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CHT Overheating issue

727TT

Member
We installed the Dynon HDX in our RV7A over 2 years ago. We also redid all of the engine baffling. The CHT on #4 always ran hot on takeoff. We replaced a number of components and traced it to a wire. The probe itself was ok.

Now, on take off 2,3 and 4 all show hot at 410 to 425 on #4.

Any thoughts?

It was suggested I install cowling Louvers to provide more air flow.

The carburetor was overhauled last year and I installed a new fuel pump. It came out of annual this past spring.

Thanks.......

Steven
Superior 0-360 with fix pitch
 
CHT

I just went through it myself. I can share what my issues were.

History: O320 and installed a Dynon system. New probes, wires. Conditioned in May 19. No issues found.
Issue: High CHT (405-425)

Solution:

1) Timed both mags. They were advanced a few degrees. Ok, maybe a lot of degrees. Lol. It was a certified new engine w 250 TTSN.
2) Dynamically balanced the prop. It was .034”. It’s now .006”

The timing dropped my CHT so much that I can now climb at Max gross at 1,900 ft/min through 5,000. Then 1,200 ft/min through 9,500 and stay below 400. Cruise is now 360s.

And my fuel flow went from 7.5 to 6.7 gph at 65% / 142kt.

Not sure if it helps, but if you haven’t checked timing in the last 50 hours, I’d start there. Good luck with your search.
 
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Check your magneto timing.

If you have P-mags, PM me your number and I'll walk you through it.

If you have even a small leak in your baffles, it can make a surprisingly large difference in CHT's.

Louvers are a last ditch effort and should be avoided.
 
I forgot a third change that helped a lot

3) Cut the bottom cowl back at a 45 degree

Can you post a pic of the cut you made? I have a 9 and yesterday in cruise 1:44 flight my #4 was at 411. Reduced power from 70% to 64% and #4 CHT only dropped to 401. Burning 9 gph at 6000?, OAT 70 degrees.
 
Can you post a pic of the cut you made? I have a 9 and yesterday in cruise 1:44 flight my #4 was at 411. Reduced power from 70% to 64% and #4 CHT only dropped to 401. Burning 9 gph at 6000?, OAT 70 degrees.

There is something else going on there. More mass flow isn't what you're looking for here.
 
Other temps and info required

What are your other CHTs, oil temp, are you running
rich, at or lean of peak, carb or injection, what injection, ignition? Engine model? Thx.
 
What are your other CHTs, oil temp, are you running
rich, at or lean of peak, carb or injection, what injection, ignition? Engine model? Thx.
I wrote them all down but left my pad in the airplane but other CHT?s were well below 400 rich of peak, carbureted and all EGT?s were below 1400. O-360 Barrett engine with less than 200 hours since new.
 
There is something else going on there. More mass flow isn't what you're looking for here.

+1

especially since this is a new development for the OP. Best to look for what changed and resolve it. Cutting more opening will just create more drag. MANY planes like yours run under 400 in cruise without cutting the cowl, so best to look for other issues causing the overheating condition.

Larry
 
Intake flange leak is one possible cause. Also intake tube coupler hoses and clamps can leak.

Pretty common problem.
 
What are your other CHTs, oil temp, are you running
rich, at or lean of peak, carb or injection, what injection, ignition? Engine model? Thx.
Here is the data from two flights.
Sam James plenum
Carburetor
ROP

Flight #1
CHT's at cruise
-375 - 395 - 394 - 411
EGT's at cruise - 1261 - 1327 - 1267 - 1328
6000 feet 72% power 9.0 gph 71 degrees F
14.5 gph at take off power (full throttle)

Flight #2
CHT's at cruise
365 - 384 - 380 - 401
EGT's at cruise - 1259 - 1318 - 1263 - 1330
7,000 feet 70% power 9 gph 67 degrees F


Any thoughts about CHT on #4 cylinder?
Each of these flights lasted right at 2 hours. The numbers above did not change much. On the second flight the #4 CHT did eventually get down to the 390's.
 
Baffle

I would try making a curved aluminum piece to wrap over the number four cylinder and reduce the air lost over the oil cooler. By slowing the air down with a curved blocking plate, you'll increase the pressure available for cylinder cooling and also not adversely affect oil temps because the less air is slowed down and spends more time in the cooler.
 
I added a small air dam in front of my #2 to balance the #2/4 cylinders. Not sure if it?ll help you much since your #2 is also a little warm.

https://photos.app.goo.gl/CVvbLtQsJKH1ruuN8

For my lower cowl mod, here are a few pics. This was very helpful And worth 10-20 degrees.

https://photos.app.goo.gl/jJnxB4WmZxrdQjoN6

https://photos.app.goo.gl/TAgmEaLKyiC5ZGv27
Hard to make out your first two,pics. The last pic showing the exhaust was clearly understandable. I have a Sam James plenum. Any idea how your suggestions would help/affect my CHT?s with my plenum?
Thanks for posting
 
Hard to make out your first two,pics. The last pic showing the exhaust was clearly understandable. I have a Sam James plenum. Any idea how your suggestions would help/affect my CHT?s with my plenum?
Thanks for posting

The first pic is looking into the air intake at cylinder #2 (Lyc). The second is the side profile of the cowl cut back 45 degrees to open it up more.

