Appropriate TIT Temperature for M20K 231/252

[joefriel]

I fly a M20K 231 with Merlyn intercooler and wastegate making it fly like a 252.  We recently had an engine overhaul that caused the TIT to run considerably hotter than on our "old" engine.  The "new" engine is in fact the same core, just overhauled to new specifications.  The temperature probes are the same as well, so they may be inaccurate, but they should be measuring the relative temperature differences correctly.  We had a Mooney Service Center set the fuel flow system at the high and low end per the Continental Service Instruction.  This supposedly is very important because an out of whack setup could create many problems.

 

At full power on takeoff (36 inches due to the intercooler), we are seeing 22 gph.  That is about 2-4 gph lower than before the overhaul.  In cruise (28 inches, 2500 rpm, mixture full rich) we get 15 gph, about 1 gph more than on the old engine.  The really concerning thing though is that with the mixture full rich, we see TIT temperatures between 1520 and 1560.  We have no ability to decrease that temperature by running significantly rich of peak.  On the previous engine with what was probably a poor fuel flow setup, we could cool the TIT to less than 1480 degrees by enrichening the mixture.

 

So my question to the community is 1) should we be concerned with flying the airplane in cruise at 1560 TIT, and 2) should we be concerned that we don't have any "mixture authority" at normal cruise conditions in case CHTs start rising or something like that.

 

The STC that added the intercooler was approved without any change to the performance charts, so we don't know exactly what fuel flow would be expected with certain power settings.  If anyone has a copy of a 252 power chart, it would be most appreciated.  

 

Thank you.

[FBCK]

Joe I have the same setup you do, the Merlyn waste gate  and a Turboplus intercooler with about 200 hours since overhaul.  On takeoff, usally between 36-37 inches the flue flow is between 22-23 gph and my TIT would be approx 1475.  I will say a one gph difference in flue flow seems to make a big difference in the TIT.  When I first got the plane I followed the previous own engine management guidelines and ran the engine ROP, and I always had temp issue on the CHT and the TIT.  After a year of that I went to running the plane LOP and it was a whole different experience.  Lower TIT and CHT.  Try running the plane at 11 GPH at 30 Inches in crews and you will find the TIT at 1500 or so and the CHT will be lower as well when compared to 28 Inches and 2500, and your speed will be about the same. You do have to monitor the FF a little more LOP in my plane, it seem to drift .2-.3 GPH up or down, which again makes a big differnce in the TIT.

[jlunseth]

I got interrupted when I was doing this post last night, am going to edit it a little.

 

In the OP you said it was a Merlyn intercooler and I don't think they make one, I think you meant a Merlyn wastegate and a turboplus intercooler, which is my setup also. Assuming that, here's what I can tell you.

 

There is a specification for setting fuel flows, SID97-3E, and that should be followed.  The fuel flow at 100%HP and 2700 RPM should be 22-24.5 per the POH, but here's the catch.  100%HP at takeoff is 40" in the factory engine.  I do not know all the details about how the fuel flow system works, but according to my A&P the 22-24.5 number is predicated on the full 40" and 2700 RPM's.  That is the result that the mechanic gets if they follow SID97-3E in setting up your engine.  I would say that 22 is about right at 36", mine is usually somewhere between 22 and 23 and I do not have cooling issues with my engine at any altitude or setting.  There is a ringer though.  I was just going through the engine log and saw that when the intercooler was first installed the log entry said that the fuel flow was set up per the turboplus STC.  So I am suspicious that there is an STC the modifies the fuel flow settings to make them somewhat richer - at 100% HP anyway.  In other words, the STC may modify the SID97 settings so that you would get to 24.5 at 36", I do not know if it does.  I did talk to Turboplus quite awhile ago, and they said they could send me the STC but I never received it so have just let it go.  22 GPH seems to give me plenty of cooling at 36".

 

As for power settings, it is not accurate that the POH tables can be used without modification.  Turboplus provides a table that requires the setting from the factory POH table to be modified depending on "differential temperature."  DT in this case is the difference between Compressor Discharge Temp. (the temp of the air leaving the turbo) and Induction Air Temp (the temp of the air entering the induction system).  The difference is the cooling effect from the turboplus and it can be as much as 100 degrees C.  I believe I have an electronic copy of the table that I got from Turboplus, if you want one send me a PM with your email and I will try to locate it.  Generally speaking, the Turboplus table results in manifold pressures lower than the POH table.  That is because the air going into the induction system is cooler and therefore denser than if the turboplus were not in the system.

 

Last, the 15 you see at cruise power is not too far off if I am reading you correctly.  Standard cruise for me at low altitude would be about 29" and 2450, and I would see a fuel flow before leaning of about 16 something.  What puzzles me a little is your TIT.  I lean to 13.3 gph, which is around 100 ROP.  At that fuel setting I usually see a TIT in the high 1400 range, say 1490.  There is nothing wrong with the 1520-60 that you appear to be seeing, at least as an absolute number, it is well within the operating range of the engine.  But it is just higher than I would expect to see at a fuel flow that is that rich (15 gph).  I would suggest trying to lean it back and see if the TIT doesn't get better. 

 

My thought for what it is worth is that your TIT gauge may be off.  I would have that checked out.  If it is ok, then you should take the plane to a mechanic who can follow SID97, which requires some specific instruments to set the fuel flows.  Most Mooney MSC's will have it.   

[apriav]

A horse power is a horse power, you burn fuel to produce horse power. The inercooler is providing a cooler intake air charge therefore you should see the same fuel flow at a lower manifold pressure to produce the same horse power.

The unmetered fuel fuel pressure is not setup correctly for an intercooler.

I have a 231 with a Merylin and Turboplus intercooler and the benifits are engine and turbo life with lower intake tempretures and less trubo speed. Nothing is free, we just get benifits from the addons.

[Z W]

My 252 engine is almost always at 1550-1600 TIT in cruise, whether LOP or ROP.  Never considered that a problem.  Redline is 1650 continuous, and you supposedly can go up to 1700 for a brief period while leaning, not to exceed 30 seconds.  

 

I do try to keep it under 1600 TIT in cruise.

 

I've never tried going full rich in contemplation of a "runaway engine" scenario, but I think that in that scenario, I'm pointed at the nearest airport, and probably not touching mixture or throttle until I've got the runway made.  Just me though.

[jlunseth]

I climb to altitude full power full rich 100% of the time, and that is how a turbo should be operated.  When I level off I leave the engine at full power full rich while I set the OAT and altitude in the TAS, close the cowl flaps, and let the plane accelerate to a little above what I think my cruise speed will be.  This means that it is running full rich in level flight for several minutes.  I haven't seen it run off on me although I am getting a little older maybe it has runoff and is up ahead somewhere scouting the best route for me and the remainder of the aircraft to follow.

 

Serioulsy, what "runaway engine" scenario are you referring to?  You can run your TSIO360 full power full rich and it is not going anywhere on you, although you might get better fuel mileage if you lean it back to something other than full rich once you are in cruise.  Full rich is a good place for a turbo engine in a climb, it keeps the engine the coolest.  There were some bootstrapping problems with the factory TSIO-360-GB and LB as I understand, but the controller on the MB is much better and won't bootstrap.

[Parker_Woodruff]

I would get the takeoff fuel flow higher. Also, changing to Tempest Fine Wire spark plugs slightly decreased my TIT.

1500+ is much too high for full rich/takeoff power.

I wouldn't want to see anything more than the low 1300s at sea level/full rich/36"

What are your CHTs running?