The 1710 Allison was a better designed core engine per se, the Merlins
had the two stage two speed blower which gave altitude performance. The
ultimate recip fighter engine was-no, not the RR Griffon with its
byzantine redrive and quill shafts, nor the Centaurus, now generally
replaced by 3350 Wrights-the late Allison fitted with essentially the
Merlin blower in the Twin Mustang , because RR refused Packard
continued license to build Merlins, despite-or perhaps- Packard having
substantially improved the engine.