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Sascha
The reason they never really caught on in the states was, they were never used in military/commercial there, while in the UK they were used to power late Spitfires, Shackleton bombers, Hoover crafts, and in some odd military supply service until the mid-80s ( I remember my friend taking a distributer apart that had a 1981 overhaul mark in it).
The Griffon is 36.5 liters and blessed with an enormous, two-stage supercharger, up to date by the time it came out, that I have seen do 60 psi on ground levels.
That supercharger (as most aircraft superchargers) is actually being designed as an "altitude adjuster" has (boost-operated) hydraulic mechanisms, that would prevent "over-boost" in lower altitudes by operating the air inlet to the supercharger / decoupling second speed.
You lock that mechanism, allow the supercharger to breathe in freely with a funnel instead of that boost-controlled mechanism and keep that thing from backfiring and detonating (add lots of water or run it on methanol) and you have a 2200ish cui monster getting 4 bars of boost.
Disadvantages: It's kinda heavy because it's bigger, the cylinder head design is pre WW2 and so is the rest of the main design of the engine. Plus it's a Rolls Royce and these guys seem to have made costume-fit threads on custom-made bolts for every application. Perfect - until you need to replace them or you think you can rebuild it in a week and figure out there are three different types of rod bolts used in it at random quantities.
The Allison on the other hand is a smaller but more modern design - but plagued with a poor-performing single-stage supercharger, as in the development it was expected to have the altitude compensation by a big turbo supplied by GE, which it only got in the P38.
The Allison Supercharger will make 30 psi on a good day (you do the valves and port the heads and it does 22) and will only start to show great boost pressures, when the air on the impeller tips reaches stall speed and its heating up your intake air to 900F (hence the "boost") - so you then have to throw lots of water or methanol in it, too keep it running and not backfiring / detonating / kicking rods out.
On the other hand, the Allison is pretty light for its power output and was built with standardized bolt sizes.
Hey Sascha, did the Griffon have the pent roof combustion chamber and four valves like the Allison???
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