M20E low CHT's on #1

[pinerunner]

Hello all,

 

I've had a 1964 M20E for a year now and with UBG-16 (started out with USG-8).  It's got the original "guppy mouth" cowling). The CHT's for cylinder #1 have always read about 30 degrees lower than the others. In general I think the CHT's overall are a little lower than I'd like (targeting 340 degrees on a hot day in normal cruise), I have work to get them into the low 300's and if I drop to lower power its down into the mid-200's. Special attention was given to the baffling at annual but no difference.

 

Is this simply normal for these older Mooneys to have #1 lower and are they in general overcooled? My money's tight but I've considered getting that lower cowl closure mod.

 

While cooler is generally better I'm aware the lead scavenger Mike Busch has talked about not doing as good a job at very low temperature. I thought it might be the ring gauge instead of bayonet on #1 but in one of Busch's webinars a question about that was asked and the answer was that the ring type should read a little higher, not a lot lower.

 

I'm not very worried about this but I'd like to know more.

[Marauder]

My F model with the lower cowl enclosure will run in cruise in the 320 to 340 range and will dip to the high 200s on descent. On hotter days, I may see the 350 to 360 range and low 300s in descent. The lower cowl enclosure I believe is supposed to improve cooling -- which it did in my case as well as providing some speed increase. http://www.lasar.com/mod-details.asp?id=8 Are you running ROP or LOP and getting these numbers?

[captainglen]

  You are right about getting the CHT's right, notice the red line above and below the green arc on the factory CHT indicator. The piston/cylinder assemblies are designed to opperate in a narrow temprature range. Too low and the piston is too small, leakage and skirt chafing occurs. Too hot and friction increases and ring galling could occur.

  You were also right to check the baffeling although baffeling problems usually result in high CHT's. At suspect could be the accuracy of the indication system and a rapid check after shutdown with an laser infra-red thermometer could resolve this. Also tyhe rig to the cowl flaps could be suspect as well.

[pinerunner]

My F model with the lower cowl enclosure will run in cruise in the 320 to 340 range and will dip to the high 200s on descent. On hotter days, I may see the 350 to 360 range and low 300s in descent. The lower cowl enclosure I believe is supposed to improve cooling -- which it did in my case as well as providing some speed increase. http://www.lasar.com/mod-details.asp?id=8 Are you running ROP or LOP and getting these numbers?

I'm generaly running LOP with 8 gph 2500 rpm and 23 inches. My GAMI spread is about 0.9 gph which makes LOP possible but has me thinking about springing for GAMI injectors. This puts #1, my richest cylinder about 30 LOP on #1 and #3 and 80 LOP on #2 and #4. The resulting CHT's are 270,315,290,and 300 so the two front cylinders are both the lowest (right) and highest (left, pilot side). I've read about some funny airflow around that guppy-mouthed cowling with actual flow reversal and air gushing out.

[KSMooniac]

I suspect you have some instrumentation errors.  I'd start by trying to calibrate/check your gauges with boiling water if possible.  Your leanest jug should be the coolest while LOP, not the other way around.

 

The cowl closure is a great mod for all guppy mouth cowls too.

[Marauder]

I agree with Scott that something is a little amiss with the temps. I would find a way like he is suggesting to determine whether your probes are reading correctly.

[takair]

My #1 cylinder usually hangs just below the 300deg green arc. There is no lower red arc on mine. The others typically run no higher than 320-330. I run ROP. Have not found any combinations that get me much hotter except in a climb. I suspect it is the guppy mouth just dumping lots of air in there. The dog house on the old airplanes is also very good at keeping the cylinders cool. Oil temp is a little harder to manage. I don't think a couple of degrees below 300 is a big deal. I suspect the cowl closure might help balance things out. Have not added that yet, someday....

[mike_elliott]

As Scott suggested, check your probes for accuracy. If it is a JPI probe, there is a star washer that needs to be between the eyelet leads of the probe when the probe is attached to the wiring harness. I have seen a number of these installed incorrectly. While this might not be the root cause, get it right while your at it if need be. Assuming your probes check out and are accurate, why not switch injectors from your leanest cyl with your richest cyl and see what you now get on your gami spread.

 

Mike Elliott

N9427v@FA40

[pinerunner]

My #1 cylinder usually hangs just below the 300deg green arc. There is no lower red arc on mine. The others typically run no higher than 320-330. I run ROP. Have not found any combinations that get me much hotter except in a climb. I suspect it is the guppy mouth just dumping lots of air in there. The dog house on the old airplanes is also very good at keeping the cylinders cool. Oil temp is a little harder to manage. I don't think a couple of degrees below 300 is a big deal. I suspect the cowl closure might help balance things out. Have not added that yet, someday....

