I looked up the merlin info. A 2,640HP version of the merlin was available, but the 130/131 was more aerodynamic (lower frontal area) and the higher power was not needed. lower boost and more reliability. The griffin 2,00 hp to 2400 hp (I was surprised that the Griffin was even close in HP to the later versions of the Merlin which is considerably smaller)
Production engines
The Merlin II and III series were the first main production versions of the engine. The Merlin III was the first version to incorporate a "universal" propeller shaft, allowing either
de Havilland or
Rotol manufactured propellers to be used.
[22]
The first major version to incorporate changes brought about through experience in operational service was the XX, which was designed to run on 100
octane fuel.
[nb 2] This fuel allowed higher
manifold pressures, which were achieved by increasing the boost from the
centrifugal supercharger. The Merlin XX also utilised the two-speed superchargers designed by Rolls-Royce, resulting in increased power at higher altitudes than previous versions. Another improvement, introduced with the Merlin X, was the use of a 70%–30% water-glycol coolant mix rather than the 100% glycol of the earlier versions. This substantially improved engine life and reliability, removed the fire hazard of the flammable
ethylene glycol, and reduced the oil leaks that had been a problem with the early Merlin I, II and III series.
[24]
The process of improvement continued, with later versions running on higher octane ratings, delivering more power. Fundamental design changes were also made to all key components, again increasing the engine's life and reliability. By the end of the war the "little" engine was delivering over 1,600 horsepower (1,200 kW) in common versions, and as much as 2,030 horsepower (1,540 kW) in the Merlin 130/131 versions specifically designed for the
de Havilland Hornet.
[25] Ultimately, during tests conducted by Rolls-Royce at
Derby, an RM.17.SM (the high altitude version of the Merlin 100-Series) achieved 2,640 horsepower (1,969 kW) at 36 lb boost (103"Hg) on 150 octane fuel with water injection.
[26]
With the end of the war, work on improving Merlin power output was halted and the development effort was concentrated on civil derivatives of the Merlin.
[27] Development of what became the "Transport Merlin" (TML)
[28] commenced with the Merlin 102 (the first Merlin to complete the new civil
type-test requirements) and was aimed at improving reliability and service overhaul periods for airline operators using airliner and transport aircraft such as the
Avro Lancastrian,
Avro York (Merlin 500-series),
Avro Tudor II & IV (Merlin 621), Tudor IVB & V (Merlin 623),
TCA Canadair North Star (Merlin 724) and
BOAC Argonaut (Merlin 724-IC).
[29] By 1951
time between overhaul (TBO) was typically 650–800 hours depending on use.
[30][31] By then single-stage engines had accumulated 2,615,000 engine hours in civil operation, and two-stage engines 1,169,000.
[32]
In addition, an exhaust system to reduce noise levels to below those from ejector exhausts was devised for the North Star/Argonaut. This "cross-over" system took the exhaust flow from the inboard bank of cylinders up-and-over the engine before discharging the exhaust stream on the outboard side of the
UPP nacelle. As a result, sound levels were reduced by between 5 and 8
decibels. The modified exhaust also conferred an increase in horsepower over the unmodified system of 38 hp (28 kW), resulting in a 5 knot improvement in true air speed. Still-air range of the aircraft was also improved by around 4 per cent.
[28] The modified engine was designated the "TMO" and the modified exhaust system was supplied as kit that could be installed on existing engines either by the operator or by Rolls-Royce.
[28]
Power ratings for the civil Merlin 600, 620, and 621-series was 1,160 hp (870 kW) continuous cruising at 23,500 feet (7,200 m), and 1,725 hp (1,286 kW) for take-off. Merlins 622–626 were rated at 1,420 hp (1,060 kW) continuous cruising at 18,700 feet (5,700 m), and 1,760 hp (1,310 kW) for take-off. Engines were available with single-stage, two-speed supercharging (500-series), two-stage, two-speed supercharging (600-series), and with full intercooling, or with half intercooling/charge heating, charge heating being employed for cold area use such as in Canada.
[29] Civil Merlin engines in airline service flew 7,818,000 air miles in 1946, 17,455,000 in 1947, and 24,850,000 miles in 1948.
[33]