Dexta special

[Emiel]

https://www.tractorfan.nl/picture/1524778/recent/


Does anybody know what’s special about it?

[mathias1]

New performance dexta?

[Billy26F5]

The Dexta Special is a Dexta with all the extras sold only in Continental Europe, Stuart Gibbard says it was to sell off surplus Dexta's in 1964 as production was run down (in the winter 2021 edition of Heritage Tractor), but Allan T Condie (in the Dexta book) says it was a special German Dexta; in any case it was only sold in the later period and featured a live clutch, oversize tyres and of course the special badge or decal (and I'm not sure any badges were actually made).
Sandy

[Emiel]

Then I opt for the Stuart gibbard explanation. The above dexta is not a German dexta. The German tractors do have wide mudguards

[Billy26F5]

I would agree by what I've seen, but in that magazine there's a Dexta Special with live clutch stickers and an orange wheatsheaf (I think it could be a pre-production tractor), which had me wondering. Also in the first film in Ford and Fordson on film 16 there are some German tractors that seem to have std. mudguards, but that's in 58, so maybe too early for the wide ones.
Sandy

[Billy26F5]

Looking around I found this: https://www.tractorfan.nl/picture/636620/type/96/ https://www.tractorfan.nl/picture/636619/type/96/
Looks like there might have been orange ones too but I can't see enough to be sure.
Sandy

[shepp]

There never was such a thing as a "New Performance" Dexta, no new or changed performance was claimed for the standard Dexta it was simply a colour change and was called the blue/grey Dexta. Incidentally the "New Performance" claimed for the Super Dexta came only from increasing the rated engine speed from 2000 rpm to 2250 rpm and quoting the power output at the higher rated speed, the speed of 1800 engine rpm to give 540rpm at the PTO was retained and the blue/orange and blue/grey Super Dexta had identical PTO horsepower at 540 PTO rpm! There was a change in the model number of the minimec pump due to internal modifications but the fuelling was unchanged. From memory the blue/orange model was claimed to have 39.5 bhp at 2000 rpm and the New Performance model 44 bhp or sometimes 44.5 bhp at 2250 rpm.
Massey Ferguson had previously done a similar exercise with the move from the 3 cylinder 35 to the 35X. The 3 cylinder 35 was rated at 39.5 bhp at 2000 rpm, when Ford launched the Super Dexta in April 1962 MF had to respond, as everyone knows the MF and Ford engines were basically the same. So later in 1962 MF launched the 35X by increasing the rated engine speed to 2250 rpm and quoting the power output as 44.5 bhp at that speed. However the engine speed that gave 540 rpm at the PTO was 1500 rpm for both the 3 cylinder 35 AND the 35X and both models had identical PTO horsepower at 540 rpm PTO speed!! The 1963 New Performance Super Dexta launch was a response to the 35X. Playing around with rated engine speeds was a favourite trick of manufacturers at that time to claim an advantage over rivals.
Also the same engine could be rated at different speeds for different applications, for example the mark 1 Major engine was rated at 60 bhp at 2400 rpm when fitted in the ET6 truck and the mark 2 and 3 engines were rated at 65 bhp at 2500 rpm when fitted in the Trader range.

Going back to the "Dexta Special", it seems that there were both blue/orange and blue/grey versions of this tractor, when the Super Dexta was launched in April 1962 it was less than £50 more than the standard Dexta so most UK customers opted for the Super model and sales of the standard model declined from that point onwards. I had always thought that the Dexta Special models were minimec pump equipped tractors marketed for export territories only to try and dispose of surplus stock, probably loaded with " extras" as standard equipment to tempt buyers?

