The type YJ or Ventura was developed in the mid-1950s as Davey, Paxman's first high-speed diesel engine.
[4] With a view to the forthcoming modernisation and dieselisation of British Railways (BR), it was intended as a successor to Paxman's existing medium-speed engine, the direct injection YH.
[5] High-speed engines offered higher power-to-weight ratios, which in turn allowed locomotives to have a lower axle loading and greater route availability.
[2][4] Their construction and reliable use though required more sophisticated manufacture than previously, with better metallurgy and balancing of the moving parts for faster running and a stiffer crankcase to avoid vibration.
The aluminium alloy bed-frame of the YH and ZH engines had previously given trouble with cracking, and had to be replaced in the Class 17 with a cast iron frame, at Paxman's expense.
[3] The YJ engines were primarily developed for diesel electric locomotives (however Paxmans main design philosophy was maximum power for a given size and weight) and so their crankshafts have the unusual feature of an additional main bearing at the drive end, to help support the weight of the generator armature, where a single-bearing generator can be otherwise unsupported at one end.