RV-6 normal CHTs

I've looked over numerous post on this subject and see a wide variation in opinions. I test flew a 6 with 455 since new O-360 hartzel CS prop. CHT on 110 knot climb 89 degree day got up to 428 by 2000 AGL. on # 3&4. They did come down at level off to 380 or so. I know this doesn't exceed Lycoming limits, but is this fairly normal or not.

428 is too high for me. My o360 hits about 390 on a steady 110 kt climb on a hot day. This only o occurs on the #3 cylinder. Once I level off, it goes down to about 340 with the rest of them. The other cylinders max out at about 370. I am swinging a c/s prop.

tracy said:

428 is too high for me. My o360 hits about 390 on a steady 110 kt climb on a hot day. This only o occurs on the #3 cylinder. Once I level off, it goes down to about 340 with the rest of them. The other cylinders max out at about 370. I am swinging a c/s prop.

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Have you tried removing the cabin heat muff and blocking off the 2" vent hole over the number 3 cylinder?

Are your CHT probes the kind that are a ring that sits under the spark plugs, or the bayonet style that "plug in" to a hole in the cylinder?

Mine are the "ring" style, and I also can see just over 400 on climbout, but they drop to 350-380 (depending on power setting) once I level off.

428 isn't horrible, but I generally climb at higher airspeeds unless I'm doing short field work or trying to clear an obstacle. Higher airspeeds = lower temperatures.

428 is horrible. We know that from this point it is a launch pad for undesirable things if left unchecked or other variables get out of hand.

Accepting something like this, sort of like saying it is OK to steal from big corporations as they can afford it but not from a small private business.

Which cylinders? All or just one?

Fuel flow? Is it enough?

Spark timing?

Baffles or more importantly exit air?

RV10inOz said:

428 is horrible. We know that from this point it is a launch pad for undesirable things if left unchecked or other variables get out of hand.

Accepting something like this, sort of like saying it is OK to steal from big corporations as they can afford it but not from a small private business.

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I submit this as an example of how not to respond on an internet forum.

If you want to disagree, fine.

But c'mon...

So you think accepting 428dF in a climb is a good thing and normal?


I understand you and many folk in the US don't get the Aussie folk when it comes to a shot across the bow like that, and thats OK, I will take your criticism. Down here it would not get the raising of an eyebrow, but all that aside, back to the facts, do you think this is a good scenario?

I promise to try to keep analogies to a minimum.

Kyle Boatright said:

428 isn't horrible, but I generally climb at higher airspeeds unless I'm doing short field work or trying to clear an obstacle. Higher airspeeds = lower temperatures.

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I'm with Kyle on this, and with everyone flying in all parts of the world, the definition of a 'hot' day will vary. Living here in here in the Mojave desert, I regularly see (on a climb) CHTs above 420°F on what I consider a 'hot day'; above 115°F. My field elevation is 2400' with the Sierra Mountains less than 8 minutes to the north, a generous climb rate is desirable and necessary.

On hot days, I will climb out at 430°F and will always level by 450°F to allow the O-360 engine to cool (#4 is my hottest); full throttle, full rich. All of my cylinder temps will settle below 400 in cruise.

As for engine data points, my first engine went 2501 hours with ZERO (Lycoming) cylinder issues, and I'm now almost 1400 hours into engine #2 with no issues (ECI cylinders).

Of course, your mileage will vary Rosie

Rv 6 CHTs

Thank you for all the replies. I know this is a hot topic, haha,but not trying to start a fight here. Not sure what type probes it has. I think bayonet. It was not that hot of a day for Georgia 89 degrees F. What compounded my concern was it came back with a not so good of an oil sample from Blackstone. Twice the iron and aluminum. This engine may have been running warm a while. No visible metal in the filter though?

You guys with the magic cooling touch need to pay me a visit and help me with my baffles.

Mine look good but apparently are lacking somewhat.

My climb performance is ALWAYS CHT limited.

Rosie's hard evidence!

On hot days, I will climb out at 430?F and will always level by 450?F to allow the O-360 engine to cool (#4 is my hottest); full throttle, full rich. All of my cylinder temps will settle below 400 in cruise.

