M20C high CHTs during TO/Climb

[Phil123]

Hi Everybody!

My Co-Owner and I recently purchased a 1962 M20C. The airplane's been great so far, but one thing concerns us - high CHTs. The engine is a pretty new overhaul with one E-Mag and the PowerFlow exhaust. The carb is the high fuel flow version (10-4164-1). 

During cruise at altitude we are able to keep the warmest (usually #4 or #2) pretty much at 380°F (2400 - 2500 RPM, WOT, leaned out to about 12gal/hr). Spread between the CHTs is around 40°F. Higher power settings or lower fuel flow tends to increase the temps to levels we wouldn't be comfortable constantly. It would be nice if we could lean it further at higher power settings (low RPM LOP works great), but overall cruise isn't a concern.

We are more concerned about take-off and initial climb. All of the CHTs will come up to 400 - 420 °F pretty fast and we see 440 - 450 °F on the hottest (again #2 and #4) if we don't take immediate action. We take-off WOT, prop full forward, full rich of course and will see about 16.5 - 17.5 gal/hrs. We try to get gear and flaps up soon and pitch for Vy until we reach about 500AGL. At this point the CHTs are usually in the range as described above and we pitch more for a cruise climb at around 120MPH and reduce RPM to about 2500. From this point one the CHTs come down again and we are able to keep them below 400°F within a couple of minutes. 

We tried the improve our operation and tested different prop / throttle / pitch settings - so far the described above seems to work best. 

We discussed the problem with several A&Ps and so far the feedback was mainly that the CHTs we see are pretty high, but that the cooling system on the old Mooneys with the dog house is just not great. The baffling is apparently in pretty good shape and we improved the seals especially around the alternator and the starter with the help of a local A&P.

We are both new to Mooneys and are pretty worried about doing harm to the new engine. At the same time, we are not sure, if we really have a problem or if that is just how it is with the M20Cs.
 

What do you think? Anybody else with the same problem? What do folks see in their M20C?

Thank you
Philip

[Hank]

Our Cs tend to run a bit warm. The Powerflo exhaust will make this tendency worse.

What happens if you keep 2700 in the climb? 

Cruising at 8000 and up, I generally get 8.5-9 gph.

For LOP, there are several things you can do to help:

• Reduce throttle just enough to make the MP needle start to move. This can be a surprising amount of movement, to cock the throttle plate and create turbulence inside the carb for better fuel atomization and mixing.
• Add a little Carb Heat. When things are going well, this lets me reach beyond 25 LOP smoothly. 
• Go over your doghouse and make it seal as tightly as possible. This can involve sheet metal repair as well as high-temp RTV.
• Add a guppy mouth closure to that huge opening behind the prop, if it hasn't already been done. Studies have shown reversed airflow out the front of the cowl if the opening is not partially closed. 

Good luck with everything, and enjoy your new ride!

[Matthew P]

Check to see what your mag timing is, when I bought mine the mag timing was still set to 25 degrees even though Lycoming put out a SB to reduce the timing to 20 degrees, once the timing was changed that not only lowered my CHTs but also improved engine starting.

 

V/r

 

Matt

[hammdo]

Does yours have the guppy mouth closure? That helps. Also plugging every opening in the dog house is also important. My 61 B was a air leaker for sure.

My 66 now has a much better arrangement but with all the recent work (new cylinders, complete IRAN with new cam and DLC lifters) she ran hot and I had to lower the nose and climb out 25 square to keep it cooler…

Mine is still timed to 25° on my C for the o-360

-Don

[47U]

1 hour ago, Phil123 said:

The airplane's been great so far, but one thing concerns us - high CHTs.

Welcome, Phillip.

Between the PowerFlow and E mag, these CHTs might be what you get.  It’s hot in Texas.

How many hours on the overhaul?  If you’re still breaking in cylinders, there might be some relief yet to come.  Was the overhaul done with chrome cylinders?  Chrome might run somewhat hotter CHTs.  Is the oil temp within expected range?

Be sure the flexible baffles around the upper and lower front cowling are in good shape, seated in the groove.

