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Suggested Next Step for Cooling Issues?

oxothnk

Active Member
Hi All,

New to the forum as a friend and I just partnered in an RV-6 a few months ago. (What a great plane!)

We installed a 4-channel engine monitor on our Superior O-360, and noticed that CHTs ran quite hot. We regularly saw average temps at or above 425, sometimes past 450. Also our rear cylinders ran about ~50deg hotter than the front.

After reading many of the posts here, we completely went through our baffles and sealed any leaks we could find (there were several). We also added some aluminum ramps(?) to the front cylinders to balance the temps.

We're now seeing very uniform temps around 400 deg on all cylinders. This latest data point was at 86F, 3000' MSL, 23", 2300rpm, ~8.5gph, 152kts TAS.

But that still seems too hot. So any suggestions on our next step? Options we have are:

(1) Adjust the timing -- we're at 25 degrees now with higher compression 9:1 cylinders. (We read that moving that closer to 20deg could help)

(2) Open up our cowl exit to get more airflow

(3) Fine tune the baffles more. The items we haven't done are (a) blocking the gap between the cylinders and (b) filing the flashing between the fins by the plugs (we inspected this area and actually have minimal flashing here, but of course even small protrusions into the airflow could have a large effect).


Any suggestions on how much the timing might be an issue? (i.e., we don't want to cut open our cowl exit if the timing is the dominant cause of high temps).

Any suggestions on what measurements we can perform (cowl pressures, etc) that could help point to the best fix?

Thanks!
James
 
Aero Sport did a 320 for us a while back with 9 to 1, it came back to us set at 20 degrees, you might try it out, can?t tell how much it will help but it?s an easy thing to try.
 
We installed a 4-channel engine monitor on our Superior O-360, and noticed that CHTs ran quite hot.

Quick...uninstall the monitor! ;)

We're now seeing very uniform temps around 400 deg on all cylinders. This latest data point was at 86F, 3000' MSL, 23", 2300rpm, ~8.5gph, 152kts TAS.

Returning to serious mode, 86F at cruise altitude is a tough day.

(1) Adjust the timing -- we're at 25 degrees now with higher compression 9:1 cylinders. (We read that moving that closer to 20deg could help)

Please change the timing and report back here with the results. It's so easy, and you cannot hurt anything.
 
Welcome to VAF!

Hi All,

New to the forum as a friend and I just partnered in an RV-6 a few months ago. (What a great plane!)


James

James, welcome to VAF:D

Agree with Dan (usually do, he seems to be right most of the time;) ) try retarding the timing.

I have a similar issue on my high compression RV 10-----retarding helped.
 
Update with Timing adjustment

Hi Everyone,
Thanks for the all the input! I finally had a chance today to adjust the timing. In the meantime we've been logging data so we have a good basis for comparison. (Since we're using a GEM 602 that only gives us 25deg markings on CHTs, and no absolute scale for EGTs, we're having to do a lot of averaging)

Previously, with timing at 25deg BTDC, we were running about 334deg above ambient, at 23" MP, 2300 RPM. (so, ~400F CHT at 66F outside air temp). (We're pretty confident in this value since we've been averaging over many flights)

Now, with timing at 20deg BTDC, we're running about 319deg above ambient at the same power settings (so, ~385F CHT at 66F outside air temp). (this was just one flight today, so consider it an approximation for now)

So it looks like retarding the timing has helped us run about 15 degrees cooler. Is that about what you would expect?

Another thing I noticed flying today after the timing adjustment was that at full power (and full rich) on takeoff, EGTs seemed higher than usual. Maybe 50-75F or so? (guessing here, since the GEM monitor doesn't have an absolute scale for EGTs). In flight, they seemed similar to before (but maybe slightly higher), so not sure how much of an issue this is.
Fuel consumption and true airspeed seem similar to before. (Maybe a a few knots less on true airspeed? But hard to tell until we flight test more).

It was nice seeing the cylinders below 400. But I wouldn't mind getting them cooler for hotter days and for takeoff (on departure they still push to 425~450 until I can bring the power back).

Let us know if this sparks any other thoughts or ideas. We're happy to start trimming the cowl exhaust outlet if needed...

Thanks for all the help!
James.
 
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RV7A IO360MI with 9.1 pistons, I was approaching 400 Cht on climb out. Installed Van's louvers on the lower cowling Temps went to 335 on climb out. Very happy with that. I have put tape over half the opening on the inside and still get 335 on climb out and all within 5 degrees.
 
Larry, these numbers seem out of character with mags, are they new?
You may consider changing to fuel injection?
 
Another few degrees . .

Do you have an exit guide attached to the lower firewall? I don't know how many delta degrees it will help but it definitely will increase the airflow (decrease the exit resistance).

Do you have dual or quad exhaust?

