earlier Marks of tanks except Mark 1's were upgraded to Chieftain Mk.5 standard as part of the 1975 "Totem Pole" programme. "Exercise Totem Pole" was carried out in six-to-nine phases depending on the Mark of vehicle being modified (Chieftain Mk.5's already had some of the required changes incorporated at the factory) between 1975 and 1979 and included fitment of the Marconi Improved Fire Control System (IFCS), replacement of the searchlight with the Barr & Stroud Thermal Observation Gunnery System (TOGS), along with modifications for using FSAPDS ammunition. Upon completion of each phase the vehicle received an additional suffix to the designation, e.g., "Chieftain Mk.3/S(Y)2" denoting a Mark 3/S having completed the first three phases of "Totem Pole". including addition of Clansman radios, fitting of TLS, fitment of Muzzle Reference System (MRS) upon replacement of L11A3 barrel with L11A5 barrel, and fitment of 750 hp L60 Mark 8A. These vehicles were re-designated Chieftain Mk's.6 to Mk.8. Chieftain Mk.6 Chieftain Mk.2 upgraded to Chieftain Mk.5 standard, 1975 Chieftain Mk.7 Chieftain Mk.3 and Chieftain Mk 3/S upgraded to Chieftain Mk.5 standard, 1975 Chieftain Mk.7C Chieftain Mk.3 upgraded to Chieftain Mk.5 standard for Oman Chieftain Mk.8 Chieftain Mk.3/3 upgraded to Chieftain Mk.5 standard, 1975 In 1977 the engines of all British Army vehicles were upgraded to the 750 hp L60 Mark 9A, followed by further upgrading with the L60 Mark 11A or L60 Mark 12A in 1978 as part of the 1977 "Dark Morn" and the 1978 "Sundance" programmes. "Exercise Sundance" concerned improvements in engine power, reliability, and other power train improvements, and was carried out in five main phases between 1976 and 1979. These, in themselves, had been preceded by "Dark Morn", "High Noon", and the initial "Fleetfoot" engine development programme which had completed in October 1971. Vehicles on which "Sundance" modifications had been carried out received an additional 'Z' suffix appended to their designations, the "Sundance" engines themselves were signified with a distinctive orange/yellow-coloured upper crankcase. Chieftain Mk.9 Chieftain Mk.6 after completing all phases of "Totem Pole", 1979. Chieftain Mk.10 Chieftain Mk.7 after completing all phases of "Totem Pole". Later upgraded with addition of Stillbrew Crew Protection Package which added additional armour to the turret front and turret ring to increase turret frontal protection against HEAT rounds, specifically, the improved variants of the Soviet-made RPG-7, etc., and consisted of conventional steel armour mounted on rubber pads to prevent vibration, 1984–86 Chieftain Mk.11