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"Baffled" by High CHTs


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Having experienced high CHTs on T/O before and after the overhaul on my carburated O-360  plus everything I have done in between.  So I have stopped worrying about it because there is nothing else I can do.  Certainly if Lycoming says red line is at 500F to experience 410-20F for a few minutes is not going to melt the the cylinder heads.  In addition I have installed new silicon baffling and closed every gap possible and installed cowl flaps which I did not not have before the overhaul (I already had the smaller front cowl mod).  And yes I t/o with flaps (why stay on the runway longer that I have to), climb at vy or faster for better visibility and reduce rpm to 2500 below 1000'.  This so far has been the most effective way to keep the CHT low and there is nothing wrong with reducing rpm, at least according to Mike Bush the same guy who says that at above 400F cylinder heads begin to "deform".  As to reducing power as well I have done it in the past and don't know about that enrichment feature but I know that when I reduced RPM and MP, CHT temperatures did not increase.

Once I am at cruising altitude (normally between 6-8K) everything is OK, C#3 always the coolest (365-75F) followed by #1, #2 and #4 (390F always the hottest by 10-15F over#1).  Engine running smoothly, oil consumption 1Qt/8-9hrs.

Life is good.  

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I have been getting crazy with my CHT temperatures on Cylinder 3. It so happens that my LASAR ignition advances the timing of the ignition by 25 degrees. Consequence the CHT is 40 degrees higher than normal!!! There is nothing you can do about it... but try to manage the temperatures to avoid that they go above 440... I talked to Champion who inherited the patent and they said relax it is normal...

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Rate of Heat generation vs Heat dissipation...

Increase cooling...

- improve flow around cylinders (repair the dog house)

- increase airspeed

- change the design of the dog house (cowl enclosure)

- increase mixture (more ROP)

Lower the rate of heat generation

- lower the MP

- lower the rpm

- decrease timing DBTDC

These are the choices that come to mind from an engineering point of view...

Overall, the standard C has crummy cooling and aerodynamics related to the cowl.

Shared ideas,

-a-

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  • 3 weeks later...

Based on the variety of replies, it appears that above 400 Degree CHT in a vintage, carbuerated Mooney is perhaps more common than I realized.  Let's look at what I have done to decrease CHT and increase cooling in my 1968 G Model:

 

1.  Installed LASAR cowl mod

2.  Rebuit carbuerator

3.  Checked timing

4.  Checked doghouse

5.  Increased airspeed in climb

6.  Kept throttle full open with no change in results

 

And I still have over 400 Degrees on #3 cylinder in the summer. Of note, once I transition to the 120 MPH climb, my CHT drops precipitously, so it is obvious that the engine is cooling properly in the climb...

 

I will have my mechanic re-check the doghouse and sway out probes.  I don't think that this is going to produce the desire effect, though.  Has anybody ever re-located their oil cooler or added a second one to fix this problem? 

 

Clear Skies!

 

Sean

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Same situation here. I routinely see 400-405 during climb out keeping airspeed as high as possible (120 or so) while still being able to climb. If I don't temps will go to 450. Ambient temp doesn't seem to make a huge difference but it was about 75 here yesterday when I flew and saw these temps. I've gone through everything I can think of over the past two years with no positive result. I think relocating the oil cooler would make a difference since moving that would lower the air pressure in the lower cowl which should increase airflow across the cylinders. One thing I discovered is in an article written on the MAPA site called " Aircraft Evaluation Report" . This was written about a test flight of a well maintained M20C and I noticed in the engine parameter tables they posted the CHT's range from 400-440 in their climb tests. Based on all the research I've done I conclude these high temps likely result from a poor baffle design. I wonder how long the recommendation to keep CHT's below 380 has been around. When the C model was made, below 500 was acceptable hence the redline of 500. I can say my C has made it to TBO twice with no cylinder issues after regularly reaching CHT's over 400 (i'm friends with the previous owner who is an IA and have discussed this with him extensively). I'm curious if there's ANYONE on this site with a C model equipped with a 4 channel engine monitor that isn't seeing high CHT's during takeoff and climb. If so, it would be worthwhile to analyze their setup to see what is different.

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On mid 60's M20C's unless you have a very special modded version, you just have to accept the temps going over 400 df for a short period of time. The trick is to get your speed up to at least 120MPH ASAP. I generally at 1000' AGL, bring the power pack to 25 squared and limit my climb out to 500 FPM. Once reaching 120MPH, I slowly start bringing the power back up and monitor the CHTs and adjust accordingly.

