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.

[kevin]

Yes Mike, tons of folks have had this problem. Search for the word "turbo" using the message board search function, and you will find lots of material on this.

You should be able to maintain power at 2500 RPM and 65% power. The book has lower RPM settings shown at 20K, but most people can't reach them no matter how tight their turbo system is. If you can't maintain power at 65%/2500, then you have an issue. (On my P337, I used 67%/2450 all the time with no problem, but could not use 2400).

It is probably NOT the turbo. 90% of the time, this problem is caused by exhaust leaks. You should look into that immediately, because a small exhaust leak can become a big one, and become a fire hazard in some situations.

Check out the other messages for other things that folks have tried. But if your engine "used to be" fine, and has deterioriated without any maintenance activity, exhaust leaks are probably your culprit.

You say you have had the hoses and fittings checked. Have they pressure checked the exhaust?

Others on board who are mechanics (I am not an A&P, although I'll be happy to fix your F-111 for you as I did in the Air Force ;-) will I hope comment on other possibilities such as the turbo controller/waste gate, etc.

One other thing, you should have your mechanic check your turbo for play in the bearings. If your turbo truly is getting "worn", you should fix it right now, before it comes apart and sprays metal through your engine.

Regardless, if it were my airplane I would solve this problem now, and not let it fester.

Kevin

[KyleTownsend]

I have this problem too.

In trouble-shooting the problem, one thing I learned was that my tachs were reading in the range of 100 (rear) to 175 (front) RPM too high. So, when it was telling me it was turning 2500 RPM, I was actually turning 2325 RPM. If I set the true RPM at 2450, I don't have any problem.

Kyle Townsend

[Kimber]

Mike,

I, too, have had this problem. In addition, at the 17,000 to flight level 200 altitudes, bootstrapping was a huge issue.

I started with a turbo overhaul...something big had evidently gone through the intake at some point, breaking several fins.

That greatly helped my lower altitude engine characteristics...allowing lower RPMs (read quieter) with same (about 65%) settings.

But...I still had the bootstrapping at the low flight levels.

My A&P just sent the exhaust system off for work, with numerous leaks found. Theoretically, that should fix the problem. I'll be able to let you know if that does prove to be the solution in reality next week, after I get my bird back and fly to SBS and then OSH.

Good luck.

Kimber

[Ron Moore]

I fly a Riley Rocket, zero time engines, etc. Above about 15k, it requires 2500 to 2550 to maintain 65% map. I can work it down to 2530, but it's still higher than the book says. The RPM is on separate, digital, counters and show lower than the tach so they're right on the money. I basically take what I get when they finally stabilize......
Ron

[Mark McConaughy]

If you look in your owners manual it should discuss this (it does in the manual for the 1974 Pressurized Skymaster, in a discussion of the Fuel System). Check Page 2-3.

[Guy Paris]

Mark, could you scan the said pages and e-mail them to me? thanks, guy (the) old72driver@msn.com