P-39 Expert
Non-Expert
No turbo for the P-39, best thing that ever happened to it was deleting the turbo. To improve performance just delete some non-essential equipment to make them lighter (.30 cal wing guns and nose armor plate) until the mechanical second stage was in production in April '43.
The internal supercharger step-up gears could accommodate ratios from 6:1 up to 11.6:1 by simply changing the number of teeth in the drive gear and the driven gear. The two stage engines (turbo and mechanical) used lower ratios (6.44, 7.48 and 8.1) since the second stage was supplying sea level air to the internal stage. Single stage engines used the 8.8 and 9.6 gears and it was hoped that the 9.6 gears could be introduced in late '41 to increase critical altitude from 12000' to 15000' and provide about 100 additional horsepower above about 9000' as compared to the 8.8 gears. The 9.6 gears proved unable to take the increased manifold pressure and could not complete the standard (for the U.S) 150 hour test. Possibly because port backfire screens were still in use (until mid '42) that reduced the manifold pressure at the port into the cylinder. Had these backfire screens been deleted the unrestricted flow from the supercharger to the intake ports may have reduced the load on the gears enough for them to pass the test. But the backfire screens were not eliminated until mid'42 while Allison was in the middle of redesigning the gear housing to accommodate wider gears to handle the increased pressure. The wider gears were in production from August '42 and were in production P-39Ms from November '42. So about a year was lost fixing the 9.6 gear problems.
The internal supercharger step-up gears could accommodate ratios from 6:1 up to 11.6:1 by simply changing the number of teeth in the drive gear and the driven gear. The two stage engines (turbo and mechanical) used lower ratios (6.44, 7.48 and 8.1) since the second stage was supplying sea level air to the internal stage. Single stage engines used the 8.8 and 9.6 gears and it was hoped that the 9.6 gears could be introduced in late '41 to increase critical altitude from 12000' to 15000' and provide about 100 additional horsepower above about 9000' as compared to the 8.8 gears. The 9.6 gears proved unable to take the increased manifold pressure and could not complete the standard (for the U.S) 150 hour test. Possibly because port backfire screens were still in use (until mid '42) that reduced the manifold pressure at the port into the cylinder. Had these backfire screens been deleted the unrestricted flow from the supercharger to the intake ports may have reduced the load on the gears enough for them to pass the test. But the backfire screens were not eliminated until mid'42 while Allison was in the middle of redesigning the gear housing to accommodate wider gears to handle the increased pressure. The wider gears were in production from August '42 and were in production P-39Ms from November '42. So about a year was lost fixing the 9.6 gear problems.
Last edited: