Searching on internet, I found some performance charts of the Merlin Engine BHP output:

enter image description here

As you can see there is an increase in BHP,(mainting a constant Boost Setting), until reaching the critical altitude of the supercharger in Low Gear.

Is this increase in BHP due to a higher compression as there is a reduction in pressure at higher altitudes?

If so, should a higher IAS (Dynamic pressure) be obtained in level flight? (because more BHP are available).

Thanks for the Help!

  • 2
    $\begingroup$ Mechanical superchargers suffer from “throttling losses” at lower altitudes until the aircraft reaches the critical altitude where max power is created with a wide open throttle. For a detailed explanation try this video. youtu.be/ULLsIo1VzTw $\endgroup$ Sep 12, 2021 at 22:52
  • $\begingroup$ Thanks for the answer! All clear now. $\endgroup$
    – Spitfire01
    Sep 14, 2021 at 23:24

1 Answer 1


As noted in Airplane Aerodynamics by Dommasch, Sherby, and Connolly, 1967, Pitman Publishing Corporation, New York...

It is a relatively simple matter to fit to an engine a large supercharger that is capable of providing excessive MAP (manifold absolute pressure) at full throttle at sea level. Ascent to altitude permits opening the throttle to compensate for reduction in air density without exceeding the rated MAP. In fact, if MAP is kept constant by gradually opening the throttle as altitude is gained, the reduction in exhaust back pressure will cause an increase in airflow that actually increases engine power. The altitude at which the throttle must be fully opened to maintain a given MAP at a given RPM is known as the critical altitude of the engine for the given RPM-MAP condition. Above this altitude the mass air flow, and consequently the power output, diminishes because of the reduction in air density. A gear shift providing a higher speed for the supercharger impeller, or an auxiliary stage of supercharging, simply extends the above conditions to a higher altitude.

This explanation is given and detailed on page 251 and pages following in the source given above.


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