Low CHT

I notice that my #3 and #4 CHTs are consistently low with #3 being consistently lowest. It is not by much about 20F low but enough to be noticeable to me. I realize that the middle cylinders typically run coolest because of the middle location.

Thus, I was wondering if anyone can share a technique for raising the CHT on a single cylinder?

I considered foil tape on top of cylinder. But applied where? Not keen on doing this without knowing it would make things worse.

Thanks for any advice,
Jae

I'm ignorant on this so I'll ask the silly question: what's the issue with them being lower? IOW what benefit do you gain by raising the CHT's for those cylinders?

Yeah, i agree, effects would probably be fairly marginal. I added a screenshot showing the low cylinder #3. Mainly, it would be just for consistency and fine tuning. Knowing that all cylinders are being operated to as close to the same conditions as possible.

I wish I had your temps. Mine tend to be in the 340's to 360's in cruise and I can get into the low 400's for 5 & 6 on climbouts. Still working on that.

Are you still in Phase 1? If so, i think what you describe is normal. Everyone thinks their temps are too high, but in reality, it's probably fine and normal. I think we eventually figure out what causes what kinds of temps.

I wish there was more collective wisdom on this in one place. The only place close was during a visit to the RV-10 camp at OSH where we could all compare notes in one place. I need to go back.

Jae,

My CHTs are similarly uneven (#4 is lower than the others) and I was wondering about the same thing a while ago. According to some posts I read on this forum it's much more effective to change the exit area below the cylinder than taping up the top. Haven't tried it yet though. If you do let us know!

Lenny

Accurate?

Have you checked the probes for accuracy?
I've seen them read low out of the box
Tim Andres

Hi Lenny, yes, in general 3 and 4 are my lowest as well. The way i justified #3 being slightly lower than 4 was i have more stuff in the even side (left): oil cooler opening and beneath the engine, i have the air duct running from the front baffle to the heat muff.

Of course, i haven't been able to correlate CHTs with the oil cooler door open or closed at all, i am pretty sure. I just assumed the scat duct was the main cause, but again i am not sure.

I have "looked" at baffling the exit area. As you know, it gets tight in there so i quickly lost motivation. Ill have to take another look again.

Another thing i have done, to even out temps from even to odd cylinders, was to play with the lower cooling louvres. Thus, i opened up the angle more on the left side vs the right side, to balance the unevenness between left and right cylinders because of the effects mentioned above. No idea how effective this exercise was but it seemed to even things out.

Check your intercylinder baffles underneath. If you nudge them towards 3&4 (close the gap), they'll go up a little and the outer cylinders will cool down a little.

I don't have louvers on the bottom at all (James cowl). I do have them on the sides though.
On the bottom of my #4 cylinder the gap in the baffling is about 1/4 inch wider than the others. Can you check yours when you get a chance?

Lenny

Yeah, i need to verify the CHTs. Maybe do a cylinder swap.

Aerhed, is there much wiggle room? I'll have to check that then.

I'll check the inter-cylinder baffles more carefully. Thanks!

CHT's

In my view you have absolutely nothing to worry about, those are great temperatures. I am willing to bet that you are running LOP with those values, they are very similar to mine at that altitude and power setting.

One problem with modern avionics is that they show us temperature values in one degree increments. For us to expect CHT's of 6 cylinders to be closer than twenty degrees is unrealistic and also unnecessary.

If you continue to cruise LOP and otherwise take good care of your engine with frequent use and oil changes your engine will go 3000 hours + before needing overhaul.

I owned a 1968 Comanche 260B for 12 years and flew it 1500 hours. This aircraft has exactly the same Lycoming IO-540 D4A5 engine as the RV-10. I remember when I installed an engine analyzer with probes in each cylinder for EGT and CHT. A very significant improvement from having this information on only one cylinder I might add.

In those days the temperature increments were 25 degrees and it was a rare time that we could get the vertical bar graphs to show a straight line across.

Based on your screen shot above your engine is running perfectly. Sit back and enjoy, concentrate on "aviating" and not be distracted on minor discrepancies between cylinder temperatures. Some time I think we have too much information in our modern cockpits.

CHT

Auburntsts said:

I wish I had your temps. Mine tend to be in the 340's to 360's in cruise and I can get into the low 400's for 5 & 6 on climbouts. Still working on that.

Click to expand...

Todd- same deal, you figure it out, let me know! I spoke with Lycoming at AOPA last weekend and they told me not to stress these temps, it is totally normal.

