By the carburettor deck I mean the inlet to the carburettor.
Critical altitude for the turbo is the maximum altitude at which the turbo can compress the air to sea level pressure.
Turbo equipped aircraft in WW2 used two stages of supercharging. The turbo compressed the air to 1atm abs and supplied it to the engine, which had an integral single stage single speed supercharger. The integral supercharger would then provide boost to the engine.
WW2 engines were rated for certain altitudes. The Merlin 45, for example, was rated at 18,000ft at +12psi boost. Below that altitude boost was maintained by throttling, while above the boost fell away.This altitude was called the Full Throttle Height (FTH) by the British and Critical Altitude by the Americans. Thus the maximum power of an engine at a particular boost level was at its FTH, power being less either side of it.
An example:
http://www.wwiiaircr...Power_Chart.jpg
In contrast, the engines coupled with turbos had a rated altitude of 0ft. The turbo provided compensation for altitude.
The difference can clearly be seen between the Spitfire V (single stage single speed), Spitfir IX (2 stage 2 speed) and the P-38 (turbo).
http://www.wwiiaircr...l_Speed_RAE.jpg
http://www.spitfirep.../bf274speed.jpg
http://www.wwiiaircr...t-2338-1400.jpg
The Spitfire IX chart also shows the boost - how it is maintained constant to a certain altitude and then falls off.
Edited by Wuzak, 22 January 2015 - 12:23.