Sorry, no experience with a Sam. They do look sexy though.
 
We installed the Dynon HDX in our RV7A over 2 years ago. We also redid all of the engine baffling. The CHT on #4 always ran hot on takeoff. We replaced a number of components and traced it to a wire. The probe itself was ok.

Now, on take off 2,3 and 4 all show hot at 410 to 425 on #4.

Any thoughts?

It was suggested I install cowling Louvers to provide more air flow.

The carburetor was overhauled last year and I installed a new fuel pump. It came out of annual this past spring.

Thanks....... Steven Superior 0-360 with fix pitch

Seems we have two conversations going. I have some free advice and worth every penny (repeat of other comments in no particular order of importance).

425F is within Lyc limits but I am a big fan of sub 400F, and Lyc recommends operations sub 400F for long life. On take off or hot day climbs you can expect over 400F for short time periods. WHAT IS THE AMBIENT TEMP? Hot humid days higher CHT.

If you have mags check timing is under 25 degrees. If you set 23 degrees it will have little affect on overall performance and will reduce CHT. Try it. The factory certified 25 degrees is required only for certified planes. Try 23 degrees. I would do this last as you should be able to run 25 degrees, but people do report solving CHT issue with a few degrees less than 25 degrees BTDC. The angle valve IO360 has 20 Degree BTDC Spec, but when it first came out Lycoming ignition spec was 25 degrees. Lycoming backed off that with a Service Bulletin on the early engines. YES I KNOW we are talking parallel valves but just saying... If you have electronic ignition that can cause higher CHT, but from what I know about most EI's they are very conservative and top out at 25 for high power and only timing advance at low power settings. Some of the EI's have fixed timing maps, some allow you to set timing advance map (based on manifold pressure and RPM). This can be dangerous unless you know what you are doing.

Baffle leak - Yes be super OCD picky... Make sure there are no leaks. Red RTV is your friend... have a cheap bore scope or camera (you can buy them cheap for your phone)... See how the baffle material sits with cowl on. Sometimes one part gets pushed down or buckles and opens up a grand canyon gap in another..... I am assuming you have stock cowl and baffle, the deflector on 1 and 2 is a good idea. Also on one side of the cylinder the cooling fins are shallow. If your baffle is tight up against that area it chokes airflow off. You need to have a gap.

Oil Cooler If you have oil cooler on back of baffle it could be starving some air from that cylinder. Make sure that is all sealed. If you have the weight of Oil Cooler supported only on baffle it is a matter of time before it will crack (either oil cooler or baffle or both). OIL COOLER - The bigger the better... a big one will not fit on the baffle and must be remote mounted with a min of 4" SCAT or custom plenum. Lower OT lower CHT. What is your Oil Temps?

TEST and Calibrate your probes. First I assume you have the bayonet or well type CHT probes NOT spark plug gasket type. The latter reads about 40F higher. Lyc numbers are based on the well/bayonet type. There are plenty of articles on how to calibrate. Don't assume they're accurate.

MIXTURE - Yes it could be LEAN - What do the PLUGS look like? My old sold O-320 RV-4 ran very lean at first... because of the efficiency of the induction and free flowing exhaust; My plugs were white even at lower heat range. I had to ream the main jet ever so slightly.... IT DID WOUNDERS. It ran cooler, had more power and plugs were the perfect color. HOWEVER you should ALWAYS aggressively lean on the ground, after start, taxi, any time at lower power, or under 75% power (as long as OT and CHT are within limits). You may need to do a JET job on the Carb. I recall the biggest stock jet available off the shelf for my O320 RV-4 was not enough, so the drill bit and Boeing lube came out. I carefully stepped the diameter up a few thousands at a time. It took a few tiny steps. Edge of the jet orifice has a profile I tried to maintain. I forgot the drill size I ended up with, but I was very happy with that modification. Marvel Schebler makes many versions of the same CARB for O-360's with different jets. If you have a certified shop do your Carb they put the stock jet in it. From the part number you can find out if it's the LEANER version of that Carb. Talk to the A&P or Shop who "overhauled" it, ask them to find the jet PN in your carb and if there are richer jets for this Carb. If they look at you funny get another mechanic. Search "The Googles" or these forums, you'll likely find the info. Changing or modification of the main jet is caveman simple. I recall you can do it with Carb still mounted to the engine. Once I did it three times iterating my main jet size up a little at a time, it took me less than a hour including taking cowl off to "rejet" it, including drilling the jet. If you are not sure about your skills, have a knowledge careful mechanic do it. You don't want to screw up the carburetor or leave something undone. Carbs are very simple and why I have one on my current project... You may need safety wire, new lock tabs for bowl fasteners and main jet... may be a bowl gasket, but if newly rebuilt gasket is probably good to reuse. If you do modify the Carb you may want to make a mark on the data tag... make it "experimental". This is the grey line of certified appliances on experimental aircraft... If this ever went back into service on a certified plane it would have to be completely "conformed" to the certified Spec...

