I was just having a little google about for any facts on the highest ever recorded top speed of a Deltic locomotive.
Well there were plenty of " i heard from..:" , or "read somewhere" from neds and "needle off the scale" from actual ECML drivers.
Also there are a few HST idiots who think deltics were a bit rubbisy and needed to be withdrawn for the zinging vermin to take over. The Deltic loco fleet was a small fleet of locos, the smallest of any commissioned main line diesel loco, and was built to supply the top express services on the ECML with improved running times on diagrams as a stop-gap for a decade while electrification was completed on the WCML and later the ECML in that timescale. Improved diagrams over exisiting sulzer powered type 4s entailed sustained 100mph operation on the new continuously welded sections. "3000 HP was needed under the bonnet" as the quote goes. So that the deltics lasted 21 years in mainline express service is to their credit.
Top speed however is much more a matter of debate, and you then encounter the "short mile" calculation error etc. Firstly let us agree on one thing : 100mph was the maximum permitted speed and not that capable of being achieved. A loco being driver limited in those days means that a higher velocity could be envisaged and that as said above 100mph was the desired cruising speed.
DP1 , the deltic prototype had been uprated in some ways and had safety clearance for 105mph in fact, probably giving this leeway for driver error in maintaining an average of 100mph. That production locos could have been designed slower is not really a consideration if in fact DP1 was not all highly over engineered in terms of electrical equipment.
Throw the next thing into the higher than 105mph camp for them to fight with: The power units were not always set correctly to their top rpm. The non turbo deltic engine 18 cylinder, was rated at 2200hp-2500hp for the admiralty for use in MTB's, first tested in former E boats versus their much larger Merceded engines. These Power Units (PU) ran at 2000 rpm where as the production deltic locos' PU's ran at 1500rpm....ish ...it would appear that the rev limiter was not so finely adjusted and after some improvements to the design and materials during the first 6 years of loco's life with for example variation in the piston head materials, then it would have been ill advised to deliver an engine running a little below 1500rpm or banging off a rev limiting ceiling perhaps so to speak.
It is alleged that D9009 had a fault in this setting while under the cherised protection of the deltic-preservation-society, with an actual power output of c. 2000hp for one of the two PU's!
"secret trials at 3600hp" stories abound when I was a lad too, and have perpetuated on the internet. There probably i some truth in either collusion between fitters or the manufacturer and some managers with an interest more in timings than reliability and service interval! If you rev two car engines to the same relative difference of 500rpm in a deltic, then you hardly really could tell the difference i'd say.
So here we come to a double ten percent club: firstly you have the permitted top speed being 105 in the prototype, so you could argue that a 10% safety margin was established above this: that being the 115.5mph.
114 mph seems to be quoted often on the internet and IIRC from deltic "men" in their light tweed jackets as they condescended to scurry around the ScR in post deltic 1980s.
Throw in then a second 10% in: 10% more power per PU due to slight misalignments in the limiter governor and Napier or the ertswhile Findsbury park fitter not wanting to disappoint with lower than desired power output. So 3630hp. More importantly actually in the highest field diversion a higher rpm will create a higher voltage with decreasing amps, so that is actually quite important for peak speed running: being able to extend the range before weak field is encountered by applying a higher voltage.
So now we do indeed come up to the possibility that standard deltics pushing out 3300hp, about 1.8kw at rail, could do 114 mph with their ETH off WHILE, an inadvertantly uprated deltic could then go over 120 mph.
You could just ask drivers if they had physically more power handle to pull on, but the situation is more complex due to field weakening, the electrical control system in DE locos and the momentum of the train.
The only real way is to look at timings from fast runs and deduce then the speed troughs for slowing to stop at stations and then be able to back calculate particular speed peaks which relate to the track conditions being straight and probably down hill. With runs of under an hour for 100 miles between stations it is obvious that a higher peak has been hit and maintained for a substantial period in order to allow for the acceleration and braking periods.
I'm of the opinion that a deltic did physically do 120 mph for a mile at least somewhere but however I think the 128.8 quote is probably a miscalculation based on short miles MSTS or whatever they are called.
The top speed for 37s has been quoted in pairs to me as being 114 mph noted by milepost timings on the great eastern, and 116 anecdotally. What speed they ran 37s to propell the APT stock on the ECML during high speed experiments is another story perhaps in the mythology of the blue and yellow railway.