Jae- I have a plate another -10 gave me, its about 1.5 inches high, I have it on the fore of the #3 cylinder. My reasoning, and it does work, was to move the air over the #3 so the #5 would get it. I had to do to to get the temps down on #5, but you shouldnt need to mess with this. If anything, think about lowering the plate over #1 a bit, that will bring the temps down on that cylinder and redirect some of the air differently as well.
If I had your temps, I would work on something else.. maybe paint or something

They look pretty good to me, mind you the EMS display is a little overcrowded. If anything that needs fixing, not your CHT's.

Ok, thanks everyone. I agree, that it is a minor nitpick. I thought i would ask in case i was missing something simple.

David, yes, i know i have a lot of "info" items. I know i could have them on another screen, but I just hate unecessary button clicks. Thus, it's nice to have it all on that one screen. It is especially nice when i take these snapshots to analyze for later. That way, i have as much information on the single screenshot for analysis later.

Hi Ivan, yes, i do a lot of my flying LOP. The only time i dont is actually when i am up higher. I guess on this day, i was at 8,500 so i was probably closer to peak or more. I guess i was in a hurry on this day. It's good to hear your previous D4A5 treated you so well!

Reidvaitor, cool i'll ask you about that in more detail.

With those power settings and fuel flows you are most likely rich of peak.
1. Slowly lean and determine the order in which cylinders reach peak EGT. #3 may have a lower cht just because it is more rich than the others.
2. Once you have balanced the injectors so all EGTs peak together, if #3 cht is still lower than the others try to decrease #1 cht by getting a little more airflow there, maybe cutting down the air dam. Since this is a zero sum game this will raise #3.

You can fuss with the bottom middle cylinder baffles, extending them, a bit each time you have your cowling off, it should raise the temps a bit.
With temperatures like that you are likely losing airspeed with excess cooling drag. The first thing I would do would be to blank off those draggy lovers, check your speeds vs CHTs and see if you can pick up a couple of knots. My goal is 380F on all cylinders.
Great job on your baffles and inlets, now fine tune for speed!

Any suggestions here for how to best blank off the louvers? Something more elegant than tape on the bottom? I'm thinking of drilling out those rivets and using nutplates and screws. I could then put a blank plate in for the cooler months.

To experiment, tape, then if you like how that works, aluminum blanks and nutplates. If you still like it, during the heat of the summer, then glass the bottom smooth. Also think of extending the aft bottom cowling to extend 2 to 3" aft of the firewall. This will be a start in getting the exit air to go in the right direction.

Hi Bob, youre right, another good suggestion. Looking at past flights, my #1 generally does seem hottest. The front air dam has already been cut down, but it could come down further for sure. Comment about ROP below.

Hi Tom, another good suggestion. I have never tried closing the louvers off completely. It would be an interesting experiment to see what that does. It's on the list! Amazing list of airplanes in your signature.

Also, part of the reason for going slightly more ROP here is because my temps are low. I figure i could live with 350F or more in cruise. However, i would be over temp on takeoff in certain conditions. I would have to balance the takeoffs somehow.

This gets back to what i was saying before that initially, i think most people like myself are fixated on the high temp ranges and lower is better. However, I think your point is valid that a more middle-range or higher is more acceptable, especially for speed.

Thanks all!

Tom, I did extend the aft edge of the lower cowl 2-1/2" aft of the firewall, as per a suggestion you made here some time ago. It is impossible to isolate the effects of each little mod, but my -10 is running a little faster than most, and cooler, I suspect this mod helped. Thanks for the idea.

Aerhed, turns out you were right. I was able to nudge the inter-cylinder baffles closer towards the middle cyliner. They are basically helped in place by simple clamping force between the supports. Sure enough, i did probably get about 1/4" movement one each one. I think it did help. Unfortunately, being impatient, i also did a few other things while cowl was off.

On Tom's suggestion, the other thing i did was to close down the louvre openings. They were opened up quite a bit. I would say more than 45 deg. I closed them back down a bit. Now my CHTs are higher by about 10 to 20F overall. I think my speed is up, actually. I need to play with this more.

This is my baseline. Unfortunately, it doesnt show my TAS but its probably around 167 kts.


After making changes, this is one leg.


This is the return leg.


I think the temps are headed in the right direction. I seem to maintain the same TAS while burning 1.5 GPH less. Definitely LOP now. Temps are in a good range, now.

The really funny thing is that my WOT temps were also in line. You would expect temps to be higher, however, it doesn't appear to be here. This is a full 2 minutes WOT after takeoff. CHTs never got hotter than this, which is incredible. I dont think ive ever had this experience. I usually need to throttle back to keeps CHTs from hitting 400F, etc. Hmmmm. Definitely need to investigate further. For now, i am quite happy!