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M20C O-360 CHT-related headaches, Lycoming guidance, other dogmas


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A few comments, increase timing and you will increase power, and heat generation, and increase efficiency and decrease detonation margin, there is no free lunch

Carb heat does help with fuel atomization and therefore mixture distribution, but it decreases air density and therefore power, so you went backwards, but I’ve seen carburetor equipped engines that can’t run LOP do so if some carb heat is applied, so it does seems to help mixture distribution.

The only reason to run thicker oil is if when the engine is hot the pressure is low. The thicker the oil, the lower the flow though the engine and you want high flow as most of an engine is oil cooled, the crankshaft, camshaft etc are all oil cooled etc. 

I can’t make a logical case to run straight weight oil, some think it’s thicker and will stay on the cam better etc after shutdown, but as we all shut down hot engines and straight weight oil is just  as thin as multi vis when hot, I don’t see it sticking on the cam better. Very often an oil companies premium oil has a better additive package than their less expensive oils, so run the expensive stuff. 

But people have run straight weight oils for a Century or more and if it didn’t work well we would have heard by now.

High cylinder head temps won’t normally decrease the cylinders cross hatching, stupid high temps could cause the oil to varnish on the cylinders and cause what’s called cylinder glazing, which does cause high blow by, high oil temp and oil consumption, excessive ground running will cause cylinder glazing, I’ve not see it from inflight myself, jugs don’t get that hot normally, heads do because the exhaust is through the head

I really think you should eliminate baffling as a possibility before going anywhere else, old baffling can look good on the ground but pressure can make it move and you lose cooling air. Replace the old baffling, then eliminate other possibilities one at a time.

‘It could of course be a little here and a little there.

What airspeed do you climb at? Try increasing airspeed until climb drops to 500 FPM and see if that doesn’t help a whole lot.

 

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6 hours ago, DXB said:I imagine a stop could be placed on your mixture cable, either at the panel side (ideally an easily removable one) or a change in how it attaches to the carb, that would give you a very slight decrease in fuel flow at the full rich setting?  It doesn't seem worth the time, expense, hassle of pulling your carb again. 

Also this is very interesting information for me - my 17-18gph leaves my #2 and particularly #4 running hotin , but on the O-360 it sounds like enriching to 18-19gph under many conditions causes other problems.  It makes me less inclined to pull my carb.  

What are the take off EGTs on #2 and #4?  The CHTs you’ve described are not optimal but they’re not necessarily something to get too worked up about either. Higher CHTs are not uncommon for the carbureted Mooneys, but they’re not eating cylinders on a regular basis. My WAG is that your two hot cylinders are simply not as rich as would be ideal. To get them there may create a less than optimal set up for the other two.

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16 hours ago, Shadrach said:

What are the take off EGTs on #2 and #4?  The CHTs you’ve described are not optimal but they’re not necessarily something to get too worked up about either. Higher CHTs are not uncommon for the carbureted Mooneys, but they’re not eating cylinders on a regular basis. My WAG is that your two hot cylinders are simply not as rich as would be ideal. To get them there may create a less than optimal set up for the other two.

I can say this for sure after a lot of time looking at my engine monitor data:  4 is the leanest, usually followed by 3.  1 and 2 are richer than either - but since I can't run those two to peak EGT without roughness, I don't know which is richest.   I can cool 4 in cruise by running it LOP, but then CHT on 2 skyrockets, so I try to find a compromise mixture where 2 rises to the low 400s and 4 falls to the low 400s - it's a PITA. The extra power from the Powerflow exhaust really seems to have exacerbated the problem for 4, less so for 2.  But because 2 and 4 have always the hottest regardless of mixture setting, climb, cruise, airspeed, or power setting, timing advance, or exhaust type,  I do think that airflow over 1 and 2 is much worse for some structural reason I just can't seem to nail down.  

Edited by DXB
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21 minutes ago, DXB said:

I can say this for sure after a lot of time looking at my engine monitor data:  4 is the leanest, usually followed by 3.  1 and 2 are richer than either - but since I can't run those two to peak EGT without roughness, I don't know which is richest.   I can cool 4 in cruise by running it LOP, but then CHT on 2 skyrockets, so I try to find a compromise mixture where 2 rises to the low 400s and 4 falls to the low 400s - it's a PITA. The extra power from the Powerflow exhaust really seems to have exacerbated the problem for 4, less so for 2.  But because 2 and 4 have always the hottest regardless of mixture setting, climb, cruise, airspeed, or power setting, timing advance, or exhaust type,  I do think that airflow over 1 and 2 is much worse for some structural reason I just can't seem to nail down.  

