"Jim Macklin" wrote in message
...
In a non-pressurized aircraft, the breathing of pure oxygen
is not adequate for sustained pilot survival much above
37,000 feet. They can use oxygen delivered under pressure
to the mask, but only about 1/4 psi of "boost" to the pilot
or it will rupture the lungs.
Engines used mechanical air pumps, large turbochargers and
later they had turbo-compound engines.

Also, many of the big high altitude engines had multiple speed (at least two
speed, that I know of) gearboxes running their superchargers. Spinning the
supercharger's impellers even faster helped to build the necessary boost.

Don't forget (to the OP) that less power is needed to keep the plane going in
the air up there, because of the lessened friction.
--
Jim in NC