A very interesting thread.
Still working the original question through in my head.
I was taught that "supercharging" is an effect of valve timing, rather than just the blower per se. Blowers or superchargers are also used on two stroke diesel engines for more effective scavanging with no increase in MAP.
"Supercharging" is caused by allowing the inlet valve to stay open slightly longer than normal during intake stroke - with the exhaust valve closed, hence not valve overlap as some one has suggested.
Can anyone tell me where the MAP sense line is tapped from on these "big" engines?
My thoughts are that the MAP gauge should be sensing the Blower output.
No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.
I find this confusing. Back pressure assists to hold the exhaust valve closed, so I can't see why less back pressure on a dead engine would affect cylinder pressure.
I imagine there would still be some back pressure even with a dead engine.
As the piston swings past the top of exhaust stroke it will induce a slight negative pressure in the cylinder, my thoughts are that this would be the same with a dead engine so there is a nil net affect.
All ways learning.
Cheers
BH