Check to make sure the inter-cylinder baffles (below the cylinders) are installed correctly.  And check that the baffle tie rods are installed.  

There are some felt pieces on the back side of the doghouse that close up some gaps.  Make sure those are doing their job.

The ‘62 has adjustable cowl flaps (I think)… to see if it makes a difference, I wouldn’t be against adjusting them to be open an inch more than spec’d in the mx manual.  To see if it makes a difference.

Get the speed up as soon as possible after takeoff.  120 is a good target.  I understand the urge to gain some altitude on initial climb, but see if emphasis on acceleration makes a difference in lowering CHTs.

Enjoy the new ride… let us know how it goes… post some pics!

[C.J.]

@Phil123 

I had similar numbers to yours, however my C has std mags, Power Flow & Lasar inlet mod. I’d recommend WOT in climb even if you choose to reduce rpm temporarily for noise abatement. I’m cleaned up and pushing to 120 mph by 500’ - 600’ AGL as well.  It seems all Cs run hot. 

Do the flashlight test & ensure any doghouse gaps are sealed with rtv. That step brought my cht’s down quite a bit in climb & cruise. I had my 10-4164-1 reamed by 08% & got better climb cht’s. An O360 can’t be timed to 20* degrees, only the IO-360 is authorized, but your A&P could be within 1* degree, iow 24* and still be legal. Here’s a recent run-up at 65*F & 1500’ DA.

[Phil123]

23 hours ago, Hank said:

Our Cs tend to run a bit warm. The Powerflo exhaust will make this tendency worse.

What happens if you keep 2700 in the climb? 

Cruising at 8000 and up, I generally get 8.5-9 gph.

For LOP, there are several things you can do to help:

• Reduce throttle just enough to make the MP needle start to move. This can be a surprising amount of movement, to cock the throttle plate and create turbulence inside the carb for better fuel atomization and mixing.
• Add a little Carb Heat. When things are going well, this lets me reach beyond 25 LOP smoothly. 
• Go over your doghouse and make it seal as tightly as possible. This can involve sheet metal repair as well as high-temp RTV.
• Add a guppy mouth closure to that huge opening behind the prop, if it hasn't already been done. Studies have shown reversed airflow out the front of the cowl if the opening is not partially closed. 

Good luck with everything, and enjoy your new ride!

 

I am still trying with RPM in climb and I want to go back to the engine monitor data. Airspeed definitely helps, but I am not sure what effect RPM really has. I want to stay WOT, because of the extra fuel flow, but to do so, I have to bring the RPM back, otherwise the CHTs are hard to get back down.

What RPM are you at for those 8.5-9p gph? Full throttle?
 

[Phil123]

23 hours ago, Matthew P said:

Check to see what your mag timing is, when I bought mine the mag timing was still set to 25 degrees even though Lycoming put out a SB to reduce the timing to 20 degrees, once the timing was changed that not only lowered my CHTs but also improved engine starting.

 

V/r

 

Matt

Mag timing is definitely on the list for the next shop visit. I am not really sure about the Electroair E-Mag. Those definitely advance the timing, but I haven't found any info on when and how they do that.

[Hank]

6 minutes ago, Phil123 said:

What RPM are you at for those 8.5-9p gph? Full throttle?

I climb WOT / 2700 at Vy all the way to cruise altitudes, whether 3000 or 9500 msl.

I retyped the checklists from the old, brown book into Excel, and selected "Print in Booklet format," which fits perfectly onto my Sporty's kneeboard. It also left enough blank pages to retype most of the Performance Tables, which I refer to when setting power (including ATC level-offs when climbing and descending), but after a couple or three years I found myself using pretty much the same ones:

• 4000 and below:  23" / 2300
• 4500 - 7000:  22" / 2400
• 7500 and up:  WOT- / 2500

The "WOT-" is what i described above, pull throttle back until the MP needle wiggles. Up high, I lean to peak then richen just a bit, as peak gives best efficiency. But I don't have fuel flow, I measure at the fuel pump after landing and divide by flight time. The single CHT needle stays near the top of the green band. Like in this old photo (I don't have many that show the gages at the top of the panel):

 

[Phil123]

21 hours ago, 47U said:

Welcome, Phillip.

