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High CHTs on engine break in

carrollcw

carrollcw

Well Known Member
So, I hve high chts on cylinders 1 and 3 during my break in making it difficult to really run the engine hard. Someone mentioned I may want to trim down the vertical piece of aluminum in front of cylinder 1 (part of the baffle kit). My understanding is it helps hold in the cooling air, but on the other hand it makes sense that it might allow more air flow across that side of the engine if trimmed down. Thoughts?
 
Yes - those need to be trimmed to get the temperatures closer to one another. The tricky part is that the balance is very dependent on attitude/speed, so getting it perfect for cruise will make it off-point for climb. The early baffle kits didn't have the dams at all - and they actually worked pretty well.
 
CHT 3 is 30 deg hotter than 2 and 4, and 10 deg more than 1, so I think I'll trim it just a little. Sound like a good first step? Having trouble staying below 425.
 
carrollcw said:
CHT 3 is 30 deg hotter than 2 and 4, and 10 deg more than 1, so I think I'll trim it just a little. Sound like a good first step? Having trouble staying below 425.
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What altitude/OAT/fuel flow?

FWIW, I removed mine entirely (they're removable via screws into nutplates rather than riveted on), and 2+ years later, have yet to put them back on...
 
RV7A Flyer said:
What altitude/OAT/fuel flow?

FWIW, I removed mine entirely (they're removable via screws into nutplates rather than riveted on), and 2+ years later, have yet to put them back on...
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All altitudes, oat, and fuel flows. I am trying to push the engine as hard as possible to break it in, but am having to run really rich and less than 23 man pres to keep CHT 3 below 425. The only time I don't have to worry about it is when I pull the power back. The engine seems to be starting to break in, I can run higher MP than before (first few flights had to run around 21 inches MP), but it is still running much hotter than the other side.
 
Do you have the washer between the rear baffle and #3?

I never put on the front dams on. I do have a small piece of AL tape on the face of #1 to balance temps.
 
carrollcw said:
CHT 3 is 30 deg hotter than 2 and 4, and 10 deg more than 1, so I think I'll trim it just a little. Sound like a good first step? Having trouble staying below 425.
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At least in the short term, 425F CHT's will not hurt your engine. However, running the engine gently those first few hours will. For break-in, I suggest running it hard and keeping the CHT's below Lycoming's 450 limit. That said, make sure you look for and eliminate leaks which waste cooling air.

Regarding the dams, the bigger they are, the cooler 3 and 4 will run and the warmer 1 & 2 will run. Sounds like you need to leave the dams alone or 3 will get even warmer.

As a data point, what type of CHT thermocouples do you have? The ring type tend to read higher than the bayonet type.
 
N941WR said:
That was going to be my recommendation.

Also, have you sealed all the gaps in the baffles? Even a small hole or gap will let a LOT of cooling air out.
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I'm going to double check tomorrow. Thought I had sealed everything pretty well, but who knows.
 
carrollcw said:
I'm going to double check tomorrow. Thought I had sealed everything pretty well, but who knows.
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One way to know is by doing a light test with the hangar dark.
Seal every light pathway that doesn't travel between cooling fins.

The most common leak locations are where the baffles interface to the engine case ( particularly at the back), and the inner cyl baffles.
 
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rvbuilder2002 said:
One way to know is by doing a light test with the hangar dark.
Seal every light pathway that doesn't travel between cooling fins.

The most common leak locations are where the baffles interface to the engine case ( particularly at the back), and the inner cyl baffles.
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Not just for Scott, but I am curious...

While I strive to seal all gaps not going through cooling fins, and believe I understand the theory, one question I've always had is how does sealing up a couple pea size leaks do much when there is a 2" diameter hole for the heater scat tube leaking tons of air into the bottom of the cowl? Would it be worthwhile to block off this opening in the baffles during the summer months? Again just curious.
 
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alpinelakespilot2000 said:
Not just for Scott, but I am curious...

While I strive to seal all gaps not going through cooling fins, and believe I understand the theory, one question I've always had is how does sealing up a couple pea size leaks do much when there is a 2" diameter hole for the heater scat tube leaking tons of air into the bottom of the cowl? Would it be worthwhile to block off this opening in the baffles during the summer months? Again just curious.
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Everything has an influence.
Some parts of the system are a compromise, but it is obvious that air flowing through an undesirable leak is 100% waste.