As for engine data points, my first engine went 2501 hours with ZERO (Lycoming) cylinder issues, and I'm now almost 1400 hours into engine #2 with no issues (ECI cylinders).

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Can't argue with real life experience.
While mine run nowhere near 400F ever, I cannot argue with Rosie since I only have 270 hours on mine.
I do believe that flying your Lycoming often and consistently as Rosie does, contributes greatly to going past TBO.
Flying skydivers for many years we consistently ran CHTs near red line
and many of our engines reached 2500hrs before overhaul.

Brantel said:

You guys with the magic cooling touch need to pay me a visit and help me with my baffles.

Mine look good but apparently are lacking somewhat.

My climb performance is ALWAYS CHT limited.

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Likewise, I'm still piddling with lowering temps. With 90* OAT, If I climb at 100 knts IAS I see 400-409 on 3+4 while 1+2 are 350-370. In cruise everything drops below 390, LOP I see about 330 on every cylinder. I don't think I can seal the baffling any tighter and more cooling is going to come from somewhere else. More little projects and piddling to keep my avgas budget in check...

Geico266 said:

Have you tried removing the cabin heat muff and blocking off the 2" vent hole over the number 3 cylinder?

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No, Im ok with these temps and dont want to fool with muffs

CHT Probes

I have bayonnet probes for my EFIS and ring probes to a backup EMS. I think the temp readings on the backup inst. tend to run about 20 deg lower than on the EFIS. I trust the bayonnet probes.

Temps

I heard somewhere many years ago that light aircraft certification only requires testing to 100 degrees F.
The Cessna Turbocharged twins and some of the Pipers, here in the desert will run red line throughout the climb. 450 CHT on the Continentals, 500 on the Pipers(Lycoming)
I have seen no indication of cylinder problems caused by high CHT. I personally believe that the normally aspirated Lycomings cannot be harmed by high CHT AS LONG AS THE MIXTURE IS NOT LEANED IMPREPERLY above 75% power.
The worst "problem airplane" I flew was the Cessna 421. Taxiing out at Phoenix on a 100 degree plus day, the oil temp would be at red line halfway to the runway and would stay there until level cruise. CHT close to red line.
Heavily loaded in a 421 on a hot day, rate of climb would be 500' min or less. Any attempt to increase speed in a climb, the airplane would stop climbing.

CHT

There are some issues regarding CHT that I don't recall ever seeing discussed on this forum.
The baffling below the cylinders is rarely discussed. I personally believe that the safety wire means of securing the baffles below the cylinders is hopelessly inadequate. A mild steel or 4130 tie rod, 1/8" in diameter, threaded both ends is the optimum for this location. Total of four rods for a four cylinder. This allows the lower baffles to be drawn up tight to the cylinder fins, with zero gap if done correctly. An all metal lock nut will not lock tight on a typical home made thread, so I use a plain nut along with an all metal lock nut as a jam nut.
The inter cylinder baffles, Lycoming parts, leave significant gaps adjacent to the crankcase. An additional extension there will help. This extension will likely have to be a separate piece, installed after the main inter cylinder baffle.
I firmly believe that at the upper baffle/cowl interface, that it is beneficial to angle the aluminum inboard on the sides and forward on the rear, where the baffle rubber attaches. Cessna has been doing this for many years and Cessna does not do anything that makes production more difficult without a very good reason. As well as being more efficient this virtually eliminates the possibility of the rear baffle rubber folding back at high speeds.
The area behind the spinner-this area is VERY important. Any leakage in this area can create airflow that pressurizes the area below the engine and possibly even reverse flow/disturbed flow behind the spinner.
Any leakage in the interface between the typical induction scoop and the lower cowl will also allow airflow that will pressurize the area below the engine.
If you look carefully at the Lycoming engine you will find many areas where additional baffle support can be attached with no significant increase in weight. Just one example-on the upper left engine mount attach point-the "ear' cast into the crankcase, you will find a hole about 5/16" diameter thru the 'ear'. I use this as an additional support point for the rear baffle. it will typically require a spacer about 5/16 thick on generic(non RV) baffles.
The optimum baffle system would be one that totally isolates the two sides. This requires a vertical baffle at the base of the cylinders extending upward to the cowl. It is very rare to see this on a Lycoming but it has been done. It requires a lot of small pieces to go around the pushrod shroud tubes. This is fairly common on the four cylinder Continentals where is relatively easy to do because the pushrods over below the cylinders.
Examples of many of these ideas can be found in the Tony Bingelis books, still available from EAA. IIRC, both engine books discuss baffling in considerable detail.

jrs14855 said:

There are some issues regarding CHT that I don't recall ever seeing discussed on this forum...

