What is the difference between the AH Spit9 and Spit16?
In RL, the only real difference was the Spit9 used a R-R merlin and the Spit16 used a Packard Merlin.
Depends on the Spitfire Mk IX.
The Spitfire Mk IX used the Merlin 61, 63, 66 and 70. The Spitfire Mk XVI used the Merlin 266, a Packard built version of the Merlin 66.
The Spitfire Mk IX in AH uses the earliest Mk IX engine, the Merlin 61 and thus represents the Spitfire Mk IX as introduced in July of 1942. For the later, more common Merlin 66 powered Mk IXs the Spitfire Mk VIII (Merlin 66 in AH) or Spitfire Mk XVI (Merlin 66 in AH, not 266) ought to be used instead.