Here is a representative thread. http://www.vansairforce.com/community/showthread.php?t=112941&page=2

RVBuilder2002, Scott, has recommended a dark hangar and take a flashlight and find air leaks in baffles to lower cowl. That would help the leakage part.
 
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I curious on how much performance loss there is with 5 degrees of timing change? I know I was messing around with slowly reducing my timing by 2 degrees and when I saw ~100 RPM drop in static runup I elected to go back to 25 degrees. On really hot days I just monitor my temps and cannot climb for long periods of time.

My next step is looking a fabricating a cowling exit fairing, similar to what some others have done. https://www.youtube.com/watch?v=CrvQeOGzJ7s

I assume that if you smooth the airflow exiting the cowl the cooling will also be better.
 
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(We're pretty confident in this value since we've been averaging over many flights)

(this was just one flight today, so consider it an approximation for now)


James.

Welcome to VAF James!

I like you approach to collecting real data......use information collected over several flights and don't just rush to a conclusion over a single data point.

Keep it up!
 
We're now seeing very uniform temps around 400 deg on all cylinders. This latest data point was at 86F, 3000' MSL, 23", 2300rpm, ~8.5gph, 152kts TAS.

Your fuel flow seems to be right about at peak power (and peak cht...) for those map/rpm settings. How did you arrive at 8.5 gph? What happens if you dial fuel back a bit at the same settings? Mixture makes a huge difference in chts. There are volumes of good information in the archives about checking mixture distribution among the cylinders, etc. All fun stuff!

152 ktas seems a bit slow for 8.5 gph at that altitude, but I'd have to go try it to be sure. If I'm right, that isn't helping the cause either. Is the TAS indication calibrated?
 
I curious on how much performance loss there is with 5 degrees of timing change? I know I was messing around with slowly reducing my timing by 2 degrees and when I saw ~100 RPM drop in static runup I elected to go back to 25 degrees.

There is undoubtedly some power loss. In-cylinder peak pressure is lower, and later after TDC.

The same thing can be (and often is) done with extra fuel. Retarding the timing is a lot cheaper, and less dirty.
 
What does your cowl exit look like? Its not unusual to open the exit an inch or so by taking away some of the cowl at the exit to allow for more flow out.
 
What does your cowl exit look like? Its not unusual to open the exit an inch or so by taking away some of the cowl at the exit to allow for more flow out.

Not sure what you are talking about? Are you talking about shortening the cowl exit from the aft end by an inch or so? This would be a whole lot easier from my standpoint vs making a fiberglass exit fairing.
 
I trimmed mine from corner to corner on the bottom of the exhaust exit in a curved manner. In the middle it was 1". It brought my cruise temps down so I could live with them. I then installed a cowl flap from Anti-Splat and that brought my climb temps down.
 
I have an RV4 built by a good builder, that has an AEIO360 with 9.5:1 pistons, timing 26 BTDC, fuel injected, electronic ignition. The airplane is 25 years old and had an engine change 20 years ago after the original builder had sold it. Was experiencing CHT issues similar to what you described. Replaced rubber on baffling, sealed gaps, installed a lip on the aft end of the lower cowl, on the bottom which formed a low pressure area to draw more air through. A all that seemed to make about a 15 degree difference. On hot days I was still having to manage my climbs with higher airspeed and lower power settings. Turns out, the major culprit, and I'm embarrassed to admit it, was I was missing an entire section of baffling that goes across the front, between the two air inlets. My airplane has been worked on by mechanics and others who are familiar with RVs, but the fact I was missing the piece of baffling was not noticed. This was brought to my attention by a very smart and observant friend who knows much about building. The piece is part of the RV8 baffling kit but is the one that is supposed to be used for RV4s that have a 360 engine. Once this additional piece of baffling was installed by another friend of mine (I have zero mechanical aptitude), the CHTs dropped 45 degrees. Now, on a 103 degree day, with two, 200 pound guys in the airplane, a 120 mph climb caused the hottest CHT to top out at 400 in the climb, and come back down significantly in cruise.
 
I would do more research before retarding your timing. My experience from building automotive hot rods is that retarded timing = increased engine heat. In fact in the late 70's the automakers all reduced timing to reduce emissions as the increased engine heat reduced the one of emission components. You should only reduce timing to offset detonation issues. The lycomings are already very conservative and leave a lot of power on the table to deliver a large detonation margin. That is why the elec. ign. systems increase power or efficiency (they increase timing in low load scenarios). Lycoming can't run the optimum timing because the mag's are fixed timing devices. most engines manipulate timing based upon both RPM and load (i.e. MAP) to deliver optimum timing in each load condition.

In addition to creating MORE heat, you will also reduce power output at any given RPM/MAP setting, causing you to run the engine harder and produce more heat to reach the same propeller thrust level.

