress on engines which were not always even made at Sulzer/winthenthur. Infact 2350 hp with intecooling would have been a more sensible rating and many actually question if duffs and intercooled peaks did on average any more than this. The order for the "second generation higher horse power type 4" should have been split between several manufacturers, possible with a Falcon style loco being a concession to some standardisation of PU for the western region. ( twin high speed engines had become comparably reliable to the 12LDA and of course both the deltics, the blue pullman and "Falcon" showed that the approach was a very valid way of offering an added degree of redundancy in getting an ailing loco to pull its load home on one PU. Also it must be noted that there is a total voltage / ampage in terms of overcoming the single PU weak field : deltics do not require field diversion until they are at around 50mph, the second engine dialling in the additional ampage while mechanical power delivery is still then comparable to a type 4 (1650hp then pulling in the second engine slowly to eventually a presumed 3300hp give or take how the loco is set up and how quickly it accelerates to a point where the first field weakens due to back EMF) This must not be overlooked because realistically it is the number of cycles an engine makes which determine its service interval. Higher speed engines have proven to have a shorter service interval, but mainly because they were chosen as somewhat exotic high power output to cc and weight in the late 50s early 60s.
The SVT / RK unit was inherently a lighter and in vee form, a more advanced power unit to upgrade over time. Even in the late 90s, replacement 37 pistons were those common stock to class 58 and worn out class 56s had their engine blocks surrogate mothered into some 50s! The V form, multi turbo approach in a light engine which had proven marine and traction application in the early 1950s, should have been ordered in numbers as part of the second (or third) phase of dieselisaiton instead of 1) continuing to 1964 with the D200 type 4 production 2) buying only 50, over advanced EE type 4 D400s.
The ideal situation would have been for Brush and Sulzer, in fully evaluating the 12LVA engine. Although in the fullness of time, it only offered 70 hp over the duff engine, getting this engine right at 2650hp would have been a better use of time and resources than derating all the duffs made by the time the decision had to be made in the 1960s. In the fullness of time though, the LVA is a 1100rpm engine and this may have proven a weakness. Also it is more complex to maintain. For we wailway enfoosiasts, it would have been a welcome variation to the endless boredom of joe strumming and winney whining!
Despite a hugely expensive rectification programme on the runs of duffs made, to put into perspective, more than the purchase price of the deltic fleet, duffs and Peaks were never all that reliable in terms of miles per casualty. When you compare like for like with say the "unreliable" class 50 post HGR in the 80s, duffs and peaks working express services had similar MPC's. The eastfield /haymarket 47/7s only maintained a reasonable MPC on the very hard service twixt the two native cities of the respective depots by a series of proactive maintenance, often on night shift hours.
This is the crux of the matter: Duffs lasted so long and were really such a good investment long term because they were darn easy to fix. A single turbo, with simple crank shafts, basically a mirror copy of working on a 6LDA with a bigger single turbo. Leaks, cracks, valve wear, timing /push rod system etc could all be kept on top of, or fixed rapidly if there wasnt a major fail like valve seat destruction or crank case / cylinder lining decompression.
Further to the MPC rate, a duff will grind away with her high ampage producing much lower engine HP and do a grand job. Indeed on many services 2500 hp with no ETH is complete overkill. I would also contest that duff's produce much more than 2000hp on no-E-heat services due to the punchy use of amps, the arrangement and rather pedestrian field diversion shifts and the speed limits on many routes or frieght services. When duffs did go bang, they would get hauled in usually by an EE type 4 or 3 and they were quick to fix most often. This was the secret to a relatively high class availability in comparison to the actual MPC's_: they went bang more often than EE type 4s for example, but could be fixed quicker, and depots got used to proactive maintenance as stated, which meant that "B" exams were really a bit more like a more involved bit of engineering.
However, given that a "DP2" earlier 2350hp loco had replaced the last 50% of the d200 fleet for example, it may have been that EE recommended an auxiliary generator as was pencilled in the drawings for class 37s. ( D6700 EE type 3's)
Class 40s and 37s show very much that you can bang away on a 70mph average speed timetable, producing pretty much full power through each field all day long and not have any significant increase in miles per casualty. Brakes etc wear out quicker, and oil needs replacing, timing chains adjusting, but that is simple maintenance.
Where of course duffs and ETH rancid wagons had their strength was in heating ETH stock. there the engine was revving higher in outset and power at rail required pretty much maximum output I'd say. The evil trade of "coffin" stock. Another strong point being freight where they could chunder away with a lot of mechanical pulling power delivered by virtue of Brush choosing high ampage.
One of the big advantages which BR saw once the class had been de-rated, was safety in numbers, there being economic spare parts, depot maintenance legacy, driver training etc etc to make the class highly available ( although class availability of 90% still means that there were seven or eight times as many duffs unavailable than the "unreliable" 50s in the late 1980s) and with the build quality of bogies, electrical systems couple to the ease of Power Unit maintenance, the class paid themselves back many, many times over in a life time which far exceeded any other of the major LDA derived locomotive class anywhere.