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This seems like a really great idea, I can't think of anything wrong with it. My thought was that the ideal thing to do would be to torque it down from the center out, then retorque when hot like you would with a cylinder head. But the shape, interference problems and high heat would make it basically impossible to retorque. This seems like a nice alternative. Are you using any particular torque value or just "firm but not too tight"?
On Fri, Mar 11, 2011 at 7:41 PM, Mark Steitle <msteitle@gmail.com> wrote:
The old 225 cu. in. slant six Dodges were famous for cracked exhaust manifolds. It was critical to torque them to factory specs in order to prevent the new replacement manifold from cracking. Regarding rotaries, I've seen more than one broken exhaust stud on the limited number of rotary engines I've worked on. I may be totally out in left field, but it is my guess that over-torquing the exhaust nuts may be at least partially responsible for the cracked exhaust manifolds. On my home spun exhaust for my 3-rotor, I tighten the exhaust studs for the center rotor, but only snug down the bolts for #1 and #3 in order to allow the exhaust manifold to grow and shrink as necessary. With 1600-1700* EGT's, it seems obvious that the manifold will grow more than the engine and if not allowed to move, it will crack. By torquing them only enough to prevent leakage, they are allowed to grow as needed, preventing cracks. So far, no broken studs or cracked exhaust manifold on my 3-rotor.
Mark S. The fasteners and turbo impeller are Inconel because steel will deform at those temperatures, but I think the manifold itself is still cast iron. Inconel would just be too expensive for the whole manifold, not to mention the manufacturing difficulty of trying to form it into the necessary shape. I assume the 20B is the same way, but Idon't really know.
On Fri, Mar 11, 2011 at 3:25 PM, Marc Wiese <cardmarc@charter.net> wrote:
And the 3rd gen tubo manifolds, bolts, studs, and nuts are Inconel as well! (Is the 20B like that?)
Marc Wiese From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of William Wilson
Sent: Friday, March 11, 2011 7:22 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Crankcase ventilation
Rotary engines crack exhaust manifolds fairly often. The 3rd gen RX-7 has it worse (IMO, because of the complex shape of the manifold), but it happens to the 2nd gen motors too on occasion. The key factor is whether it is cracked far enough to cause an exhaust leak or not. The car guys usually don't fix the crack until it starts to leak. A crack that doesn't cause a leak probably doesn't present a safety issue. I wouldn't at all be surprised if a worn exhaust gasket accelerates the cracking process as it may change the temperature distribution over the manifold, increasing the stress on it.
If you are using a custom manifold, there is a balance between the mass of the manifold, and the resistance to cracking. Cast iron is heavy. Mazda's manifolds are as heavy as the engine block, or at least it seems that way. Aftermarket manifolds, which are usually lighter weight, crack more often. You have to think Mazda knew what they were doing, since the RX-7 is a very light car with a very light engine and yet Mazda put these enormously heavy duty manifolds on it, which still crack sometimes. For whatever reason, quality headers on naturally aspirated cars never seem to have problems, it's only with turbos. More air going through the manifold maybe, or maybe having the turbo attached causes more stress. Maybe Racing Beat would know.
It turns out they are talking about this exact issue on one of the car forums right now, with pictures:
http://rotarycarclub.com/rotary_forum/showthread.php?t=13552
My gut says that oxidation won't significantly affect the cracking situation. It's cast iron, after all, it's always oxidizing. I believe the cracking process is entirely physical in nature. RX-7s always have oxygen in their exhaust but only the thermally and mechanically stressed ones crack.
On Fri, Mar 11, 2011 at 5:15 AM, David Leonard <wdleonard@gmail.com> wrote: My single EGT probe is just down stream of the turbo. I do my measurements in Celsius. Peak is about 915 or so. I usually climb out ROP and with the reduced RPMs, EGT are in the mid 700's to low 800's. MP on take off is anywhere between 30 and 45" depending on my mood and who I am trying to impress - though I usually try to keep it under 40". Even then it is for only the first 1000' or so, then I usually maintain about 28-30" through most of climb, and cruse at 22-30" depending on my mood. ROP cruse EGTs are 750 to 900 depending on power, RPM and how rich I make things. I usually lean it out just enough to keep EGT's around 870. (that is appx 100 deg ROP). If I am using a lot of boost or flying formation (when I cant keep an eye on the EGT) I will richen it up and keep the EGT's in the low 800's.
I have done a lot of LOP flying. In cruse it is difficult to get the temps much below about 890 without the engine running a little rough (occasional misfire). Climbing LOP is actually better because the lower RPM's really help keep those EGT's down. Descending is the opposite, the EGTs really climb with the higher RPM. That is how I lost turbo #2, a long high-power descent at essentially peak EGT (about 920 or so) (Hey, I was late and didn't know any better - you guys can thank me for doing the testing :-)
I have been on a kick lately to keep my EGT's down so I have been running mostly ROP. At the last annual I was getting some leaking around the stock exhaust manifold. Turns out that the gasket had worn out and that the manifold itself was getting some of the typical cracks that those cast iron manifolds are known to get. It looked to me as though at least part of the problem was oxidation. I replaced the manifold with one I had sitting around, but they have suddenly become harder to find. So my current plan was to eliminate the oxygen in the exhaust and keep the temps lower by running ROP.
The problem is, that ROP flying really uses up a lot more fuel. Yesterday, flying home from Mammoth. I was hitting a headwind and would have maybe had to stop for fuel if I didn't lean it out (ASOS on departure was reporting wiinds 25 gusting 38mph - tough to get the plane loaded and untied in those winds). Anyway, 9 gal/hr LOP will get me about the same power and EGT as 11 gal/hr ROP (different MAP setting). Add more fuel and get lower EGT's but power starts dropping a little while the cost goes up fast. So, I have decided to go back to running LOP during cruse on my longer trips, especially if I fly at lower MAP settings.
On Thu, Mar 10, 2011 at 11:05 AM, Kelly Troyer <keltro@att.net> wrote:
Glad to hear from you..........Reminded me to ask you the same questions about your
turboed 13B as I did John Slade..........
Would like to know your typical EGT's and MP (boost) at takeoff , climb and cruise
and if you lean "lean of peak" (or lean at all)........... Kelly Troyer
"DYKE DELTA JD2" (Eventually)
"13B ROTARY"_ Engine
"RWS"_RD1C/EC2/EM2
"MISTRAL"_Backplate/Oil Manifold "TURBONETICS"_TO4E50 Turbo
Interesting Idea, though you still probably want some other crankcase vent functional in case the injection ports don't suck enough. BTW, VERY little oil, if any, comes out the vent unless there is something wrong with one of the side seals. probably best to just plug the oil injection and vent the crank case.
--
David Leonard
Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net
I've not provided for clean way to vent the crankcase, yet. And I'm also not going to use the oil injection ports. As I understand it, the injection ports should always be at a negative pressure. Would it be a crazy idea to have the ports pull the dirty air out of the crankcase and not spread it across my airplane's belly?
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