 

Thanks Rob,

 

Your numbers are very similar to mine. The folks at EI where I got my probes said they've seen this frequently on the front cylinders, in particular where the oil cooler shields the cylinder right behind it. On their spark plug ring probes the actual thermocouple is not in the ring but a attached a little ways away. It can be either cooled or heated depending on how close to the exhaust. Its comforting to see your numbers. I hope others chime in.

 

Dave

[pinerunner]

I suspect you have some instrumentation errors.  I'd start by trying to calibrate/check your gauges with boiling water if possible.  Your leanest jug should be the coolest while LOP, not the other way around.

 

The cowl closure is a great mod for all guppy mouth cowls too.

I'm away from the plane until the weekend. Will post more data then.

[pinerunner]

I'm away from the plane until the weekend. Will post more data then.

I did look at the temperatures before start-up that had been recorded. All EGT's and CHT's were within a degree of each other at 84-85 degrees so there's one data point to feel comfortable about. Talked to the folks at EI and they explained that the spark-plug ring probe has the actual thermocouple outside the ring that doubles as a washer for the spark plug. Either cooling air or hot air from the exhaust manifold can change the temperature the thermocouple see's. All calibration is done at the factory and can't be changed. Basicly the response is strictly a function of the two disimilar metals used and thermocouple gauges either work or fail completely. I had one fail and it would drop out entirely and then come back and I replaced it.  Since I bought the UBG-16 to upgrade my USB-8 I have a complete extra set of probes. I do have an extra ring probe so maybe I'll swap and report. I was behind schedule on a lot of things last weekend so the only thing I boiled was some hot dogs.

[pinerunner]

At the end of this pilot report on a newer M20J they quote the monitor numbers

 

www.mooneypilots.com/mapalog/M20J%20Evaluation/M20J_evaluation_report.html

 

and they look a lot like my numbers even on the newer plane. I'm beginning to think its normal with a ring probe on #1, instead of a probe. I laughed when I saw the author quoting 475 degrees as the max.

[Shadrach]

In temperate weather LOP CP cruise, #1 on my F runs between 290-305, #2 runs 310-325, #3 runs hottest @330-350 and #4 305-320.

I don't buy that mid to high 200s will cause any issues with rings or pistons... My only concern is that lead scavenging requires a certain amount of heat. I've yet to lead foul a plug, so I'm guessing its running plenty hot.

[mikesalman]

I suspect you have some instrumentation errors.  I'd start by trying to calibrate/check your gauges with boiling water if possible.  Your leanest jug should be the coolest while LOP, not the other way around.

 

The cowl closure is a great mod for all guppy mouth cowls too.

Just don't use water, oil is recommended. I just had to replace all 4 probes because the last mechanic or the mechanic before pinched all 4 ring probes in the spark plugs. #1 cylinder would disappear until run up on my 830 #3 ran 410-420 while the rest were 360-380 during cruise. Now I ordered the ones that go in the block and allow for my Tanis heater. We are doing the install along with the annual this week. Hopefully we will be finished by Thursday.

[pinerunner]

In temperate weather LOP CP cruise, #1 on my F runs between 290-305, #2 runs 310-325, #3 runs hottest @330-350 and #4 305-320.

I don't buy that mid to high 200s will cause any issues with rings or pistons... My only concern is that led scavenging requires a certain amount of heat. I've yet to lead foul a plug, so I'm guessing its running plenty hot.

Just like mine, pretty much. I'm feeling better and better about what I've been seeing all along.

 

Dave

[pinerunner]

Just don't use water, oil is recommended. I just had to replace all 4 probes because the last mechanic or the mechanic before pinched all 4 ring probes in the spark plugs. #1 cylinder would disappear until run up on my 830 #3 ran 410-420 while the rest were 360-380 during cruise. Now I ordered the ones that go in the block and allow for my Tanis heater. We are doing the install along with the annual this week. Hopefully we will be finished by Thursday.

I had one of my CHT probes fail. It kept dropping out more and more but would come back to same numbers. It was either dead or alive and giving good numbers. Thermocouples are like that. We used to make them in grad school for an antique instrument we were building an addition for using thermocouple wire. Just cut off the length you need, strip insulation, twist the two dissimilar metals together and connect up to the meter and bingo you've got a thermocouple gauge. You can find it online and buy some if you want to play with it as a hobby. The tech guy at EI confirmed all this and explained that in their ring probes the actual thermocouple is not in the ring but close by in the lead going away. They often get reports of them reading high because they get heated by the exhaust manifold. So if yours is dropping out you probably do need a new one if its not just a simple loose connection. How the ring probes are oriented and what they're near could matter, otherwise temps over 400 degrees would scare me.

