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, designated Series IIA, (E115) was produced in May 1943 which increased the combat boost to + 9 psi and raised the maximum power output further to 2,220 B.H.P. This engine 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 The Series IIA engine was eventually superseded in 1944 by the Series IIB (E107A) engine.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 called for an Improved performance at low altitudes and this was achieved by increasing the combat rating boost to +11 psi. This raised the maximum power output to 2,420 B.H.P. enabling the Hawker Tempest fighter to catch and destroy large numbers of flying bombs.
The Series IIC design E107B was evolved in 1945 with an improved type of carburettor with a weaker mixture setting. This 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 achieved by raising the ratio of the "M" gear whilst leaving the "S" gear unchanged,
A modified version of the Series II engine incorporating balanced crankshafts and strengthened sleeve drives was evolved in 1941 to meet the special requirements of a Fleet Air Arm Fighter. This engine, designated the Series III, had a maximum power rating of 2,305 B.H.P., and a take-off power of 2,250 B.H.P. at 4,000 C.R.P.M. the high take-off R.P.M. being in the interests of propeller thrust. About 30 of these engines were built between 1942 and 1943 were fitted (as a complete power unit) to the Blackburn "Firebrand" aircraft. This aircraft never became fully operational, but was used extensively for training purposes.
A further range of Sabre engines employing bulk injection carburettors commenced with the Series IV. They had a single entry supercharger as well as strengthened supercharger and reduction gear drives to withstand the higher gross powers which resulted from the use of higher boost pressures coupled with higher ratio supercharger drives. They additionally incorporated sundry technical improvements including boost and R.P.M. corrected ignition, as well as an improved supercharger clutch.
The Series IV engine first utilised a S.U. twin-choke suction carburettor but this was soon changed to a Hobson-R.A.E. Injection Carburettor. Intensive development of this unit followed. In 1943 the Series V (E107C) engine superseded the Series IV. Both series of engines commenced life with fully balanced crankshafts and developed 2,180 B.H.P. at 8,500 foot (at 4,000 C.R.P.M. with + 9 psi boost). As a result of torsiograph tests it was decided to remove the balance weights from the crankshafts and reduce the maximum R.P.M. slightly owing to the presence of a certain amount of torsional vibration at the top end of the speed range.
Gradually the power output of the Sabre V was stepped up until, in November 1944, a development type-test was passed at a combat power rating, of 2,600 B.H.P. Technical improvements to the engine were continually added and ultimately the the Series VA production engine passed an official type-test at the N. S.81.S.M combat power rating of 2,600 B.H.P. in June 1945
The Sabre VA incorporated the following features;
(i) Single lever control
(ii) Improved oil sump
(iii) Improved auxiliary drive casing
(iv) Increased coolant flow through cylinder blocks
(v) Strengthened reduction gears
(vi) Single piece sleeve drive idler’ gears