Engine break in is going to produce the highest need for cooling performance. Blocking the cabin heat flow path during that time would be one way to improve overall cooling. If you do that, the heat muff should be removed so that it and the pipe in that location don't get overheated (no cooling flow through the heat muff).
In summer I often fly high enough under an overcast to have a desire for some cabin heat so disabling would not be something I would do on my airplane, but if someones summer operation don't need it, it would be a way to gain a slight improvement in overall cooling (oil temp, CHT, etc.).
 
Kyle Boatright said:
At least in the short term, 425F CHT's will not hurt your engine. However, running the engine gently those first few hours will. For break-in, I suggest running it hard and keeping the CHT's below Lycoming's 450 limit. That said, make sure you look for and eliminate leaks which waste cooling air.

Regarding the dams, the bigger they are, the cooler 3 and 4 will run and the warmer 1 & 2 will run. Sounds like you need to leave the dams alone or 3 will get even warmer.

As a data point, what type of CHT thermocouples do you have? The ring type tend to read higher than the bayonet type.
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I agree with everything you said but Lycomings CHT limit is 500 degrees.
 
You could tape over most of the cabin heat scat hole, which would allow some cabin heat and heat muff air, but let most of the air that would otherwise be going through there go through the cylinder fins instead.
 
carrollcw said:
All altitudes, oat, and fuel flows. I am trying to push the engine as hard as possible to break it in, but am having to run really rich and less than 23 man pres to keep CHT 3 below 425.
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Is "really rich" based on an F/A meter, or a relationship to peak EGT? Can you offer some numbers?

EFii ignition has two available timing maps. You would have specified a compression ratio when ordering. What CR do you have?
 
DanH said:
Is "really rich" based on an F/A meter, or a relationship to peak EGT? Can you offer some numbers?

EFii ignition has two available timing maps. You would have specified a compression ratio when ordering. What CR do you have?
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Clark has not installed the F/A meter yet, so all numbers are coming from EGT and FF. EGT @ 2000 MSL / 17 GPH are 1050ish, highest CHT #3 at 415. if you bring the FF back to 13GPH and the EGT up to 1350 the CHT will get stable around 430.

Timing is 25deg WOT, CR: Clark will have to chime in here, but I believe it is standard 8.5:1

My concern is so much the CHT's we are seeing, more the large split between 1/3 and 2/4. I don't believe the break-in has happened yet. The temps we saw today are only marginally better than we had on First Flight. I think another few hours will make a world of difference.
 
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Nick said:
Clark has not installed the F/A meter yet, so all numbers are coming from EGT and FF. EGT @ 2000 MSL / 17 GPH are 1050ish, highest CHT #3 at 415.
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Got it. Yep, that's rich, sho'nuf. Have you done a GAMI spread?

Timing is 25deg WOT, CR: Clark will have to chime in here, but I believe it is standard 8.5:1
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Ok, good. That makes max advance 30 with reduced MP, which is sensible, and you can dial that back later if all else fails. For now, you may wish to experiment by pulling the hose off the MP sensor, which should lock in fixed 25 degree timing, no advance. Don't forget to plug the hose.

BTW, timing is a function of magnet installation in the flywheel. It's fair to assume no problem, but a quick running check with a timing light is SOP with similar systems. Stuff happens.

My concern is so much the CHT's we are seeing, more the large split between 1/3 and 2/4.
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Please quantify the split. We're not seen all four.

BTW, what sort of cowl does Clark have?
 
taping the leading edge between the top and bottom cowl pieces helped drop my chts about 7 degrees. ;)
 
carrollcw said:
Standard vans cowl. Chts on 1/3 are 30-40 deg hotter than 2/4.
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That's not horrible. Twist the little knob and do a GAMI spread. If all cylinders peak within 0.2 GPH, you're pretty much left with the usual cooling air leakage and flow management issues.

Let's assume you did a great job of sealing around the inlets. With two hot cylinders on the short inlet side, my guess would be that you've located the glue-in upper ramp too far aft (left, below). Remember, the inlet air must expand and slow to convert dynamic pressure to increased static pressure. A pinch does the opposite.

 
Update - I removed both dams which resulted in lower/balanced chts in all cylinders. Now I am working on balancing my EGTs!
 
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