Too much information? It was not long ago, say 30 years GA factory planes and most home built planes had NO CHT or EGT, may be one CHT on the "hottest" cylinder... Now we are instrumented like it is a flight test. It is a good thing especially on experimental aircraft with so many differences....
 
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#4 cylinder is leaner than the others. As you have a carburetor I would assume an air leak on the #4 inlet.

Carl
No air leak. Removed all the spark plugs and no indication of running lean. Swapped the #4 CHT probe with the #2 probe. Went for a test flight today. #4 is still the hottest. So not a probe issue.
Someone had mentioned about cutting the lower cowl. Would like more info on that.
 
Someone had mentioned about cutting the lower cowl. Would like more info on that.

Before you did something that drastic, I would recommend you try and figure out why your hottest cyl is 20 - 25 degrees hotter than the next coolest, and it isn't one of the cyl that is typically the hottest.

Why do you think it is not lean, just from looking at the plugs?
It may not be severely lean, but it still could be enough so to be causing you a temp problem.
 
Before you did something that drastic, I would recommend you try and figure out why your hottest cyl is 20 - 25 degrees hotter than the next coolest, and it isn't one of the cyl that is typically the hottest.

Why do you think it is not lean, just from looking at the plugs?
It may not be severely lean, but it still could be enough so to be causing you a temp problem.
I was told that the #4 cylinder is always the hottest. It is right in front of the oil cooler.
How else could I determine if that cylinder is really Ning lean other than the spark plugs? My engine is carbureted.
 
I was told that the #4 cylinder is always the hottest. It is right in front of the oil cooler.
How else could I determine if that cylinder is really Ning lean other than the spark plugs? My engine is carbureted.

My #4 Cylinder was always the coolest, albeit by only few degrees as my spread between all 4 cylinder was within 10 degrees from hottest to the coolest yet...
 
Flight #1
CHT's at cruise
-375 - 395 - 394 - 411
EGT's at cruise - 1261 - 1327 - 1267 - 1328
6000 feet 72% power 9.0 gph 71 degrees F
14.5 gph at take off power (full throttle)

Flight #2
CHT's at cruise
365 - 384 - 380 - 401
EGT's at cruise - 1259 - 1318 - 1263 - 1330
7,000 feet 70% power 9 gph 67 degrees F

Any thoughts about CHT on #4 cylinder?

Yes. This isn't one hot cylinder, singular. Left side warmer than the right, and the rears 15 to 20 warmer than the fronts.

This is a short James cowl?
 
How else could I determine if that cylinder is really Ning lean other than the spark plugs? My engine is carbureted.

In cruise flight, under 75% power, incrementally lean mixture and plot all EGTs. Continue leaning until engine runs rough. Plot EGTs at each lean point. Look to see if #4 peaks before other cylinders.

http://content.savvyanalysis.com/static/pdf/SavvyAnalysisFlightTestProfiles.pdf

Savvy Analysis will plot the data from most engine monitors/EFIS.
 
Yes. This isn't one hot cylinder, singular. Left side warmer than the right, and the rears 15 to 20 warmer than the fronts.

This is a short James cowl?
It is a James cowl but I don?t know if it is a short one like you said. How can I tell?
 
What happened to OP 727TT? Ha ha.

Dan what are you going for, long or short cowl. Is there some significant cooling difference? At this point nothing he can do about changing the cowl (economically) at this point except open cowl outlet.

Looking at the CH temperatures 411F is not terrible. But again retard timing a degree or two (if magnetos) and open main jet mixture (even if plug color is good) will cool the engine. All the other low hanging fruit should be tried.

Where are you 727tt?
 
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How do you increase the fuel flow?
Open up the jet in the carb. Again get the carb part number (dash number will very). Find out if you have a lean version of the carb. You can buy richer main jets for the MS 4-5 carb. It does not matter what the plugs look like. If you are running the lean version of the O360 carb.... This is a well known issue.

I don’t know how to answer this. I would need pictures to identify what I have. I have a Catto three blade prop if that helps.
You have a fixed prop. How long is the prop extension. Measure the distance where the prop extension bolts to the engine crankshaft to the where prop bolts to the extension. Not sure why Dan wants this but I suspect the longer cowl has slightly better cooling. However you can as was originally suggested open up more cowl exit air area.... that will cool your engine as well.