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I'm in the middle of some extensive (and some will consider radical) inter cylinder baffle mods on the Rocket and I happen to agree with much of what you say in this post. That said, I'm curious for the reasoning behind dividing the plenum between two sides. I'd think the two inlets in the front of the cowl contribute to the upper plenum as a whole. After all, the airflow is not often equal from side to side due to propeller blade angle and relative direction, and airflow use is not the same from cylinder to cylinder, so why take away the opportunity to "average" the total air volume?

baffles

Michael-
I never really gave this a lot of thought. Just copying what the fastest Midget Racers did in the 50's. In Dec 1959 I first visited the Warwick NY Airport. It was then the home base for the prototype Cassutt and Bill Falcks famous Rivets. These were the first Midget Racers I had ever seen and the third and fourth homebuilts I ever saw.
Rivets had a very unique updraft cooling system that is not pertinent to this discussion. The Cassutt had isolated "box" plenums on each side.
Examining your comments made me think of this differently: with a plenum over the entire engine, IF there is a difference in airflow between the two air inlets, is it not possible that turbulence or even reverse flow is detrimental to overall cooling flow.
The other answer which is not new, is that the dual plenum helps isolate the area behind the spinner.
I have a single plenum for my not yet flown non RV. I had to make it from scratch. I am confident it will work well but is by no means optimum. I did not intend it to be optimum, just near optimum.
I can't remember for sure the exact configuration of the late Paul Lipps Lancair 235, but it is definitely worth examining. Also had some very exotic carbon fiber baffles.
Sam James also has some very interesting information on intercylinder baffles as well as the plenums he sells.

I don't think it can be argued that lower CHT's are probably better. However, the argument I have seen most by the well respected article writers, is that aluminum loses something like 1/2 it's tensile strength at 450 deg., or something like that. Well, who cares. The engineers that designed the engine in the first place put the limits there and I would imagine the tolerance of the parts for strength where designed well beyond 2x.
I fish for Salmon in BC every year. One of the guides uses a certain flasher that nobody else uses. I asked him if he thought it caught more fish. He said he didn't know if it caught more, but he knew it didn't hurt.

I think it is wise to do what you can to make your plenum work as well as it can. But I don't think you need to kill yourself doing it or obsess over it if it is within the manufacturers published limits. That is only my opinion.

For the OP. I dropped my CHT's 10-20 deg. in climb simply by finding every nook and cranny and filling it. Most of that was on the lower and inter cylinder baffles. My next step will be to improve my servo inlet to snorkel interface, but I am in no big hurry. I see 410 on a hard climb, hot day (not Rosey hot), heat soaked engine. Easily managed by lowering the nose. My LOP numbers are 280-310.

I know I can do better, and it won't hurt a thing.

CHT

The number that I recall is 50 % loss in strength of aluminum at 400 degrees.
When filling those gaps, if you use silicone, make sure you don't get any between the fins.
I seem to recall some Mike Busch info on too cold in cruise but I don't remember any numbers. I do think 280 is a bit on the cool side. Why I am planning a cowl flap.
If any one can point to any information on what is too cool I would like to see it.

As I reported yesterday, I did a phase one climb test to 10,000' at WOT to see what the temps would do. OAT was 78F at take off, hottest CHT was 377 max oil temp was 175.

What I did not mention was that on the very first taxi test, hottest CHT was 395 just taxiing around for a half hour. That was unacceptable and immediately decided to open exit area as a 4 pipe exhaust system does take up some of the existing area.

The bottom of the cowl was opened forward of the standard area 4"x12" and in effect nearly doubled the exit area. I did not see anything like 395 again during taxi tests.

