IO-540, middle cylinders too cool

[vlittle]

I have an AeroSport/Lycoming IO-540-D4A5 on my HRII. It has about 10 hours on it, and all of the cylinder temperatures are below 400 in cruise and sometimes just above in full power climb.

However, there is a large (40 degrees) temperature difference between cylinders 3, 4 and the other cylinders. I've been taping off cooling fins on 3 and 4 to raise the temperatures, but this has a minor effect at this point.

These two cylinders obviously have the best cooling flow of the six cylinders, but it's not obvious how to effectively block the cooling air. Taping the fins just creates ducts for the air to flow in. I could squirt some rtv in the fins between the plugs to block the airflow because I know that this is a particularly effective cooling area.

Has anyone else dealt with this and found an effective solution?

[DanH]

The CHT probe actually measures temperature on the underside of the head. Just for the heck of it, take a look at the cylinder head baffle exit gaps. 3-4 are the same as 1-2-5-6?

[BobTurner]

You didn't mention it, so I assume all EGT's peak at about the same time, when you lean? e.g., 3 and 4 are not running excessively rich?

[David-aviator]

Sonja Englert, frequent forum speaker at OSH, talked about this problem a couple years ago.

She was contracted by a manufacturer, tufted the upper cowl intake area above the cylinders, installed a camera, and discovered an area of turbulence. She then designed a baffle of sorts to straighten out the air flow and the issue was resolved.

I know that does not solve your problem, but illustrates how it was done by her. I thoroughly enjoy her talks and have her booklet, Efficient Powerplant Installation.

[vlittle]

Originally Posted by BobTurner

You didn't mention it, so I assume all EGT's peak at about the same time, when you lean? e.g., 3 and 4 are not running excessively rich?

Don't assume anything. This engine is 10 hours old and I've been running ROP most of the time. Even a couple of LOP operating times has the same results in CHT spreads.

I'm pretty sure it's a cooling airflow management issue. I have success in balancing the other 4 cylinders, but the center two are more difficult.
Perhaps I should work on cooling the other 4 cylinders more rather than increasing the middle two. At least there are some adjustments I can make on these.

[RV10inOz]

Vern

There is no such thing as running too cool. There is in a true sense of the word, but the only time your cylinders are too cool (for flight power) is shortly after start up, by the time you get going they are all warm enough.

Your problem really will be getting the others cooler.

If you see CHTs in the 370 range now running in ROP, and that is full rich at low level, then all is good. Later on the CHT in flight at altitude will be about 360-370 when ROP and 300-340 when LOP.

A better way to run in now is actually WOT 2500-2600 RPM and around 80dF LOP when at 1000-2000 AMSL.

Enjoy.

[Tom Martin]

Vern
This is typical for the rockets that I have owned/ worked on. Right now I think it is too early, at ten hours, to do any real fine tuning unless things are way out of range. I have always felt that my engines are really broken in at 50 hours.
Also at this time of year it is likely not hot enough to tune the system for worst case cooling conditions.
Ideally you would like to be able to fly at your normal cruise power settings with the least amount of cooling air, drag, on the crappiest cooling day of the year with CHTs in the 360 to 380 range.
This will dictate whether you open the lower baffles on the warmer cylinders, or close the cooler middle ones.
Drop me at note at fairlea@xplornet.ca and we can set up a time to have a chat.

[Kahuna]

Common.
My middle cylinders are almost completely wrapped in speed tape. Have been for ~2k hours. The heat will move around when you do this. Meaning the coolness does not automatically go to the cylinder behind it. Tape is causing both flow and pressure changes. You will chase the heat around until you get it close enough.
You can just see it at the bottom of this picture. Or high res here.

[vlittle]

Tom, I will take you up on your offer, but let me chase a couple of other things right now. I'm going to try to drop the temperature of the corner cylinders first. I've dropped #5 by about 10 degrees, I think I can get more. #1 is easy to adjust, I have a 1/2" dam in front of it. #2 will need a bit of fussing and #6 will be the challenge.

Kahuna, your cylinders look like mine. I'm wrapping them in tape to see if they will warm up.

My experience from previous aircraft is that if you warm one up, the others will cool and vice versa because the air flow changes. I'm not going to go crazy until after full break-in, but just get things closer for now.

BTW, I have a plenum with no leaks except for the cabin heat duct, oil cooler duct with a positionable door and blast tubes. This is an interesting challenge, should be educational.

[vlittle]

Originally Posted by BobTurner

You didn't mention it, so I assume all EGT's peak at about the same time, when you lean? e.g., 3 and 4 are not running excessively rich?

All six cylinders reach peak EGT within 1 liter/hr flow (46-47 liter/hr in my test).