 

Oil vs water? I prefer bacon grease with onions in it and then the hot dogs with the natural casings on them that go snap when you bite them. And if they crack open when you bite them. And if they crack open lengthwise when you're cooking even better cause it makes a long canyon in the hot dog that you can fill with ketchup. 

[garytex]

Lead scavenging by the bromine compound in 100 LL is effective above 1200df EGT. Below that one needs tri crecle (spelling?) phosphate. TCP Or just lean for 1200 for low power ops

My understanding is that CHT is not the determining factor.

[danb35]

I understand that spark plug washer probes typically read higher than the bayonet-style probes.  I have a UBG-16 as well, with a "piggyback" style probe on #1--it's a washer-style probe that goes under the factory CHT probe.  It consistently reads about 100 deg F cooler than the other three cylinders.  I've considered just pulling the factory probe and installing an EI screw-in probe, but of course that wouldn't be legal...

[pinerunner]

I understand that spark plug washer probes typically read higher than the bayonet-style probes.  I have a UBG-16 as well, with a "piggyback" style probe on #1--it's a washer-style probe that goes under the factory CHT probe.  It consistently reads about 100 deg F cooler than the other three cylinders.  I've considered just pulling the factory probe and installing an EI screw-in probe, but of course that wouldn't be legal...

I talked to the folks at EI about it and they told me its because the actual thermocouple is not in the ring but in the lead going away from it. It will read different depending on what its near and the air going over it, typically lower if its in the cooler air on top and maybe hotter if its on a spark plug on bottom and getting heated extra by the exhaust manifold. I'd just get comfortable with the displaced values, save those old data dumps to help detect changes in the future, and try to orient the same way if you should work around it.  It is a pain that we can't just replace the old one. Hmm.  

[Schinderhannes]

Just don't use water, oil is recommended. I just had to replace all 4 probes because the last mechanic or the mechanic before pinched all 4 ring probes in the spark plugs. #1 cylinder would disappear until run up on my 830 #3 ran 410-420 while the rest were 360-380 during cruise. Now I ordered the ones that go in the block and allow for my Tanis heater. We are doing the install along with the annual this week. Hopefully we will be finished by Thursday.

Mike your numbers look very similar to my readings using spark plug probes on all cylinders. Did you have a chance to fly the airplane after you installed the new probes. I am considering changing my probes as well, and would love to hear about your results.  I am wondering if the duct attachment behind #3 supplying cooling air to the fuel pump is one of the reasons why this cylinder is the hottest.

[danb35]

Just don't use water, oil is recommended.

The reason you'd use boiling water to check your probes is that it's a known temperature.  With hot oil, or anything else, you could confirm that your probes were all reading the same, but you'd need another thermometer of known accuracy to confirm whether they were accurate.

[Shadrach]

 

Oil vs water? I prefer bacon grease with onions in it and then the hot dogs with the natural casings on them that go snap when you bite them. And if they crack open when you bite them. And if they crack open lengthwise when you're cooking even better cause it makes a long canyon in the hot dog that you can fill with ketchup. 

We agree on our hot dog preference if not preparation and condiments!

 

Little known fact that I just discovered. Maine has the best hot-dogs in the country!  Almost every Hot-dog stand uses the natural casing dogs that "snap" when you bite them and almost all have a grilled onions available (well done if you ask).  To top it off, the use the same flat-sided, top loading buns that they use for Lobster rolls... so the buns get buttered and browned on the grill as well!  It's almost worth the 3hr Mooney flight to Rockland just to have one!!!

[pinerunner]

We agree on our hot dog preference if not preparation and condiments!

 

Little known fact that I just discovered. Maine has the best hot-dogs in the country!  Almost every Hot-dog stand uses the natural casing dogs that "snap" when you bite them and almost all have a grilled onions available (well done if you ask).  To top it off, the use the same flat-sided, top loading buns that they use for Lobster rolls... so the buns get buttered and browned on the grill as well!  It's almost worth the 3hr Mooney flight to Rockland just to have one!!!

Yes, when I moved to Illinois for 10 years I discovered I had been spoiled. And nobody understood why I was such a  hot dog snob.