Whilst researching the background of a Merlin XX that we run very occasionally it seems it may have been in a Hurricane at one point in its career... this little piece seems relevant to the thread...
In his book "Hurricane: Victor of the Battle of Britain" Leo McKinstry, writes on pages 228-230:
While the RAF struggled to find the ideal night-fighter [page 220: though the Hurricane was not designed for the role...it was felt that "The Hurricane is a simple aeroplane to fly by night. there is no glare in the cockpit, either open or closed, from the cockpit light or luminous instruments. The steady, steep glide at slow speed which is characteristic of this type makes landing extremely simple." But in practice the aeroplane proved woefully ineffective against German night intruders during the long winter of 1940-41.... by far the the biggest difficulty was the lack of any method of guiding the fighters onto their targets...without any navigational aids, the Hurricane pilots could do nothing more than grope in the dark...] a much more fundamental drive was underway to improve the overall performance of the Hurricane. As far back as the outbreak of the war, the Air Ministry and Sidney Camm himself had recognised that the plane would soon head for obsolescence unless changes were made. Indeed, in early 1940 the government briefly discussed the idea of phasing out the hurricane altogether in 1941, forestalling the need for any design improvements. 'As policy was to concentrate on the Spitfire and the Hurricane production programme was to be completed by July 1941, it was not necessary to increase the performance of the Hurricane as it had been done with the Spitfire,' concluded a research meeting of the Air Ministry in February 1940. But this negative outlook never gained any traction, particularly as the Hurricane was proving itself in combat and the Spitfire production was so badly behind schedule. The more realsistic alternative to pahsing out the Hawker palne was to enhance its capability through the introduction of a more powerful engine and high-calibre weaponry.
By early 1940 Rolls-Royce had developed a more advanced version of the Merlin, featuring a two-stage supercharger. Called the Merlin XX, it delivered 1300 hp, compared to 1,030 from the original. It also gave a stronger performance at altitude and raised the efficiency of the engine because it used a mix of 30% glycol and 70% water, rather than pure glycol.... Camm sent the Air Ministry a proposal to install the engine on the Hurricane. He emphasised that no design changes were needed in the plane to accommodate the new power-plant, apart from slightly longer front fuselage and a larger coolant radiator. The Air Ministry approved the proposal and told Hawker to produce the prototype of what soon was officially entitled the Hurricane Mark II Series A.
In May the arrival of Lord Beaverbrook at the Ministry of Aircraft Production (MAP), combined with the growing intensity in fighter combat, gave new emphasis to the Mark II project. On 24th May 1940, MAP agreed that the Hurricane II must have first claim over the Spitfire and Beaufighter on Rolls-Royce's production of the Merlin XX because of the vital need to improve the Hawker fighter's performance...
the first [production] models emerged from the Langley works in September 1940, going into service initially with 111 'Treble One' Squadron and then replacing Mark Is in other Fighter Command units over the coming months.
...Bob Foster of 605 Squadron recalled his satisfaction 'it is no exaggeration to say that everyone was pleased with them. They had a Merlin XX engine which gave us higher speeds and a much better rate of climb. In the air the superiority of the Mark II was evident. [Tom Neil of 249 Squadron reported] 'The manually controlled two-speed supercharger, when engaged, came in with a disconcerting thud but increased the performance above 20,000 ft very substantially.' In the Mark I , Neil had never been optimistic going into combat above 25,000 feet whereas the Mark IIA 'was still pretty active at something over 30,000 feet'.