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  #1  
Old 04-29-2020, 11:51 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Default #1 and #2 CHTs high

Just rolled 10 hours on N8PH. The engine seems to be settled but I?m chasing hight CHTs. This is a stock IO-360-M1B with Hartzell prop, using these stock Van?s cowl with the snorkel.

#1 runs ~10 degrees hotter than #3 and #4, and #2 runs 20 degrees hotter than 3 and 4. I did a GAMI spread and all cylinders are between 0.0 and 0.1gph, so I do not think I have an air leak or an injector that needs to be changed.

I did the standard Van?s baffles on this project. Both #1 and #2 have air dams in front. I have now three times trimmed them down chasing this issue. #1 is now about 3/4? high and #2 is about 1/2? high. The last trim of 1/2? did not do much to improve the CHTs.

Is there a secret on this setup that I don?t know about? What are other builders seeing and doing?

Carl
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  #2  
Old 04-29-2020, 12:57 PM
JDA_BTR JDA_BTR is offline
 
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Location: Baton Rouge, LA
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Default

Carl, I think this is something that is related to having a snorkel. On my 14 with an IO390 my #1 is always 5-10 hotter than 3/4, and #2 is always 20-30 hotter than 1. #2 never over 365 on climb and usually lower in cruise.

So its all in spec. A big part of it is the way the baffling and snorkel obstruct the lower fins on the #2 cylinder. Even with a little washer to space out the baffle to let more air get down past the front lip of the cylinder the snorkel just keeps it from getting more air there.

My Arrow had a 1" tube there that purposely brought air to the lower fins of #2.

I think some people add some baffling to force more air forward so the back two cylinders get a little less air and the front a little more. I probably won't be doing anything myself. I've decided to live with it.

I've perfect the baffling at the inlet to not dump air in front of #2 inadvertently. I've decided more improvement at the inlet isn't the problem, because #3/4 are so good. But the air is pushed to the back of the plenum and down, and I think it skips 1/2 a little bit. Hence the idea of a wind fence if you will between the front and back cylinders.
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  #3  
Old 04-29-2020, 05:02 PM
BillL BillL is offline
 
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Location: Central IL
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Carl, after extensive testing and tweaking on my plenum equipped M1B, I have concluded the standard items like bypassing the fins, flash removal around the fins around the spark plug are certainly areas than can balance, but it is the gap under the heads/barrels that are the primary flow restriction for air. You might get under there and made some consistent measurements and see if the gaps match your temperature deltas.

Now, having said that 10-20F is really not very much, and if the other items mentioned are already addressed you are doing pretty well.

As an aside, the barrels effect on CHT is virtually nothing, wide airflow changes on the barrels could not be traced to any CHT change -so- back to be lower baffle gap.
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  #4  
Old 04-29-2020, 05:14 PM
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N941WR N941WR is offline
 
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I made my front air dams removable (Plate nuts rather than rivets) and to get my front two cylinder balanced, I removed them and trimmed them down a little. About an 1/8" at a time until I had it sorted out.

Keep in mind that by reducing the front deflectors, the rear cylinder temps will probably go up a little bit.
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  #5  
Old 04-29-2020, 05:29 PM
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Carl Froehlich Carl Froehlich is offline
 
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Yep - I did a bunch of trimming as well, and will probably do more. I note however that now with most of the dam gone the last trim did not do much.

Here is #1:


Here is #2:
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  #6  
Old 04-29-2020, 08:45 PM
JDA_BTR JDA_BTR is offline
 
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On the 14 these have screws, so you can take the whole thing off. Trimming it down more won't make the temp better, but it will let bugs get down the fins.
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  #7  
Old 04-29-2020, 08:46 PM
JDA_BTR JDA_BTR is offline
 
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I suppose you made sure not to put sealant on the front edge of the cylinder-to-baffle location? With all the sealing around the sides it is awful tempting to seal the front of the cylinder to the baffle; but you need air to go down there too.
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  #8  
Old 04-30-2020, 05:42 AM
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Larry DeCamp Larry DeCamp is offline
 
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Default One more possibility

I am always impressed by guys that are so anal about speeds, temps, flows and pressures. However, I am not sure how reliable some of the data is from my own experience. For example, how does one validate an EGT sensor at 1200F ? Does a CHT in boiling water stay accurate at 350F . Do they all read the same in boiling water when connected in the harness and instrument of the air plane ?
In Carls specific issue, I read a suggestion to swap CHT sensors front to back etc, to take sensor variance into consideration.
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  #9  
Old 04-30-2020, 08:33 AM
JDA_BTR JDA_BTR is offline
 
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There are so many planes in the threads here in exactly the same situation. I'd believe the indications and not fuss with the probes.
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  #10  
Old 04-30-2020, 09:04 AM
Bavafa Bavafa is offline
 
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Improving and perfecting is just as much fun and satisfying as building. I am in the camp of if I can improve it, why not.

My RV7A with IO360-M1B had the delta of 8 degrees between the hottest and coolest CHT. Sadly my 14-A is more like 20 and it is #1&2 to be the hottest just like the 7A. I yet to find a way of improving it.
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