I asked before what is your OIL TEMP? If you are running high oil temp as well check to see how many rows your cooler is. You may have insufficient cooler size. Oil keeps the CHT down.

Did you check your Mag timing? 25 degree is the spec... I would make sure it is at 25 deg or less. Again seriously 24-23 degree will help CHT.
 
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I believe it does. Not at the airplane right now To confirm but that is what I believe it looks like.

Measure the prop extension (Saber).between the flywheel (really the ring gear assembly) and back of spinner plate sandwitched between extension and prop.

Long = 2.25"
Short = 1.75"

Why Dan needs this not sure. Good luck and when you get serious about re jet of carb Jet me know. Also check timing (or not up to you).
 
Open up the jet in the carb. Again get the carb part number (dash number will very). Find out if you have a lean version of the carb. You can buy richer main jets for the MS 4-5 carb. It does not matter what the plugs look like. If you are running the lean version of the O360 carb.... This is a well known issue.

You have a fixed prop. How long is the prop extension. Measure the distance where the prop extension bolts to the engine crankshaft to the where prop bolts to the extension. Not sure why Dan wants this but I suspect the longer cowl has slightly better cooling. However you can as was originally suggested open up more cowl exit air area.... that will cool your engine as well.

I asked before what is your OIL TEMP? If you are running high oil temp as well check to see how many rows your cooler is. You may have insufficient cooler size. Oil keeps the CHT down.

Did you check your Mag timing? 25 degree is the spec... I would make sure it is at 25 deg or less. Again seriously 24-23 degree will help CHT.

I will check the carb part number
My oil is definitely not running hot.
Not looking to change the plenum/cowl so the distance is irrelevant to my attempts to cool my CHT down.
We did check the mag timing and it is right on spec.
Looks like the carb is the next area to look at.
Thanks for your help
 
Open up the jet in the carb. Again get the carb part number (dash number will very). Find out if you have a lean version of the carb. You can buy richer main jets for the MS 4-5 carb. It does not matter what the plugs look like. If you are running the lean version of the O360 carb.... This is a well known issue.

You have a fixed prop. How long is the prop extension. Measure the distance where the prop extension bolts to the engine crankshaft to the where prop bolts to the extension. Not sure why Dan wants this but I suspect the longer cowl has slightly better cooling. However you can as was originally suggested open up more cowl exit air area.... that will cool your engine as well.

I asked before what is your OIL TEMP? If you are running high oil temp as well check to see how many rows your cooler is. You may have insufficient cooler size. Oil keeps the CHT down.

Did you check your Mag timing? 25 degree is the spec... I would make sure it is at 25 deg or less. Again seriously 24-23 degree will help CHT.

The carb had the enrichment kit done to it so it is not the lean version. When discussing the EGT?s it seems fuel is not the cause. So I guess I am left with air. Cowl flap or trim a little off of the bottom cowl? I am told to do this trimming where the exhaust exits the cowl. How much to trim off?
 
The carb had the enrichment kit done to it so it is not the lean version. When discussing the EGT?s it seems fuel is not the cause. So I guess I am left with air. Cowl flap or trim a little off of the bottom cowl? I am told to do this trimming where the exhaust exits the cowl. How much to trim off?

Increasing fuel flow on takeoff will help with CHTs; its effect on EGTs is secondary. Re-jetting the carb to get FF to 18GPH-ish seems to be a standard tool to reduce CHT.
 
Cowl Louvers

I checked timing, new baffling and RTV. Ill check the oil cooler again.

Installing newly primed and panted cowl louvers from Vans.

Last flight before install number 4 on climb out was 407 with OAT of about 80 degrees here in Texas.

We'll see if they make a difference. Thanks.....good informative thread!
 
....Last flight before install number 4 on climb out was 407 with OAT of about 80 degrees here in Texas.

We'll see if they make a difference. Thanks.....good informative thread!

That is well within Lycoming's limits for continuous operation, not to mention climb.

What is your CHT in cruise?
 
Last flight before install number 4 on climb out was 407 with OAT of about 80 degrees here in Texas.

We'll see if they make a difference. Thanks.....good informative thread!

That is well within Lycoming's limits for continuous operation, not to mention climb.

What is your CHT in cruise?
Tango Tango you have been at for 9 months. Good for you, glad you are back. There was some thread drift. As Whisky Romeo said, this CT is the normal range. I posted before but you may want to back off timing advance; it is OK to set Mags to something less than 25 degrees BTC. If you have a carb make sure the main jet is large enough. Let is know your cruise 75% power and give full rich EGT and CHT and then lean it slowly. Note how much difference EGT rises from full rich to peak, looking at first cylinder to peak. Did you check your plugs. Are they white or toasty color.
 
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