You may very well have an airflow problem. I asked for an EGT number for each because it will at least provide an indication that the cylinders in question are 250ROP or more at full throttle. If you are seeing EGTs >1300, those cylinders are borderline on FF given your field’s altitude.

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18 minutes ago, Shadrach said:

You may very well have an airflow problem. I asked for an EGT number for each because it will at least provide an indication that the cylinders in question are 250ROP or more at full throttle. If you are seeing EGTs >1300, those cylinders are borderline on FF given your field’s altitude.

All are <1300 at WOT on takeoff from sea level. Oddly #1 EGT is always the hottest though it runs rich relative to 2 and 4 and always  has the lowest CHTs by far.  It may be just probe placement variation.

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9 minutes ago, Hank said:

1 and 2 are the two cylinders at the front of the engine. How are they not getting airflow???

It's the air pressure gradient from above each cylinder to below that cools the fins - I've hypothesized that the left side of the doghouse creates less of a gradient to cool 2 and 4 for reasons I just can't grasp.  There may be a point of air flow stagnation on the left but a favorable gradient on the right.  It may take an engineer brain like yours to solve - I'm clearly not up to the task.

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1 hour ago, DXB said:

All are <1300 at WOT on takeoff from sea level. Oddly #1 EGT is always the hottest though it runs rich relative to 2 and 4 and always  has the lowest CHTs by far.  It may be just probe placement variation.

The lower compression a cylinder has, the higher it’s EGT will be, even if you match fuel flows on an injected engine perfectly, there will always be a mis match in EGT due to variation of compression. 

‘Some variation is completely normal 

Edited by A64Pilot
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1 hour ago, DXB said:

It's the air pressure gradient from above each cylinder to below that cools the fins - I've hypothesized that the left side of the doghouse creates less of a gradient to cool 2 and 4 for reasons I just can't grasp.  There may be a point of air flow stagnation on the left but a favorable gradient on the right.  It may take an engineer brain like yours to solve - I'm clearly not up to the task.

Time to get a GoPro and some yarn...

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3 hours ago, DXB said:

It's the air pressure gradient from above each cylinder to below that cools the fins - I've hypothesized that the left side of the doghouse creates less of a gradient to cool 2 and 4 for reasons I just can't grasp.  There may be a point of air flow stagnation on the left but a favorable gradient on the right.  It may take an engineer brain like yours to solve - I'm clearly not up to the task.

The only thing that makes sense to me (my #2 & 4 are also hottest) is the rotation of the propeller.  The downward movement of the prop on the # 1 and 3 cylinder side may give that side a greater pressure differential than the 2/4 cylinder side of the engine. Given the inefficiency of our doghouse cowling, that may explain some of the cooling differences. 

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7 hours ago, A64Pilot said:

The lower compression a cylinder has, the higher it’s EGT will be, even if you match fuel flows on an injected engine perfectly, there will always be a mis match in EGT due to variation of compression. 

‘Some variation is completely normal 

I suppose high EGT/low CHT can happen if you have a sticking or poorly seating valve reducing compression, but it's hard to for me to imagine how increases in leakage past the rings during the compression or power stroke increase EGT?  When my #1 got replaced a few years ago, I was struck by how much hotter its EGT was and how its CHT ran substantially cooler. I was pretty worried about a sticking exhaust valve or poor exhaust valve seal, although it was a brand new cylinder and the engine ran smooth.  It's run that way now for a few hundred hours though with no signs of burning when I borescope.  I kinda wish my other cylinders acted the same way!

4 hours ago, Andy95W said:

The only thing that makes sense to me (my #2 & 4 are also hottest) is the rotation of the propeller.  The downward movement of the prop on the # 1 and 3 cylinder side may give that side a greater pressure differential than the 2/4 cylinder side of the engine. Given the inefficiency of our doghouse cowling, that may explain some of the cooling differences. 