Between the PowerFlow and E mag, these CHTs might be what you get.  It’s hot in Texas.

How many hours on the overhaul?  If you’re still breaking in cylinders, there might be some relief yet to come.  Was the overhaul done with chrome cylinders?  Chrome might run somewhat hotter CHTs.  Is the oil temp within expected range?

Be sure the flexible baffles around the upper and lower front cowling are in good shape, seated in the groove.

Check to make sure the inter-cylinder baffles (below the cylinders) are installed correctly.  And check that the baffle tie rods are installed.  

There are some felt pieces on the back side of the doghouse that close up some gaps.  Make sure those are doing their job.

The ‘62 has adjustable cowl flaps (I think)… to see if it makes a difference, I wouldn’t be against adjusting them to be open an inch more than spec’d in the mx manual.  To see if it makes a difference.

Get the speed up as soon as possible after takeoff.  120 is a good target.  I understand the urge to gain some altitude on initial climb, but see if emphasis on acceleration makes a difference in lowering CHTs.

Enjoy the new ride… let us know how it goes… post some pics!

Overhaul was about 40 hours ago. Oil temp is within expected range as far as I can tell (we only have to analog gauge unfortunately) and the oil consumption is very low.

Baffles have been our primary improvement target, especially around the alternator / start. I have a couple of pictures attached. During the next oil change we want to tackle the rearward pieces on the back side.

[Phil123]

17 minutes ago, Hank said:

I climb WOT / 2700 at Vy all the way to cruise altitudes, whether 3000 or 9500 msl.

I retyped the checklists from the old, brown book into Excel, and selected "Print in Booklet format," which fits perfectly onto my Sporty's kneeboard. It also left enough blank pages to retype most of the Performance Tables, which I refer to when setting power (including ATC level-offs when climbing and descending), but after a couple or three years I found myself using pretty much the same ones:

• 4000 and below:  23" / 2300
• 4500 - 7000:  22" / 2400
• 7500 and up:  WOT- / 2500

The "WOT-" is what i described above, pull throttle back until the MP needle wiggles. Up high, I lean to peak then richen just a bit, as peak gives best efficiency. But I don't have fuel flow, I measure at the fuel pump after landing and divide by flight time. The single CHT needle stays near the top of the green band. Like in this old photo (I don't have many that show the gages at the top of the panel):

 

Thank you, great insights!

 

[Phil123]

22 hours ago, C.J. said:

@Phil123 

I had similar numbers to yours, however my C has std mags, Power Flow & Lasar inlet mod. I’d recommend WOT in climb even if you choose to reduce rpm temporarily for noise abatement. I’m cleaned up and pushing to 120 mph by 500’ - 600’ AGL as well.  It seems all Cs run hot. 

Do the flashlight test & ensure any doghouse gaps are sealed with rtv. That step brought my cht’s down quite a bit in climb & cruise. I had my 10-4164-1 reamed by 08% & got better climb cht’s. An O360 can’t be timed to 20* degrees, only the IO-360 is authorized, but your A&P could be within 1* degree, iow 24* and still be legal. Here’s a recent run-up at 65*F & 1500’ DA.

Is this just a run-up on the ground? Or are you getting those low CHTs during climb? If not, what are your temps during climb?

[Ragsf15e]

@Phil123 Id definitely use the flashlight test to check the baffles and make little fixes.  Id also definitely make sure the cowl flaps are fully opening.

I think you’ll find most (or at least many) people climb at full rpm.  It likely won’t increase/decrease your chts much but you’ll get to level off altitude sooner.

[Shadrach]

On 11/17/2025 at 9:50 PM, Matthew P said:

Check to see what your mag timing is, when I bought mine the mag timing was still set to 25 degrees even though Lycoming put out a SB to reduce the timing to 20 degrees, once the timing was changed that not only lowered my CHTs but also improved engine starting.

