[MFAFF]

All depends on which mark of Spit and which model of Mustang.

A Mk1 Spit would have eaten a P-51A for breakfast and not hesitated; the same apples for all of the early non-Merlin Mustangs. They simply did not have the all round performance of the equivalent Spits in the close turn battle arena, at low level or high level. Hence the installation of the Merlin engine.

The use of the Merlin (and the bubble canopy) in the Mustang changed everything. The additional power at all levels, but especially high level ensured the more advanced aeronautical design of the Mustang's wing was exploitable more of the time. In comparison the later Mk Merlin engined Spits were beginning to reach the far right of the wing's performance envelope and the comparison is different.

As mentioned by others the pilot's training and experience was massively influential as to the outcome moreso than the abstract performance of the aircraft. At low level the Spit was possibly better, however the advantage was insufficient to ensure consistent results. At high level the Mustang in theory was better, again slim margins.

A Mk 1 Spit is a very different beast to a Mk V or Mk IX, let alone a Mk 24. Similarly a P-51A/B is not the same as a P-51D or H.

The Spit changed from 8 x .303 Brownings to heavier cannon armament depending on the Mk so the weapons side of things balances out over time. 'A' to 'E' wings and different mixes of .303, .50 and 20mm cannon depending on the role and Mk.

In addition we could consider the Griffon engined Spits (later Mks, with the XII the first) which had a longer fuselage and different wings) and were contemporary of the Mustangs. The larger and more powerful engine did add significant performance across the envelope... and these 'big' Spits may be viewed as one of the ultimate piston engined fighters (along with the Bearcat and Corsair).

In contrast further development of the Mustang was halted as the initial view was the XP-75 Eagle was the next thing; however piston- engined a/c development was curtailed by the realisation that jets were always going to be faster.

PS. The Merlin had a two stage supercharger (low and high level ) whilst the Allison only had a single stage s'charger. A bolt-on second high level charger was added later, however it lacked other features of the Merlin's installation. The final development of the Allison was to add a turbine to the lot, creating a turbo- compound engine (primarily for the XP-75) however it was not developed fully, shame as it would have been a beast. By then these high power V engines were on the way out.... radial for transports were far more reliable (!) and powerful and gas turbines were the future for fighters.