Bubblehead

Bubblehead

Well Known Member
I have standard Vans EGT/CHT gages with the 4-place switches so I can check the temperatures on each cylinder. The engine is an IO-360 parallel valve engine with about 500 hours on it. On my trip down to Sun-N-Fun and back to Illinois I happened to check all the cylinders and found the CHT on #4 to be 50 degrees colder and the EGT 100 degrees colder than the other cylinders. I'm not sure when this started, because I have gotten lazy about checking all cylinders. #3 was always slightly hotter than the rest so the switches stayed there. I am sure, however, that last fall all the cylinders were close to the same temps.

One thing that has changed since then is I have blocked off the cabin heat line that comes off the rear baffle because of problems with the heat muff. Perhaps blocking it off has caused more air to flow down over #4, but if that were the cause, wouldn't #2 also be cooler?

I'm going to pull the cowling this weekend and check compressions and the plugs but that feels like I'm just grasping at straws. The mag rpm drop checks are fine so I don't think this is a spark plug issue. It certainly could be a compression issue though.

Any thoughts? Has anyone seen this before? If so, what was the root cause?
 
When I block off the baffle holes

When I block off the baffle holes the adjacent rear cylinder temperatures go down but the EGT also going down is a concern.

Bob Axsom
 
I would not be concerned about those temp differences. What is important is when the cylinders peak. They should all peak togeather + or -. If it bothers you call Don at Airflow Performance. Down load and perform the EGT test before you call. He will recommend a restrictor changes so the cylinders peak togeather. That makes running LOP more efficent and makes the engine run cooler.

http://www.airflowperformance.com/
 
I think that if you change the temperature of the inlet air by changing the air temperature in the nacelle it will affect EGT. If you just change the airflow over the cylinders and not the airflow throughout the entire nacelle it will effect CHT but not EGT. If we reduce CHT cooling flow but really cool off the intake air, we will see lower EGT but higher CHT. Likewise, if we really heat up the intake air while increasing the CHT cooling type flow, the EGT will also heat up but the CHT would go down. So affecting both the air inlet temperature and the cylinder cooling flow could have different effects on egt and CHT depending on which way we effected the air in the nacelle.
I think.....
Good Luck,
Mahlon
 
Balance cooling and DON'T block air flow to heat muff

The reason #4 (aft left) and #1 (fwd right) typically have lower or cooler CHT's, they have deeper fins next to the baffle. The opposite is true of #2 (fwd left) & #3 (aft right), shallow fins against the baffle. Cyl are asymmetric front to back. The deeper fins are on the exhaust side.



Your solution might be to get the temps down on the other cylinders, verses get #4 up. One method of making #2 and #3 cooler is increase the gap between the baffle and the fins, allowing more air flow down around the cylinder. The shallow fins tend to choke off the air down and around to the lower part of the cylinder.

Yea 50F CHT is not a big deal but not ideal. You did not mention absolute temp? If they are all well below 400F and above say 320F than you are OK. There is such a thing as being to cool.

LAST DONT STOP THE AIR FLOW THROUGH THE HEAT MUFF. YOU WILL COOK IT. If you decide to disable the heater for the summer than take the whole muff off.
 
gmspilot,

That's great information, but now I'm confused. Don't worry, it happens all the time. I was for some reason thinking #4 was right rear, where the cabin heat air is taken from, but it is of course left rear, where the oil cooler sits. I checked my pictures of my engine and understand what you are saying about the larger fins on the side of the cylinder with the exhaust port. That makes sense.

The temps otherwise are fine. CHTs go to about 350 or 360 full rich and climbing. When I reduce power and lean the engine the CHTs on 1, 2 and 3 go to about 400 and #4 to 350. EGT's settle out about 1350 if I remember correctly.

So there are two questions.

1. what changed from last fall when all the CHTs were within 10 or 20 degrees? The only changes I've made to the engine is
a) removal of the cabin heat muff (it was burned through because the crossover pipes are too close to one another)
b) changed to an Andair fuel valve.
c) replaced the scat tubing on the air intake. I have a non-standard snorkle on the intake.

2. How do I correct what is wrong or modify airflow to raise #4. Now that I know #4 is in front of the oil cooler it is interesting that oil temps were a little cooler too.

As I said earlier I'm going to check the plugs and compression this weekend. If I'm not developing full power on #4 the temperature would be low.

Maybe I have a blockage in the injector line to #4 cylinder so that it is not getting enough fuel to run at full power? My air speeds seemed normal on the trip. Hmmm.

mahlon_r - I'll need to think about what you said a bit before responding.

Geico - I'll talk to Don. Good advice.
 
Bubblehead said:
I'm going to pull the cowling this weekend and check compressions and the plugs but that feels like I'm just grasping at straws. The mag rpm drop checks are fine so I don't think this is a spark plug issue. It certainly could be a compression issue though.

Any thoughts? Has anyone seen this before? If so, what was the root cause?
Click to expand...

Like you, I've experienced a cool running cylinder. In my case, it is #1 and about 100 degrees cooler EGT at cruise. The CHTs are all within 10 degrees at cruise. My mag checks are also normal. When I lean at cruise, I shoot for peak to LOP. After about 1/2 hour of LOP operation, I see the cool cylinder EGT gradually rise to within 30 degrees of the other 3.

When I get a chance, I'll be checking that #1 plug for any fouling. Someone on this board suggested a fouled plug that gets cleaned up as the engine runs LOP.

Mike
 
alcladrv - let's keep in touch on this. Perhaps there is a common issue. I'll post more after I do the checks this weekend.
 
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