[Billy26F5]

I think Ford did reffer to it as an NP Dexta (for selling purposes) but as you say it was just a change of colour, although the films would make you think the pressure relief type valve was also introduced then (09C-913383 approx. I can't find any conclusive information on this), when it actually appeared on 09C-909585. There were several changes from the P4588 to the P4696, the governor main spring was changed to be the type used in the industrial 6 cylinder Major engine, the delivery valve holders were the NP Super Major type, volume reducers were said to be removed initially but later this was corrected to include them, longer plunger inlet stroke and new phasing spacers, 11.6-12 ccs for 200 injections at 600 rpm (instead of the 10.4-11 of the earlier Super Dexta or the 8.6-9.2 of the Dexta), higher no load speed of 2450 rpm, new injector nozzles (Simms NH389 instead of I think NH153, the holes are 0.32mm instead of 0.35mm) were also fitted, as were a new fan cowl and a foam rubber deflector, as well as the new colours. The NP Super Dexta therefore did have more power at the same PTO rpm, but probably not very much. The Dexta Special seems to have been for selling excess stock of Dexta's when they weren't selling very well once the Super Dexta took over most of that area, and all the extras probably did help Ford achieve this.
Sandy

[shepp]

Hi Sandy, I have never seen any reference to the blue /grey Dexta by Ford as "New Performance" either in the sales brochures or the salesmen's handbooks and price lists which I have, or in the film publicity material such as is available on the "Ford & Fordson" series of dvd's . In addition Stuart Gibbard also pointedly only describes the tractor as "the blue/ grey Dexta" in his books such as Dearbourn to Dagenham and in his renowned "Fordson" DVD.

There were several internal changes to the minimec pump fitted to the NP Super Dexta which produced claimed improved performance characteristics but not I would say in absolute power output. There is a limit to how much fuel can be burned in a cylinder, and hence how much power output can be obtained, and this is decided by the amount of oxygen available. You can play about a bit with valve timings and/or valve sizes to get that bit more oxygen in as was done with the Major engine from mark 1 to marks 2 & 3, but after that you have to move on to turbocharging. I have the editions of Farm Mechanisation covering the launch of the New Performance Super Dexta and Super Major, and the technical articles clearly state that NO increase in PTO power is claimed by Ford for the NP model Super Dexta over the original Super Dexta model, the PTO power remains the same.

[Billy26F5]

At no stage do I think that Ford claimed an increase in PTO power either, but the extra fuel will make a difference, the Major went from 11.6 to 13 ccs and it remained at the same capacity, so with the extra bore the Super Dexta had there was more margin for extra fuel, and I would say that Ford used it up for the NP Super Dexta giving it a 1.1 ccs increase in max fuel which although Ford might not have claimed it would have given a bit of extra power without increasing the no load speed. Even like this I would susppect that there was still plenty of oxygen available for more, but it would probably make it smoke a lot at full power. As diesel engines use high compression there is more air than actually needed for combustion, but as the fuel doesn't atomise as much as it maybe could the result is that even more air is a good idea. I could be very wrong on this but I just did a quick calculation on the fuel air ratio in the NP Super Dexta at full power and it came to a lean mixture of 41 times the stoichiometric air for the 12 ccs fuel calibration. As I say I could be quite wrong and putting in more fuel would definitely make it smoke black, but I definitely think the NP Super Dexta will manage to run a PTO implement where an earlier Super Dexta wouldn't manage (assuming the engines have equally good running, which is not easy to find). The smaller holes in the NH389 nozzle would probably change the atomisation as well, so the NP Super Dexta might habe better combustion as a result even with that much less air available.
Sandy

[shepp]

Well, I can only say what my own experience is over the years having owned and used a number of both Super Dexta models. There is absolutely no noticeable difference in performance between the 62 blue/orange Super Dexta and the 63 NP Super Dexta that I have. The power curve graph for the NP model appears to show around the same 39/40 bhp at 2000 rpm as the blue/orange model shows at that speed. The same is true for the Super Major, the power curve for the blue/orange model shows around 51/52 bhp at 1600 rpm and 53/54 bhp at 1700 rpm, the NP model graph shows similar powers at those speeds. So if you want to claim extra engine power for your newly launched NP model simply quote and claim the 53.7 bhp shown at 1700 rpm instead of the 51.8 bhp at 1600 rpm claimed for the earlier model! Simple! Nuffield did the same with their 3 and 4 cylinder models when the rating was upgraded.

[mathias1]

Came across this one

[Billy26F5]

Nice tractor, pitty it's outside. Several strange things on it but several nice original features too.
Sandy