Larry
 
I would do more research before retarding your timing. My experience from building automotive hot rods is that retarded timing = increased engine heat. In fact in the late 70's the automakers all reduced timing to reduce emissions as the increased engine heat reduced the one of emission components. You should only reduce timing to offset detonation issues. The lycomings are already very conservative and leave a lot of power on the table to deliver a large detonation margin. That is why the elec. ign. systems increase power or efficiency (they increase timing in low load scenarios). Lycoming can't run the optimum timing because the mag's are fixed timing devices. most engines manipulate timing based upon both RPM and load (i.e. MAP) to deliver optimum timing in each load condition.

In addition to creating MORE heat, you will also reduce power output at any given RPM/MAP setting, causing you to run the engine harder and produce more heat to reach the same propeller thrust level.

Larry

This logic does not track with thermodynamics. The timing is selected for power (actually torque at a specific RPM) and its effects. The effects are with more advanced timing combustion begins earlier, that means the mean temperature (for heat rejection) is higher in the compression stroke. That will drive up piston ( and barrel) and head temperatures. It will have a higher peak cylinder pressure which is also at a higher temperature. Since it peaks earlier, then the EGT will be lower as the expansion stroke to exhaust valve opening will extract slightly more more energy and along with the added heat rejected to the components BTDC, the heat rejected just shifted from the exhaust to the oil, heads and barrels.

So, as DanH mentioned a little more final HP, but at a price. That price is higher CHT and oil temps.

Ignition systems with higher energy will promote (up to a point) more rapid combustion and for a "fixed" timing ( and operating point) will increase HP as (possibly "if") combustion is completed earlier and allows the increased PCP to fully expand (thus producing said HP increase). Faster combustion yields a MBT timing that is (relatively) less advanced. High energy ignition is especially needed with leaner AFR's that were/are employed in automotive for emissions purposes.

OK, back to work.
 
The items we haven't done are (a) blocking the gap between the cylinders
Have you done this yet? It seems to me this would help a lot.
 
Reduced Timing and Heat

I would do more research before retarding your timing. My experience from building automotive hot rods is that retarded timing = increased engine heat. In fact in the late 70's the automakers all reduced timing to reduce emissions as the increased engine heat reduced the one of emission components. You should only reduce timing to offset detonation issues. The lycomings are already very conservative and leave a lot of power on the table to deliver a large detonation margin. That is why the elec. ign. systems increase power or efficiency (they increase timing in low load scenarios). Lycoming can't run the optimum timing because the mag's are fixed timing devices. most engines manipulate timing based upon both RPM and load (i.e. MAP) to deliver optimum timing in each load condition.

In addition to creating MORE heat, you will also reduce power output at any given RPM/MAP setting, causing you to run the engine harder and produce more heat to reach the same propeller thrust level.

Larry

Reduced timing will increase exhaust gas temperature (EGT) while reducing cylinder head temperature (CHT). This is significant in the world of emission controls because catalytic converters require a certain amount of temperature to be effective.

Skylor
RV-8
 
Reduced timing will increase exhaust gas temperature (EGT) while reducing cylinder head temperature (CHT). This is significant in the world of emission controls because catalytic converters require a certain amount of temperature to be effective.

Skylor
RV-8

EGT's go up because more of the charge is burning past peak and heat is not converted to energy. This raises the exh temp and also increases heat in the cylinder (more cyl walls than heads) I have very limited experience in air cooled engines, but a water cooled engine will over-tax it's cooling system when the timing is retarded. However, a water cooled engine is taking more of it's heat from the cylinder walls and not the head. I would expect oil temps to go up with retarded timing. But the cooler may handle it and never be seen. But as I think more about this, I can understand why CHT's would go down with retarded timing.

Larry
 
EGT's go up because more of the charge is burning past peak and heat is not converted to energy. This raises the exh temp and also increases heat in the cylinder (more cyl walls than heads) I have very limited experience in air cooled engines, but a water cooled engine will over-tax it's cooling system when the timing is retarded. However, a water cooled engine is taking more of it's heat from the cylinder walls and not the head. I would expect oil temps to go up with retarded timing. But the cooler may handle it and never be seen. But as I think more about this, I can understand why CHT's would go down with retarded timing.

Larry

Pulling timing back a few degrees isn't going to overtax any liquid cooled engine's cooling system built in the last 30 years. There is huge margin built in there, like 2 to 3 times in most cases.

On a Lycoming, you're likely to see noticeably increased EGTs and slightly reduced CHTs through less hp but also some increased heating of the heads by those increased EGTs.

I probably work on sealing every air leak in the baffling first or build a plenum which seals to the inlets tightly. Reducing timing if you don't need to, just reduces efficiency.
 
Yada, yada ;)

Will somebody please go measure CHT and EGT before and after a timing change?
 
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