Yeah that is an interesting point that I haven't considered in a while - if accurate, that's a tough one to fix without total redesign of the inlet and baffle I'd think.   It's surprising then that most other C owners don't see the same pattern and mainly complain about #3 being hottest - and it seems the factory felt the same way because that's where they put the factory probe.

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6 hours ago, A64Pilot said:

The lower compression a cylinder has, the higher it’s EGT will be, even if you match fuel flows on an injected engine perfectly, there will always be a mis match in EGT due to variation of compression. 

‘Some variation is completely normal 

No one is suggesting that variation isn’t normal. Only suggesting that an O360 with an 8.5CR is likely <250ROP if WOT EGTs are above 1300.

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If you want to understand why #3 runs hotter than #4, look at the back wall of the doghouse behind each cylinder- the wall is flat against the rear cooling fins of #4 cylinder so that the airflow is forced to flow through the cooling fins as it travels from the top to the bottom of the cylinder. Look at the wall behind #3- it is not flat against it- it sits a distance away, and has an indentation in it the width of the cylinder that directs the airflow to the bottom of the cylinder- the airflow is not forced through the cooling fins, and instead may flow around the outside of the fins. Just a hypothesis. 
 

DABED18C-70EA-4E28-A2FF-78F634B20B12.jpeg

25AE6239-86BD-4BCA-B29C-A381288FB89D.jpeg

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Are you running the STC'd cowling mod with the reduced opening size?  I never knew Mooney made a model with fixed cowling flaps.  With your cruise speed numbers - I would suspect you could open those fixed cowling flaps. 

"I have the fixed cowl flaps on a '68.  I've thought about how much work it would be to convert them to adjustable.  Talking to folks with adjustable ones seems to indicate that they have to open them almost fully to get perceptible cooling benefit, so I wouldn't want them fixed in that position."  Sorry, I don't agree with this .

"Thus far, it seems too much fuel for me- the engine surges and runs rough after climbing out from a 2600 MSL airport at summer temps. Leaning it immediately after takeoff to achieve smooth running is a chore and a little worrisome regarding the possibility of leaning it too much and losing some cyl head cooling, as the cyl temps don’t immediately change on the instrument- I’ll know I’m running too lean only after having run lean for about half a minute or so… And If I lean it out for takeoff instead, well, that has been a miserable experience for passengers, airport neighbors, and my nerves….I might be sending it back to have the flow reduced if carb performance doesn’t improve during colder temps. A carb that is too rich has you trying to figure out exactly where the mixture should be set for best power + extra fuel for cooling, but not so much fuel that it reduces take off performance. It’s a bit of a guessing game. I miss my old carb!"

You thought about taking some additional flight training?  Might go along way to easing some of the anxiety regarding your engine management.

 

 

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Well, let me jump in. I have been dealing with the same problem for years! And I am happy to hear that I am not alone. My CHTs in cruise and climb are WAY to HIGH (lower 400 to 450). I have to admit I redid the doghouse, a hangar fairy without of my knowledge removed the doghouse and replaced it with more normal baffling system... Guess what, no change!. So, again one night the fairy came back and I got the doghouse again. 

My set up is similar to yours. I have a power flow exhaust, ARI cowling (no difference before and after in terms of CHTs) and I had an electronic ignition. I decided to remove the ignition and go back to the magnetos (CHTs were simply too high). I asked Savvyanalysis for advice. They were great! They helped me improve my technic, ensure that baffles were tight, adjust timing and that doghouse had NO leak.  But my plane resisted and insisted in having the CHTs in the 400s specially cylinder #3. #1, #2 and #4 are manageable, but #3... it is a real PITA.

Oil temperature are good and EGTs are also normal. 

One issue that could also play into my problem is OAT. It is always above 70, even at 5,000 feet.... 

I have not tried flying with carbheat... May be that helps...I will report back. 

Any other suggestion, please let me know.

CaptureCHT .PNG

m20c.jpg

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DXB's doghouse baffling is so airtight that you can put a compression tester on it and its 80/80.   Ive flown a few M20C's now, and they all run hot. some a little hotter, but all are 420 on climb on a few jugs and 380-410 in cruise.