 

V/r

 

Matt

1) 20° is not an option for the 0360

2) if the lag angle is set properly, the timing at startup is the same whether it’s 25° or 20°

3) I’m very surprised to learn that you had CHT issues with an angle valve (200hp) Lycoming, as they have considerably more cooling area than the parallel valve (180hp) engines.  My timing is still set 25° and I can barely keep #1 and #4 above 300° this time of year.

[C.J.]

52 minutes ago, Phil123 said:

Is this just a run-up on the ground? Or are you getting those low CHTs during climb? If not, what are your temps during climb?

This was just a ground run-up. Climb CHTs AFTER applying rtv to gaps in doghouse reduced temps by 25*F. After carb was reamed 8%, climb reduced an additional 10*-15*F. End result climb CHTs 385-390*F versus about what you reported.

Your doghouse appears to have no rtv applied to the gaps. Note the black rtv on my doghouse.

I spoke to a Marvel Schebler tech re: 4164 fuel flow specs and was told there can be significant differences in T/O fuel flows between any two carburetors chosen right off the shelf. STD Atmosphere SL fuel flow should be 18 GPH.

[47U]

2 hours ago, Phil123 said:

Overhaul was about 40 hours ago.

Thank you for the pictures.  You have a really nice vintage example.  

A field overhaul that was not run on a test stand ‘could’ still be breaking in cylinders.  If chrome cylinders, perhaps even more so.

Your baffling  around the starter/alternator shows attention to detail, but the rest of the forward flexible baffling might benefit from replacement.  There are some areas showing wear, some questionable areas, especially below the prop where if looks a little funky.  If you’re fastidious about making sure the baffle is secure in the channel, it could be fine, but there is some evidence of deterioration both upper and lower.

You’re picking up manifold pressure off both #1 and #3 cylinders, both to the firewall?  I’m not an expert… the #3 is to an engine monitor?

With the PowerFlow, 16.5 to 17.5 might not be sufficient to keep the CHTs where you want them.   @Skates97 had a good post on carbs about 5 years ago…

I am assuming your PowerFlow exhaust carb heat system is not dumping air like the stock Mooney system.  I noted the cold air intake for carb heat was moved from behind #3 cylinder to in front of #2 cylinder, which you mentioned #2 and #4 were your hottest CHTs.

Externally, your doghouse looks really good.  There’s certainly many in worse shape (including mine).  Still, it’s a good idea to do some flashlight testing for gaps.

Interesting cowl flaps for a ‘62 C… I’m wondering what event brought those about.  An upgrade from normal wear… or an unfortunate event?

Please keep us posted on developments… we’re all learning.

[lamont337]

I had high CHTs for awhile mostly on 4 and 2 as well. Eventually found and closed up a gap behind my #3 cylinder (aft side at base) that brought the other side temps down over 20 degrees. Had to remove the oil dip stick tube to get at everything. Seemed odd at first that the opposite side would come down that much but makes sense if you think of it in terms of total pressure.

[cliffy]

Have you calibrated your CHT system to see if it is reporting accurately?

[Phil123]

On 11/19/2025 at 4:25 PM, lamont337 said:

I had high CHTs for awhile mostly on 4 and 2 as well. Eventually found and closed up a gap behind my #3 cylinder (aft side at base) that brought the other side temps down over 20 degrees. Had to remove the oil dip stick tube to get at everything. Seemed odd at first that the opposite side would come down that much but makes sense if you think of it in terms of total pressure.

That‘s a great tip, will look into that!

[Phil123]

19 hours ago, cliffy said:

Have you calibrated your CHT system to see if it is reporting accurately?

It reads OAT correctly from what I can tell, but haven‘t checked in boiling water, yet

[Shadrach]

9 hours ago, 47U said:

Thank you for the pictures.  You have a really nice vintage example.  

A field overhaul that was not run on a test stand ‘could’ still be breaking in cylinders.  If chrome cylinders, perhaps even more so.

