High CHT... again. All help/experiences wanted!

[Jim Doody]

Re: High CHT... again. All help/experiences wanted!

I would move the the leads from cyl to cyl to see if the high temp moves and then the sensors from cyl to cyl to see if it moves, this will prove it is a problem on that cyl. And then I would follow the suggestions here http://www.sacskyranch.com/eng.htm Good luck. Jim

[VictorBravo]

Re: High CHT... again. All help/experiences wanted!

A little more info, and thanks again guys for the help.

When I made my new lower oil baffle, I had to install a couple of small spacers underneath the crankcase to get everything to fit (because I could not form the little depressions that were stamped into the original oil baffle). This moved the spider about 1/4 to 3/8" further down below the crankcase.

There is no spacer or change between the carb and spider.

The only net change (from the engine's perspective) is that there is a slight angular difference on the intake tube runners... a couple of degrees, nothing more. I tightened and grease-sealed the rubber hoses and verified that there were no visible gaps... the rubber hoses took up the misalignment.

Before everyone screams AHA!!! That's IT! remember these things:

The engine does not run rough.
It appears to make plenty of power.
The spark plugs on the hot cylinder are not the white-ash color that is normally associated with an excessively lean mixture.
No detonation, pinging, knocking, etc. even on hot days.

Also, the only discoloration on the cylinder is a light brown color stain near the exhaust port, but this is not soot. It looks like baked oil, but it is a thin transparent stain... not carbon chunks or a thick coating in any way.

Once again I truly appreciate the experience and advice of everyone here, this one's got me stumped!

[Michael Rodina]

Re: High CHT... again. All help/experiences wanted!

In the early 1980's I discussed the lower cowling design as it relates to cooling with none other than C.G. Taylor. I will try to paraphrase as best I can:

Incoming ram air creates high pressure air on top of the cylinder baffles in a pressure cowl. The space between the bottom front of the boot cowl and the back of the lower cowl as well as the lip on the back of the lower cowl is critical to cooling. A vacuum is created under the cylinder baffles by the space and lip of the lower cowl. This vacuum helps suck the hot air out. Small changes can have a significant impact on cooling.

This conversation was in regard to a T-craft owner who had made the space between the boot cowl and lower cowl larger to "improve cooling" and actually made it worse. This may be your problem. You could also have a mechanical problem with the engine as others have mentioned.

The #1 cylinder receives reduced ram air pressure because of it's rearward location under the cowl.

I hope this helps.

Michael Rodina

[crispy critter]

Re: High CHT... again. All help/experiences wanted!

Bill,
If you have no intake or exhaust leaks and have done the work to prove that and it is still running hot.....my advice is to remove the CHT. Another idea if nothing else works is to pull that cylinder and have it checked for a crack or maybe a pitted intake or exhaust leak. Chrome cylinders will also run a little cooler then steel. One other thing you said was you don't have any problems with plugs,i.e. always come out clean....can you richen it a little bit? We all know t-crafts don't have the greatest baffle system in the world even when in excellent condition. The #1 cylinder is the farthest back on the calm side of the prop so it will run hotter then the rest. I guess what I'm trying to say is unless it is causing a problem I wouldn't worry too much about it.

[flyguy]

Re: High CHT... again. All help/experiences wanted!

"The calm side of the prop,"
I have never heard anyone in general aviation even make reference to this. I was thinking the other day about the direction of the air flow behind the prop, into the cowling. Interesting.

I have read about effects on radiator cooling when it is near the propeller in water-cooled aircraft engines.

Would you care to say a little more on the subject Kevin?

Darryl

And I didn't remember that #1 was in the back on the right, that is my hottest cylinder also. Around 400 is not unusual on mine on climb out.

Bill. Think we went over this before: The temperature on my hottest cylinder actually went down when I covered up about 80 percent of the hole in the front cowling (bowl) to get the oil temperature up in cooler weather. That is, the hole that appears to be for cooling the oil tank and crankcase, not the grill holes. I was going to check to see what kind of pressure differential I could get but the gauges I wanted to use were overly expensive for a one shot deal.

[crispy critter]

Re: High CHT... again. All help/experiences wanted!

I messed up and wound up with 2 reply's,it would not let me delete this one so please read the next one

[crispy critter]

Re: High CHT... again. All help/experiences wanted!

[QUOTE=flyguy;33891]"The calm side of the prop,"
I have never heard anyone in general aviation even make reference to this. I was thinking the other day about the direction of the air flow behind the prop, into the cowling. Interesting.

I have read about effects on radiator cooling when it is near the propeller in water-cooled aircraft engines.

Would you care to say a little more on the subject Kevin?

Flyguy,
I was just rattling out the mouth on that one but....since you asked. Experts will tell you that you will get no affects from the prop that close to it. However the #3 cylinder will sometimes run hotter or as hot as the #2. Also, it seems that most of the birds I bring in have some type of baffle wear,it is always worse on the left side(torque side) then on the right. This tells me the torque side of the engine(left side or even numbered side) gets more air or at least more abuse from whatever then the right. Maybe I've been spending too much time alone in the hanger paint booth again with not enough things to do besides think about some of this crazy shit....but,that's what I think about it and it makes sense with evidence to back it up.

On another note. Bill,your CHT is running a little hot but within limits. What are the temps on the #2,#3,and #4 cylinders? If we are going to compare then lets try to compare them all. IMHO, I'm going to suspect you have a mixture problem some where weather it is being caused a leak or a very fine crack in the cylinder or intake system. #1 should run hotter then the rest but not enough to cause consern. What are the rest of your temps when this one it passing the 450 mark?

[flyguy]

Re: High CHT... again. All help/experiences wanted!

