As we know that the Peregrine and Vulture will be cancelled, can they not be developed in the first place, so Rolls Royce can concentrate on improving the Merlin and getting the Griffon in to service sooner? AFAIK the engine that became the Griffon was started around the same time as the Merlin, but development was suspended for several years so RR could concentrate on the Merlin.
This is from Lumsden's "British Piston Aero-Engines and their Aircraft"
"Griffon I
This engine originated from the 1925 Buzzard, whose enormous potential was finally realized in the 1931 'R' racing engine. It was natural that the engine was in line for development into the upper bracket of military power units, even more potent than the P.V.12 (Merlin) already planned. A de-rated version of the 'R', to be called the Griffon, was first run in 1933. The Griffon I never flew. There was considerable pressure to get on with the smaller engine, already planned for the new Hawker and Supermarine fighters and so active development did not continue until 1939. It was then extensively changed and made far more compact, as the Griffon II.
The Griffon II was a major redesign of the original engine, first run in November, 1939. It was generally similar to the earlier Rolls-Royce engines and the Merlin in particular, although of considerably larger capacity. It was necessary to keep its size and weight within the limits already imposed by the size and shapes of existing fighter aircraft. In order to do so, much thought went into redesigning the components so as to keep the engine's dimensions, particularly the overall length, to a minimum. As a result the camshaft drives and the first part of the of the step-up of the supercharger drive were removed from the rear of the engine to the front. The crankshaft driving them via a pinion and shafts. The supercharger drive was a long shaft running to the rear, in bearings beneath the crankshaft; the second part of the step-up drive was completed at the rear of the engine through the usual moderate and full supercharger clutches. At first, it was thought that the small diameter of this drive was adequate for the spring drive."
And it continues with other comments like
"..... The original type of supercharger drive was not altogether satisfactory and in subsequent Griffons it was changed so that both step-up gears reverted to the rear. The real trouble, in its original form, was that it was expensive to make, hence the reversion to the drive at the back of the engine....."
"....These compact arrangements thus made it possible for the Griffon to be squeezed into the Spitfire, even with two-stage supercharging, aftercooling and ultimately a third supercharger drive. The Griffon turned in the opposite direction from that of earlier engines and at a rather lower speed, having a different firing order from that of the Merlin and producing a harsher exhaust note...."
The 'R' racing engine dimensions were (length x width x height) 100 x 32 x 42 inches.
The Griffon dimensions were 72-81 (depending on version) x 30.3 x 46 inches.
Merlin prototype engine first ran 15 Oct 1933 i.e the same year as Griffon I. It passed Type A test in July 1934 and first flew 21 Feb 1935.
RR was still developing the Kestrel through to 1938 for the likes of the Miles Master I trainer (Kestrel XXX at 720hp was the most powerful version) with the Peregrine being a development from that for use in the Whirlwind. First run of the latter was 1938.
Vulture built on that development line being described by Lumsden as
"Vulture I 1,800hp (1939), resembled two Kestrel sized Vee-12 units with a common crankshaft mounted on a common crankcase, with a two speed, single stage supercharger. A 24-cylinder, 90-degree X shaped engine. Virtually two Peregrines, one above the other on a single crankcase with all four banks of connecting rods operating on a single crankshaft."
First run was in May 1937.
RR was also working on the air-cooled 24 cylinder X layout Exe engine (also known as the Boreas) which first ran in Sept 1936. Intended for the Barracuda.
I suppose one question that needs to be answered is where was the general market in 1933 for an engine of the power of the 'R' engine (designed for 1,500 hp but developed to 2,350 hp in 1931 form). The early Merlins with about 1,000hp were adequate for the new fighters. It was the specs for new heavy / medium bombers arising in 1935/36 that drove the need for more powerful engines. By that time Griffon I development had been shelved. It seems to me that it was problems with Vulture development that saw Griffon once again become of interest but in the very much modified Griffon II form.
