You can call a boost gauge a manifold pressure gauge/indicator but you can't always call a manifold pressure gauge a boost gauge. For that, the engine needs to be super/turbocharged. Many normally-aspirated light aircraft have manifold pressure gauges.
I will try again. The gauges do the same thing. They measure the pressure in the inlet manifold and convey the result to the cockpit in either lbs per square inch or inches of mercury. With everything stationary both instruments will read the ambient pressure. The checks for both are the same. You wind your altimeter to Zero and if the subscale is lower than standard 1013.2 then the boost or MAP gauge will read either either below Zero or below 29.92 ins. With an inches subscale they will read the same. The ratio of boost to MAP is 2.036, or 2 for short; so for every pound of boost pushed into the engine a MAP gauge would increase by 2 ins and when the engine goes into suction mode, vice versa. An old check on the Hercules used to be a climb at Zero boost and see what height the supercharger coudn't maintain boost any more.
Last edited by Fareastdriver; 20th Feb 2012 at 19:45.
A "boost cutout" was more accurately a boost regulator cutout. The boost regulator was part of the linkage between the throttle lever and the actual throttle valve, and limited the throttle valve opening in order to avoid over-boosting the engine in normal operations. In an emergency the boost cutout lever could be pulled and an extra couple of pounds of boost gained.
It also slowly opened the throttle, without moving the throttle lever, as the aircraft climbed in order maintain the selected manifold air pressure until you reached full throttle height.
Of course the term may have found other meanings since then.
I've slowly come round to the conclusion that the Americans were right; manifold air pressure is a much more useful term than boost pressure.
Increasing altitude will cause both instruments to reduce
A supercharger will hold a set boost up to full throttle altitude, after which it will start to reduce.
It is possible to hold MAP in a climb, but impractible. With a constant speed propeller without a turbocharger it can be done by using reduced power (rpm & MAP) in the initial climb. As altitude increases the throttle may be progressively opened, which will hold the MAP until the throttle is fully open.
Location: On the Rump of Pendle Hill Lancashire UK
I used to have a full rebuilt Merlin in my reception office when I owned a factory with assorted offices, the Merlin I had was a two stage two speed Supercharger type, just below the main air tube going into the intake manifold (looking from the rear) was a quadrant of levers that would have been connected by rods and bell cranks back to the cockpit controls, but it seemed that the second stage S/Charger would be activated by the same pushrod that activated the first stage (or speed), the same quadrant of levers were all connected as well to two cranked arms to either side of the mopping bucket sized Carburettor. So it always seemed to me that just one lever would operate the functions of stepping up and giving extra motion lotion, and more HP. All these levers and bell cranks also were connected to a water injection system as well as the a small fist sized fitting at the front of the engine that I imagine was also connected with the propeller speed.
Who ever perfected that system possibly went mad.
I had many people come to see the Merlin, or possibly my well endowed Secretary but no one ever did satisfy my question of how it was all controlled, most were old Flyers who came to touch and remember things in past years.with a few philistines who wanted to know if it would fit into a car!!.
I have been pondering the implications of both instruments.
Climbing to critical altitude, whilst maintaining constant MAP (i.e. absolute pressure), the back pressure on the exhaust system is decreasing, as is crankcase pressure and thus the force on the bottom of the pistons.
At the same time, ambient temperature is dropping, for greater air density at the same MAP.
I read these two factors as a gradual increase in power during the climb.
On the other hand, a Boost "gage" (Yankee spelling) would tend to even out the first of these factors. I've never flown a Boost engine - maybe someone can expand on this.
From a RAAF (1942) ground school training publication,
The air pressure in the induction pipe is known as boost or boost pressure, and is measured in lbs per square inch above (+) or below (-) that of the standard sea level atmosphere.
Of course, we colonials may not understand the complexities of machinery, but to me that says a boost gauge is exactly the same as a MP gauge. The boost gauge will indicate ambient pressure with the engine shut down.
Re boost cut out. The Lancastrian (transport Lancaster) engine lists a maximum of +14 boost, but with the boost cut out +18 was available. +18 was only to be used on take off if necessary, and if so used, boost was to be reduced to +14 as soon as the +14 take off safety speed (140 knots) was reached.