P337 Engine MP at high altitude

[MikeZ]

Hello all, its been a while since I posted. I have an engine performance issue I'd like some feedback on:

My 79 P337H rear engine, mid time, has been persistently acting funny at higher altitudes as follows: As I climb through say 14000, I have to raise RPM to maintain adequate manifold pressure. Up to 20,000, the other day I had to run at close to 2600 RPM. That doesn't seem to be a problem as the book numbers indicate I was only at 72% power at 31". It would seem though that I should be able to maintain full MP at lower RPM. I've had the hoses and fittings for the turbo and intake systems tightened and checked repeatedly. I think it could be just a weakening turbo that might need overhaul in due course, or a leaky wastegate.

Another funny problem: Sometimes when its holding pressure at altitude, I'll flip on the synchro, and rear will start to drift downward in MP. Fuel flow goes down along with that, so it actually is reducing power. As soon as I flip sychro off, MP restores.

I don't like running the engine at even close to red line RPM in cruise, but it seems I have to. last flight I just let it drift down and stay down and flew OK, seemed around 210Kts. TAS at 20000 according to the computer, so its performing well.

I would like to know if others have this funny problem at altitude and what they have done about it.

Regards. I hope to make the next fly in. I keep saying that but work gets in the way.

Mike.