Your baffling  around the starter/alternator shows attention to detail, but the rest of the forward flexible baffling might benefit from replacement.  There are some areas showing wear, some questionable areas, especially below the prop where if looks a little funky.  If you’re fastidious about making sure the baffle is secure in the channel, it could be fine, but there is some evidence of deterioration both upper and lower.

You’re picking up manifold pressure off both #1 and #3 cylinders, both to the firewall?  I’m not an expert… the #3 is to an engine monitor?

With the PowerFlow, 16.5 to 17.5 might not be sufficient to keep the CHTs where you want them.   @Skates97 had a good post on carbs about 5 years ago…

I am assuming your PowerFlow exhaust carb heat system is not dumping air like the stock Mooney system.  I noted the cold air intake for carb heat was moved from behind #3 cylinder to in front of #2 cylinder, which you mentioned #2 and #4 were your hottest CHTs.

Externally, your doghouse looks really good.  There’s certainly many in worse shape (including mine).  Still, it’s a good idea to do some flashlight testing for gaps.

Interesting cowl flaps for a ‘62 C… I’m wondering what event brought those about.  An upgrade from normal wear… or an unfortunate event?

Please keep us posted on developments… we’re all learning.

I agree with you about the fuel flow. the O-360s are often leaner than optimal at full power.

I am curious, do you know of an engine shop that is breaking engines in on a "test stand"?  I have always thought the test stand thing was a bit gimmicky.  There's a shop on the east coast that does this ($500 line item). I told them I thought it was cool and asked them what data parameters were provided from the test run, they sort of stammered and said none.

[EricJ]

24 minutes ago, Shadrach said:

I agree with you about the fuel flow. the O-360s are often leaner than optimal at full power.

I am curious, do you know of an engine shop that is breaking engines in on a "test stand"?  I have always thought the test stand thing was a bit gimmicky.  There's a shop on the east coast that does this ($500 line item). I told them I thought it was cool and asked them what data parameters were provided from the test run, they sort of stammered and said none.

The shop that my engine is currently at runs it for either sixty or ninety minutes (I don't remember which) on a test stand with a club.   They monitor temps, etc., and look for leaks, and I think check static RPM.   They may do more, I don't know.   I think from their perspective this helps them minimize warranty issues.    That's obviously not a full break-in, but it's a good start.

[47U]

1 hour ago, Shadrach said:

I am curious, do you know of an engine shop that is breaking engines in on a "test stand"? 

You got me, Ross.  I’m only vaguely aware that some of the bigger shops have a test stand.  I haven’t had the pleasure of experiencing a full overhaul yet, but I did top 3 cylinders in two years a couple annuals ago.  Reading the Lycoming (and others) break-in instructions, I took away some important points.  Minimizing ground run time, stipulation to have ‘calibrated’ engine gauges, initially run at high power to start, then alternating high and low power settings.  Considering that a good portion of the cylinder(s) break-in can occur in the first hour (or so), after a major overhaul, the controlled environment of the test cell seems to me like a good thing.  Even from a standpoint of maybe lowering the risk of infant mortality.  In my former life, the jet shop ran everything across the test cell before putting in on the jet.  

Lycon isn’t that far from me… this is on their website:

https://lycon.com/testing.php

[Shadrach]

14 hours ago, EricJ said:

The shop that my engine is currently at runs it for either sixty or ninety minutes (I don't remember which) on a test stand with a club.   They monitor temps, etc., and look for leaks, and I think check static RPM.   They may do more, I don't know.   I think from their perspective this helps them minimize warranty issues.    That's obviously not a full break-in, but it's a good start.

Agreed.  I don't know precisely how long it took for my last set of cylinders (nitride-hardened) to completely break in, but as I recall, temps and oil control were well within acceptable levels in 5hrs. I ran it very hard in late winter - OATs and WOTLOP at DAs that were likely around SL or lower.

[MooneyMitch]

On 11/17/2025 at 6:55 PM, hammdo said:

Does yours have the guppy mouth closure? That helps.

Paul Loewen told me the enclosure did nothing to help with cooling or performance.  It is strictly aesthetics.