As far as I am aware, there was not a significant difference in power output between the T35's and the later Mks - the redesign for the Mk3 was aimed at eliminating some rough running that had been reported by owners of the early machines. With the subsequent improvement in the fuel available, any power difference between a T35 and a Mk3 was difficult to spot. Most Douglas enthusiasts seem to agree that the Mk3 engines were the sweetest running of the postwar variety - the later mk4 and 5 machines seemed to suffer from poorer quality control (probably attributable to the tooling becoming tired). The power outputs I have seen quoted are:- T35, Mk3, Mk4, Mk5, 18 BHP. Mk3 Sports, Mk4 Sports 21BHP (this horsepower increase being gained with just a higher lift cam).
The production 'Plus' models were reputed to produce 25BHP (80 Plus) and 28BHP (90 Plus), but these figures were obtained with a further redesign of heads, cams and valve gear, and larger carbs. The last 'works' development engines were reputed to be giving 31BHP at something like 11,000 RPM!!
Since the advent of Classic Racing, several Club members have tried their hand at making a postwar Doug competitive - the most successful being George Easton. His Mk3 machine (opened out to 500cc) being a match for a similar age Manx Norton! George later went on to do the same with a Dragonfly engine (which he found to be 'over ported' - so had the ports welded up and reshaped). During the development of his engines, George also found that the higher output Plus/Dragonfly oil pump over-oiled his engines, so he reverted to the standard Mark oil pump.
Regards,
Eddie.