This is a contentious subject with cooling purists, but I believe the best way to improve cooling is to reduce the internal cowl pressure in the lower end of the engine compartment and the way to do it is increase exit area. It worked with the 7A with the Subby H6 and the conversion to the Lycoming IO360. You can increase exit area by cutting forward of the opening or by installing louvers.

I do not like CHT issues...

David-aviator said:

The bottom of the cowl was opened forward of the standard area 4"x12" and in effect nearly doubled the exit area. I did not see anything like 395 ...

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Out of curiosity, David, how far fwd of the firewall did you have to cut the cowl exit to do the above? That's my next step I think, though I only have the two pipe exhaust.

David-aviator said:

As I reported yesterday, I did a phase one climb test to 10,000' at WOT to see what the temps would do. OAT was 78F at take off, hottest CHT was 377 max oil temp was 175.

What I did not mention was that on the very first taxi test, hottest CHT was 395 just taxiing around for a half hour. That was unacceptable and immediately decided to open exit area as a 4 pipe exhaust system does take up some of the existing area.

The bottom of the cowl was opened forward of the standard area 4"x12" and in effect nearly doubled the exit area. I did not see anything like 395 again during taxi tests.

This is a contentious subject with cooling purists, but I believe the best way to improve cooling is to reduce the internal cowl pressure in the lower end of the engine compartment and the way to do it is increase exit area. It worked with the 7A with the Subby H6 and the conversion to the Lycoming IO360. You can increase exit area by cutting forward of the opening or by installing louvers.

I do not like CHT issues...

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Dave, I'm piddling my way through this now on this thread:
http://www.vansairforce.com/community/showthread.php?t=112941&highlight=Exit+bump

I'm waiting on my manometer to arrive so I can have more specific/accurate results on my upper and lower cowling pressures. Mines a 7, so mods would be different but possibly relevant.

Did you measure the lower cowling pressure before and after the cut?
What was your upper plenum pressures?
Did you have any NPTS speeds before and after?
Do you have any seal behind the spinner?
Vertical or horizontal induction and intake seal?

Thanks,
Andy

jrs14855 said:

I heard somewhere many years ago that light aircraft certification only requires testing to 100 degrees F.
The Cessna Turbocharged twins and some of the Pipers, here in the desert will run red line throughout the climb. 450 CHT on the Continentals, 500 on the Pipers(Lycoming)
I have seen no indication of cylinder problems caused by high CHT. I personally believe that the normally aspirated Lycomings cannot be harmed by high CHT AS LONG AS THE MIXTURE IS NOT LEANED IMPREPERLY above 75% power.
The worst "problem airplane" I flew was the Cessna 421. Taxiing out at Phoenix on a 100 degree plus day, the oil temp would be at red line halfway to the runway and would stay there until level cruise. CHT close to red line.
Heavily loaded in a 421 on a hot day, rate of climb would be 500' min or less. Any attempt to increase speed in a climb, the airplane would stop climbing.

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Reading your post is a breath of fresh air.
Far part33.49 Aircraft engine certification endurance testing requires. 150 hours minimum. Of this 150 hrs, 75 hrs are at max continuous power and speed and 35 hrs of this MCPS has to be at maximum redline CHT (500 degrees) and oil temperature (245 degrees) for Lycomings and their clones. Most experimental Lycoming type engines and their parts conform to this certified standard even though they are not required to.
A lot of people on this forum act as if 400 degrees is the maximum for their engines and something bad will happen if they operate above this level. It is normal for CHT?s to be high on a hot day under sustained climb conditions. It will not harm the engine to be operated up to the certified red lines. The engine certification testing procedures prove this beyond a doubt for standard compression ratio engines.
In order to achieve TBO on these engines it is best to maintain CHT?s at or below 435 degrees under high performance cruise conditions 75% and above and at or below 400 degrees for low performance cruise conditions 75% or less.

RV10inOz said:

So you think accepting 428dF in a climb is a good thing and normal?


I understand you and many folk in the US don't get the Aussie folk when it comes to a shot across the bow like that, and thats OK, I will take your criticism. Down here it would not get the raising of an eyebrow, but all that aside, back to the facts, do you think this is a good scenario?

I promise to try to keep analogies to a minimum.

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Do I think that 428F in a climb is a good thing and is normal? In a sustained low airspeed high climb rate,answer is YES!