Edited by jetdriven
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As a point of reference my 67C always runs hot on climb out here in Florida with #4 being the hottest typically 420-430 on initial climb and then settling just below 400 on cruise climb. During level cruise all CHT are in the 340-365 range. I have 1800 hours on the engine and she still has good compression with everything above 70. I do get concerned about the high temps during climb but to put it in perspective it’s been like that for 1800 hours and I see no reason I will not make TBO. So, it appears the hight temp on climb out maybe not doing as much harm as I fear.


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Not sure how relevant but I'll throw this out.  I have a Skyhawk with an STCd O-360 that ran hot as heck on climb out.  New baffling, precise timing....... Tried most all of what's been discussed.   On a whim I called Marvell Schebler to discuss the economizer circuit and how to trouble shoot it.  After listening for all of about 15 seconds he tells me he supects I have the wrong carb.   If it's been mentioned in this thread my apologies.  I skimmed before typing this and didn't see it.  My AC has the MA-4-5 carb.  They make this same model number in three versions-Lean, Middle, and Rich.  All have MA-4-5 on the data plate.  My data plate had no other nomenclature indicating which version.  He utilized the SN# to determine which version I had.  It turned out to be the Leanest.  Someone in the past had replaced the carb and ordered a MA-4-5 without any consideration to the specific version.   Another clue to the improper model was the idle setting.  When adjusting to get the desired 25-50 rpm increase on shutdown we had to back the idle mixture screw well past the normal 1 1/2-2 turns. 

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On 8/16/2021 at 11:13 PM, PilotCoyote said:

If you want to understand why #3 runs hotter than #4, look at the back wall of the doghouse behind each cylinder- the wall is flat against the rear cooling fins of #4 cylinder so that the airflow is forced to flow through the cooling fins as it travels from the top to the bottom of the cylinder. Look at the wall behind #3- it is not flat against it- it sits a distance away, and has an indentation in it the width of the cylinder that directs the airflow to the bottom of the cylinder- the airflow is not forced through the cooling fins, and instead may flow around the outside of the fins. Just a hypothesis. 
 

DABED18C-70EA-4E28-A2FF-78F634B20B12.jpeg

25AE6239-86BD-4BCA-B29C-A381288FB89D.jpeg

Cylinders are symmetrical. #1 and #3 are not mirror image but rather reversed in orientation. I think you’ll find the the fins on the back side of #3 do not provide a pathway to the bottom of the cylinder because of the position of the barrel. If the space you mentioned was not there, there would be no pathway for air to circulate around the back of cylinder 3. Put a light in he lower cowl and see if it shines through the backside of the fins on #3.

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33 minutes ago, Shadrach said:

Cylinders are symmetrical. #1 and #3 are not mirror image but rather reversed in orientation. I think you’ll find the the fins on the back side of #3 do not provide a pathway to the bottom of the cylinder because of the position of the barrel. If the space you mentioned was not there, there would be no pathway for air to circulate around the back of cylinder 3. Put a light in he lower cowl and see if it shines through the backside of the fins on #3.

That is absolutely correct. However, the RV guys have discovered that a large indentation in the rear wall behind #3 is not optimal, and creating a much smaller “bump” in the wall is optimal for airflow around that portion of the cylinder head where there are no fins. They have seen fairly significant changes in cooling from varying that gap by the thickness of a nickel, I believe…. See below:

RV builder’s posts regarding #3 cylinder and baffle building

 

Also, their lower baffling is curved and hugs the cylinder and head, unlike many of our setups.

 

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6 hours ago, PilotCoyote said:

That is absolutely correct. However, the RV guys have discovered that a large indentation in the rear wall behind #3 is not optimal, and creating a much smaller “bump” in the wall is optimal for airflow around that portion of the cylinder head where there are no fins. They have seen fairly significant changes in cooling from varying that gap by the thickness of a nickel, I believe…. See below:

RV builder’s posts regarding #3 cylinder and baffle building

 

Also, their lower baffling is curved and hugs the cylinder and head, unlike many of our setups.

 

 Mooney must have discovered it at some point in the 60s because the non doghouse planes have a small pocket in the baffle behind #3 to allow air around.

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It's the air pressure gradient from above each cylinder to below that cools the fins - I've hypothesized that the left side of the doghouse creates less of a gradient to cool 2 and 4 for reasons I just can't grasp.  There may be a point of air flow stagnation on the left but a favorable gradient on the right.  It may take an engineer brain like yours to solve - I'm clearly not up to the task.
Where's the oil cooler?

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