Quote:
Originally Posted by 41Sqn_Banks
There is no magic device needed to get +17 boost from the engine, it's simply the supercharger that is capable of producing this pressure at sea level. Of course this is a overboost condition for the engine and will kill your engine in no time. That's the problem with supercharged engines, the pilot has to keep the boost below the limits and has to adjust the throttle with every altitude change (as boost changes with altitude).
Thats why there is the automatic boost control that limits the engine boost below full throttle heigth automatically. However there might be a failure in the automatic boost control or a emergency situation ... so there was a boost control cut-out that disabled it and allowed the pilot to control the boost himself. Then however he could seriously overboost the engine. So they modified the engine that even with disabled boost control the boost pressure couldn't be raised above +12 boost.
So the "modification" didn't allow the engine to produce higher boost, in fact it limited the boost from around +17 to +12.
PS: Please use whole words to write your sentences it's hard to make sense of these abbreviations.
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This is a very good answer Tomcat, well known facts btw. Just read any articles or books about the history if the Merlin II & III engines and you'll see it really is as simple as that. The actual boost was around +16PSI, that was considered as unsafe for normal service and engine life, therefore, a 3mm hole has been drilled to get the excessive pressure out of the system limiting it to +12PSI. That was what it says - Boost Control Cut-Out so it simply overrided the rated limit by moving certain bits to block certain openings via bowden cable. The actual gauge only showed +8PSI and slightly above (was +4PSI max. only on first production Spitfires and +8PSI max on later Spits) and pressure went from +6.25 to +12PSI straigt away, eg no boost of say +10PSI was possible. You can find exact technical info about how exactly this was achieved, you can find another pieces of information about the RAF pilots flying over France well before BoB period ''pulling the plug'' all the time. You can also find info about a +25PSI rated Speed Spitfire (same Merlin III) if you really care.
No matter what your opinion is, these are simple and well researched facts, not much space to argue I am afraid.
I am a blue pilot and not a Spitfire jockey, mind you.