Kevin, now you have me going, Ha, Ha. What was that saying about something not changing it's stripes. A retired tech guy is still a tech guy. I think I'll stick some strings on the front bowl near the intake grills and see which way the air is going. Must have too much time on my hands since I finished my hot rod Camaro. LOL
DC

[Ron Coleman]

Re: High CHT... again. All help/experiences wanted!

Another potential cause for a hot spot. If you have moved the thermocouple to other plugs to verify the sensor is not faulty, use an infared thermometer to check the general head temperatures after you shut down the engine (right quick). A hot cylinder will hotter than the rest so compare them by scanning all over the heads. If the hot cylinder is not nuch hotter than the others you may have a leaking plug insert which allows hot gas to blow on the thermocouple. A fine crack in the cylinder head (usually radiating out from the sparkplug) will do the same thing.

RonC

[VictorBravo]

Re: High CHT... again. All help/experiences wanted!

Guys... my other CHT's are (if I'm remembering right) 360 390 and 410. #1 is hottest at 450+, #2 is 390, #3 is 360, #4 is 410.

The starboard rear cylinder is very hot. The starboard front cylinder is very cool. The port rear cylinder is warm, and the port forward cylinder is hot.

I have not switched the positions of the thermocouples because I installed the little stick-on color change temp dot thermometers on the #1 cylinder and verified that it is indeed running hot. I put five or six of the temp dots on it at various places. The one at the hottest point on the cylinder (between the lower spark plug and the exhaust valve/exhaust flange area) tells me the temperature at that point exceeds 435F (top of the range). Other places on the same #1 cylinder are a lot cooler of course, like the intake valve area is below 340.

I don't think there is an intake leak. Spark plugs normal, not white ash color.

There is no soot trail near the thermocouple, so I don't think an exhaust leak is bathing the thermocouple in heated air.

I do not have the equipment to do an internal airflow video recording... I wish I did.

My gut feeling is that this is an airflow problem, not a broken engine problem. It just runs too good and too smooth IMHO. I have been doing a lot of work on baffling, cooling, airflow... somewhere I think I got it wrong, or air is not moving correctly.

[LLBOYD]

Re: High CHT... again. All help/experiences wanted!

Bill, I have scanned the responses and didn't pic up on, if the inter cylinder baffels were inplace and snug to the fins of the cylinder. Years ago I had the retaining spring break and the baffel pulled away from the barrel fins. The indication was a rise in Cyl head temp and oil temp.

Larry

[VictorBravo]

Re: High CHT... again. All help/experiences wanted!

Bill, I have scanned the responses and didn't pic up on, if the inter cylinder baffels were inplace and snug to the fins of the cylinder. Years ago I had the retaining spring break and the baffel pulled away from the barrel fins. The indication was a rise in Cyl head temp and oil temp.

Larry

Yep, the inter cylinder baffles are in.

[flyguy]

Re: High CHT... again. All help/experiences wanted!

Bill, I did my little static test that I mentioned today. Put some strings in a row across the middle of the grill on both sides and ran the engine on the ground at about 700 rpm.

Surprised me a little that I saw what I expected, but the effect was much more pronounced than I thought it would be.

Using longer strings the air on the pilot's right side (#1 in back) is actually blown down and across and close under the propeller hub.

The air on the other side goes up and across the top of grill, away from the prop, or straight up, but it can be seen to be pushing the strings into the cooling intake and up over the cylinders on that side.

I then trimmed the strings on both sides to about 3 inches. On both sides they were then sucked into the grill, except on the pilots side they were turned downward towards the lower front baffle where there is nowere to go except back out the front. More short ones I added across the top of the grill were sucked into the area in front of the cylinder but they were also tilted downward, but to a lesser degree.

The short ones on the other side went right up and into the passage over the front cylinder fins.

As I said above this was static, but as with the radiator problem on the British fighters, I can see with full power, that close behind the prop, the effect may be significant at climb speed as well.

Darryl

[VictorBravo]

Re: High CHT... again. All help/experiences wanted!

The string test unfortunately cannot tell the true tale unless it is photographed in flight. The air behind the propeller in flight might be somewhats imilar to the ground run, however without the 60 or 70 mph in flight climb airflow AND angle of attack across the bottom of the cowl (to pull whatever air out that it is going to pull out), you cannot figure out what is going on.

A scientist-type guy in our EAA chapter is helping me rig up a water manometer to measure the static pressure above and below the engine in flight. While not a really perfect picture of the airflow, it will at least tell me whether there is enough "delta P" to make SOMETHING work... anything!

I pulled the lower cowl and verified (via the temperature stick-on dots) that the #1 cylinder is definitely getting up above 435F... all the little dots on the indicator had turned color.

So there is a problem, and the fact that the spark plug probes read higher, or the possibility of a bad gauge, or a bad probe... all that stuff has been removed from the equation. The cylinder is hotter than it should be, period. While within the book redline, 450 F on initial climb, on a cool day, with one person in the airplane... is outside of MY redline.

I really hate to cut up a cowling to let more air out, but unless I figure out something very obvious with the manometer or some kind of video inside the cowl, I'm going to have to do something drastic. I don't want to, but I REALLY don't want to ruin a good engine or have an off-airport landing within Los Angeles city limits.

[flyguy]

Re: High CHT... again. All help/experiences wanted!

Manomenters are awkward, but they work, and they are cheap.

I still have my TC's installed on the back cylinders and all I have to do is plug in the gauge on the next flight and check the temps again. I do know under worse conditions with the center cowl hole open that my reading on one of the cylinders was not that far from what you are getting.

I'll check it again carefully and let you know. Now that it is cooler you may get an even higher reading depending upon your reference junction temperature.

I have no doubt though that the prop plays a significant role in that side running hotter.
DC