Comparing the Allison powered P-40E with the Merlin powered P-40F reveals that below 15,000 ft. they perform pretty much the same. The F had a higher service ceiling (34,400 ft. vs. 29,000 ft.) and a better rate of climb, but that's about it. The F used the two-stage supercharged Packard built V-1650-1. Since the U.S.A.A.F. considered the P-40 primarily a low altitude fighter/bomber, and the Merlin was needed for the Mustang, most subsequent P-40's went back to the Allison. And, many of the Merlin powered P-40F and L models when removed from front line service were re-engined with V-1710's and used were as advanced trainers designated the P-40R.