In my opinion, one of the things that we tend to not pay sufficient attention to is the extent of the demand we are putting on turbos.  Without getting into the specifics of boost ratios vs ambient pressure, suffice it to say that a turbo compressor has to work considerably harder at altitude to maintain the same pressure ratio in the manifold.

 

  The ambient air pressure is of course less at 8000 MSL therefore density less as well and the turbo simply has to turn faster to create the same amount of boost pressure out of the less dense air.  The only way the turbo can do that is to turn faster at altitude.  So you have that hot rotary exhaust pushing those small turbine blades, faster and faster.

 

  As we have discovered, the stock RX-7 turbo succumbs quickly to those stresses (simply not designed for them).  Even with the TO4 compressor mod, which clearly helps by reducing the turbine rpm needed to produce X boost, the stock turbine and bearing just do not appear up to the stress for long duration – although 130 hours is better than I thought the stock bearings would do.

 

 

A precaution I would take before flying back with the turbo that condition is to disconnect and plug the oil line to the turbo.  Should the bearing have taken a worst beating 0r continues to deteriorate, you would not want to be dumping that precious engine oil into the exhaust – although there should be plenty of smoke to tell you when that condition occurred – if there is anybody around in West Texas to let you know {:>).  So I would disconnect and plug the oil line and coolant line to the turbo – just to make certain no leaks occurred. 

 

Just my 0.02.  Glad, nothing more happened to the engine or you, Dave. 

 

 

Ed.

 

 

 

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.dmack.net/mazda/index.html

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of George Lendich
Sent: Saturday, June 21, 2008 6:24 PM

To: Rotary motors in aircraft

Subject: [FlyRotary] Re: Another Turbo Bites the dust

 

 

It's amazing how fast 130 hrs goes when you are having fun.:-) 

I took off the exhaust today and too a look.  One of the blades on the turbine wheel is gone, but the axle seems otherwise intact, though it may be off kilter and that is what is causing the resistance when I try to turn it.  So I am to sure if the problem was primarily the bearing, or overheat, or overspeed.  According to my calculations, turbine speed should have been well within the green area. (way down around 60k RPM - near the bottom of the chart.)

One difference with this fligh...  I usually run well lean of peak, but this particular leg I was running a little late so I decided to push it up a bit by running a best power setting.  I figured I was OK because I was high enough that I was only showing 21" MAP.  But the failure happened in descent when I kept best power as I came lower and the engine RPM came up to about 6300 (more exhaust available).

Combination of higher than normal EGT's and 'you number is up, turbo'???

Dave Leonard

Dave,

I follow your threads carefully as you have put so much into the Turbo installation, I don't know anyone who has done more but I'm sure there are, it's just we/I am not aware of them. From what your saying, I think your right on the money - heat is the killer (in this case).

 

I personally would like you to try something remote as per suggestions by others. I think Paul's idea is sound, but may be still to close to the heat for my liking.

George (down under)

 

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