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Re: Jim's comment about the pressurization being a problem
Maybe I am thinking about this wrong, but I think I disagree. Here's why:
A pressurization system is essentially a controlled exhaust leak. It takes a larger turbo to make enough MP to overcome this leak in the P models. The turbo is not the same one as on the non-P versions, it is physically larger.
If you make the exhaust leak too large, you will be unable to maintain MP at altitude. The balance is so delicate that above 16000', at 65% power, I used to have to use 2450 or 2500 RPM instead of 2400 to maintain MP in my '73 P. Also, an incorrectly adjusted wastegate (not 100% closed) will prevent maintaining MP at high altitude.
Many times I have spoken to owners about P337's that were not tight enough to allow operation at high altitude, even at 75% power.
There is a mistaken impression that if you pull the pressurization dump valves, you are just opening the cabin to the outside air, and folks who have done this notice that the engine still makes good MP. But when you put those valves in the dump position, they also *close* a valve that seals the cabin off from the exhaust system, thus plugging the exhaust leak entirely.
I think it is quite reasonable that an improperly sealed cabin, or inproperly operating pressurization valves, could make a P337 unable to maintain manifold pressure due to what would essentially be an uncontrolled, excessive exhaust leak.
Kevin
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