Well, I am pleased to report that my turbo, stock housing with TO4 wheel and 60-1 compressor with a fixed open waste gate has lasted over 230 hrs without a single problem. The solution to good enough? Make it better. I just finished my annual during which I upgraded my turbo to a similar TO4 big shaft wheel, but now it has a TO4E 50-trim compressor and the wastegate flapper and actuator are back in place. Why the change? Either I am a glutton for punishment or I am starting to pin down this turbo issue. For those of you not familiar with my turbo history, here is a quick recap:
First turbo - the stock second generation with waste gate flapper installed, ridden hard and put away wet... it was run at full throttle at high altitudes, often at peak EGT. It lasted 70 hrs before the turbine wheel fell apart and broke off. The thought at the time was that the stock turbo was just too small for this heavy duty abuse and was overspeeding at altitude.
Second turbo - small shaft TO4 wheel with 60-1 compressor and removed waste gate flapper. It was also run mostly at peak EGT but the power produced was somewhat lower because of the bypassed exhaust. It lasted about 140 hrs until it suffered the same fate as the first turbo while I was in a long full power decent (read extra high RPM and EGT's). My thinking at this point is that the high EGT's are primarily the problem.
Third Turbo - The one I just removed after 230 hrs. Same configuration as the second but I kept the EGT's under 1600, mostly as lean as I could while keeping the engine running not too rough. It was also run at pretty low pressure ratios. It could only make about 38" MAP on take off and about 40" at full bore down lot. At 16k altitude it was down to about 22". My curiosity now is weather I can perhaps increase the boost and continue to have the turbo last, as long as I keep the EGT's under 1600. I know, glutton for punishment...
Other issues..
Lean of Peak operation. During the turbo change I discovered that my stock cast iron exhaust manifold had developed a number of serious cracks and some advanced erosion. Much of the erosion/oxidation was near the mating surface with the turbo and it had started a small leak. My concern is that all those hours of LOP operation allows too much oxygen into the exhaust to promote to oxidation and accelerates the problem. The question is, should I start running rich of peak to prevent rusting away the exhaust system components? ROP operations also allow me to keep even lower EGT's (for the turbo) and smoother engine running (for the wife). Turbos and exhaust components that I get from cars usually have the appearance of being run rich, rather than the dry orange color of the inside of my components. The only obvious downside is the obvious much increased fuel usage. I think that for the next couple hundred hours I have going to have to bite the bullet and fly ROP (while at higher power settings). Thoughts?
S.A.G. is really baffling. Spark plugs only seem to be lasting 20-30 hrs before SAG starts kicking in. Noted by a distinct and marked loss of power as I cross through about 1500' AGL after take off. The problem is usually intermittent and mildish at first becoming more consistent and remarkable as the hours accumulate on the spark plugs. The problem is always solved by changing out the spark plugs. I am switching to colder platinum plugs in order to try to get them to last longer. Perhaps running ROP will help them last long too?
Time for a new Prop? On a test flight yesterday, the engine was running great. No SAG. At 7000' and 33"MAP I was at 6500 RPM, about 1/3 throttle and doing 175KTAS. It is a 64lengthx77pitch 3-blade Cato, but that is more like an effective pitch of closer to 71". I seem to remember being able to get closer to 185KTAS at 6500 RPM in the past on the same prop but I did have a little taken off because of the low output of the previous turbo. After some more experience I may discover that it is time for another new prop. Might soon contact Paul Lipps, I liked his article in the last issue of Contact! mag. If that is my worst problem, I am going to be pretty happy.
Doing some formation flying with the guys next week that may make its way into a movie...
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