During 1941 and 1942 Service experience in the Typhoon had faults which had not been apparent during the test bed development of the engine. A further modified version of the engine was produced in May 1943 which increased the combat boost to +9 lb/sq.in. and raised the maximum power output from 2,090 to 2,220 BHP. This engine was designated Series IIA and included further improvements such as a strengthened starter gear train, fully nitrided sleeves, and strengthened sleeve drives.
The Series IIA engine was eventually superseded in 1944 by the Series IIB (E107A) engine. The advent of the V1 flying bomb in June 1944 had called for an improved performance at low altitudes and this was achieved by increasing the combat rating boost to +11 lb/sq.in. This raised the maximum power output to 2,420 BHP, and enabled the Tempest fighter to catch and destroy large numbers of flying bombs.
The design was constantly being improved upon, until finally in 1945 the Series IIC (E107B) was evolved. The production of this engine, with an improved type of carburettor with a weaker mixture setting, marked the peak in the design and performance of Sabre engines fitted with suction type carburettors. The two-speed supercharger drive was altered to give a closer ratio; this being done by raising the ratio of the “M” gear whilst leaving the “S” gear unchanged.
A modified version of the Series II engine but incorporating balanced crankshafts and strengthened sleeve drives was evolved in 1941 to meet the special requirements of the Fleet Air Arm Fighter. This engine, designated the Series III, had a maximum power rating of 2,305 BHP, and a take-off power of 2,250 BHP at 4,000 CRPM, the high take-off RPM being in the interests of propeller thrust. About 30 of these engines were built between 1942 and 1943 and were fitted (as a complete power unit) to the Blackburn B37 Firebrand aircraft. This aircraft never became fully operational mainly because the Sabre was being prioritised for the Hawker Typhoon, but was used extensively for training purposes.