Kind of a pain in the butt but it is what it is...

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I too have been battling high CHT's in my '67C . On about a 4 years old overhauled engine with ECI Cerminil Cylinders, LASAR Cowl Enclosure , No light leaks in engine dog house engine baffling, which I translate to no air leaks in the baffles ( Except where the spark plug leads go through rear baffling). Not certain what kind air leaks occur when the engine is running.

I have an Insight G3 engine monitor which says the furtherest forward cylinder #1 is the coldest cylinder #3 has dual thermo couple one for the OEM CHT and the Insight CHT probe which gives me erroneous low CHT readings. My Problem children cylinder #2 and cylinder #4. Cylinders 2 & 4 temperatures greater than 420 dof for about 5min or less usually after take-off. Last annual we boroscoped cylinder #2 and looked normal.

I am embarrassed to say the other day that when I was looking through my Mooney Service Maintenance Manual I came across cowl flaps adjustment page (3-2 & 3-3) which is a page I had seen thousand times and I was absolutely certain that I had rigged my cowl flaps correctly and that coupled with my miss-guided logic of “the larger the cowl flap exit air opening the better”. Reluctantly I reduced the cowl flaps exit air opening and did see lower CHT temperatures.

I then realize that I had been using the wrong hole on the cowl flap actuating assembly to meet 1.00 (+/-1 0.") spec.

While adjusting cowl flaps didn't solve all my CHT problems but seem to have helped.

I have also been pulling back the power to 25^2 right after take-off for a while now which also seems to help.

Unfortunately my SD card from G3 Engine monitor quit working today. Why someone doesn't make wifi file share or Blue tooth file transfer for the engine data I'll never know.

I suspect that these carbureted engine in the M20B,C/G's have been running high CHT's for a long time now. With the limited ability to legally change our baffles design without major paper work and headache. I am not too sure what else can be done.

James

'67C

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  • 5 months later...

Glad this issue came up because I have the same problem with my 0-360A4A on my C-172. I overhauled the engine and put a JPI-700 in the plane. I was alarmed about the temps. A much more experienced mechanic than I said it is possible that the carb is not atomizing the fuel properly and the fuel is not mixing well with the air. He mentioned possibly sending my carb out and having it checked. The carb works normally other than this possible problem. My baffling is in excellent shape. My #3 cylinder usually maxes at 450 or over, even with a lower rate of climb. I believe Lycomings have a max CHT of 450.

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Glad this issue came up because I have the same problem with my 0-360A4A on my C-172. I overhauled the engine and put a JPI-700 in the plane. I was alarmed about the temps. A much more experienced mechanic than I said it is possible that the carb is not atomizing the fuel properly and the fuel is not mixing well with the air. He mentioned possibly sending my carb out and having it checked. The carb works normally other than this possible problem. My baffling is in excellent shape.

 

 

I didn't think you could have an E with a carb engine. In fact, isn't that the difference between a C and an E?

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Same situation here. I routinely see 400-405 during climb out keeping airspeed as high as possible (120 or so) while still being able to climb. If I don't temps will go to 450. Ambient temp doesn't seem to make a huge difference but it was about 75 here yesterday when I flew and saw these temps. I've gone through everything I can think of over the past two years with no positive result. I think relocating the oil cooler would make a difference since moving that would lower the air pressure in the lower cowl which should increase airflow across the cylinders. One thing I discovered is in an article written on the MAPA site called " Aircraft Evaluation Report" . This was written about a test flight of a well maintained M20C and I noticed in the engine parameter tables they posted the CHT's range from 400-440 in their climb tests. Based on all the research I've done I conclude these high temps likely result from a poor baffle design. I wonder how long the recommendation to keep CHT's below 380 has been around. When the C model was made, below 500 was acceptable hence the redline of 500. I can say my C has made it to TBO twice with no cylinder issues after regularly reaching CHT's over 400 (i'm friends with the previous owner who is an IA and have discussed this with him extensively). I'm curious if there's ANYONE on this site with a C model equipped with a 4 channel engine monitor that isn't seeing high CHT's during takeoff and climb. If so, it would be worthwhile to analyze their setup to see what is different.

I have seen these O-360s running at or near 400F in cruise with long life and I, too, have learned to tolerate it. The parallel valve cylinders have less fin area and as such, perhaps are just going to run hotter. But they seem to tolerate it well.

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