1. So IOW, the Merlin XII (for instance) can not handle more than 9 pds. psi without being in danger of shutting down?
"Shutting down"... it's a bad word to describe it. Merlin XII "can" and "cannot" handle over 9 psi. It can handle it for short period of time but it costs some durability. It will wear. It cannot handle it continuously.
It will not suddenly just "shut down". Engine failure is gradual and there's no safeguard that would shut the engine down to preserve it. A safeguard like that would be life-threatening. The Boost Cut-Out (BCO) merely open when 9 psi is reached. It opens to the extent needed, to give you 9 psi of manifold pressure. If there's much excess pressure, the valve will open more and release more. If the pressure would be only slightly above 9 psi, the valve is merely cracked. When boost drop below 9 psi, the valve shuts completely and leaks no air in any direction.
What was discussed on BCO valve failing was a rare failure scenario. It's not part of normal operation of the valve... at all. But Spitfire is still equipped to handle that rare failure with BCO override (BCO-O).
2. How will I know if this piston fails or is about to fail ?
I don't even know if it's modeled in Accusim. It certainly isn't mentioned in maintenance hangar. That part of discussion was about real Spitfires... like was my post as well.
I wouldn't mind if some Core-Accusim update included boost cut-out valve failures. But considering there's some peripherals which (even if completely failed to red condition) don't affect anything, I'd like them fixed first. Supercharger and generator seem to be some of them. I've tried breaking and then fixing supercharger while flying at various altitudes but I don't see any improvement in my boost when I repair it. And a red condition generator, I get perfectly fine charge voltage and current without fixing one. I don't remember whether they broke with Accusim 1.1 update of whether they were always that static.
Accusim is not a real Spitfire but it's getting there. And development on Mark V means that systems on Mark I and II will probably improve as there's lots of shared modules.
4. And I have to now ask this further question; why is it idiotic to go full throttle at low altitudes?
Engine wear down faster with high power. Early production variants of Merlin were nondurable engines and wore out quickly.
A2A will soon(?) release P-40 Warhawk. That one has Allison V-1710 engine. It was capable of withstanding a lot more running hours, including operation at high boosts. It does have a supercharger inferior to Spitfire so it's a low-mid altitude fighter where as Spitfire is mid-high altitude.
I used to fly a Cessna 152 "back in the day" and I flew those quite well VFR and by "the seat of my pants". But all this
techno-speak escapes me - - The only attention I ever paid to the engine was whether or not to
apply Pitot or carb heat or not.
That explains a lot. Unlike a car engine, or a Cessna for that matter, Spitfire has a supercharger. Most cars don't. Cessna doesn't either.
When you apply full throttle on a freely aspirating Lycoming O-235, the manifold pressure is +0 psi (relative to standard air pressure) on sea level. When you do the same with Spitfire with BCO turned off, you get +12 psi. That's almost two times the pressure, all thanks to the supercharger Spitfire has but your Cessna was missing. And if almost twice the pressure is not enough, Spitfire has 27 liters of displacement, Cessna has less than 4. That's 7 times more displacement and a bit less than twice the pressure, meaning more than ten times the oxygen and fuel forced to the cylinders per engine revolution. O-235 rotates at 2800rpm, Merlin at 3000rpm, which are roughly same. Doing the math Spitfire uses about dozen times as much fuel and produces ten times as much horsepower and waste heat. Ok, assuming a lot about them having the same efficiency...
Now, lets check the horsepower ratings instead of guesstimating them from pressures, rpm and displacement: Lycoming O-235 produces around 115 horses depending on variant. Merlin XII produces 1,150 horsepower. Ten times more. It's obviously not quite the same engine as what's installed on Cessna. O-235 is an air-cooled engine. Being air cooled, it can a lot hotter than a watercooled engine. Basically it can be run as hot as the engine metal can handle without becoming brittle. Aluminum alloys start having issues at upward from 250 deg C of cylinder head temperature but staying at low 200's may be perfectly safe. Cessna is more than capable of keeping itself well below critical CHT with full throttle. Merlin has to keep itself below 120 deg C, and despite having ten times the amount of heat to dissipate, it has a tiny box under it's right wing. That's the cooler.
So, drive it 3000rpm +12psi and it will overheat at any altitude. It's just ridiculous amounts of heat produced. And even if you used it multiple times for short durations preventing it from having time to overheat, the high rpm takes it's toll on camshafts, bearings, etc. and +12 psi takes it's toll on piston rings, cylinder walls, creating scores, etc. This, even without overheat. While +12psi is obviously more straining than +0psi, 3000rpm on a 27-liter engine is definitely more straining than 2800rpm on a small engine that would fin in a car. Merlin doesn't. Bigger engine => bigger components => even with same rpm, speeds are much greater thus is the wear.
If you want to fly Spitfire "like a Cessna", you would never exceed +0 psi, even at low altitude. Running it like that, and using a more sensible rpm suitable for lower boost (like 2000...2400rpm), that big Merlin will last a long loooong time before overhauls (almost like a Cessna). And that's how people fly old warbirds these days. Replacement engines cost a fortune. There's no war, and there's no War Ministry funding the engine replacement or overhaul.
PS. Remember when I said Merlin doesn't fit into a car? I LIED.http://www.youtube.com/watch?feature=pl ... 5aZg#t=44s
PPS. Forgot to hit "Submit". Killratio managed to answer some of the questions before me.