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High CHTs? Going into Condition Inspection, Looking for Ideas

msaltzman14

Active Member
Hi All-

I have an RV9A, bought not built. I've had her about 2 years and put 250 hours on so far.

My CHTs are consistently higher than I'd like, but some associates with RV experience have told me they're warm, but nothing to worry about.

That said, I'm starting to not love what it means for performance, let alone potential safety factors or engine lifetime.

On spring days here in Northern California, I climb out at 105 KIAS and get about 420 CHT on my O-320, 1 p-mag and 1 mangeto. When I come back to 65% power in cruise around 7500, I'm seeing temps like 390. Last week I did a red-line (2700 rpm for fixed pitch) test at 8500 and the temps came up to 416.

I've tried to stay under 400 in cruise always, which can mean I'm not going as fast as I want, especially at altitude. I know the Lyc book says 500 and below is fine, but it just feels totally wrong. Even on climbs to high altitudes, I'm reducing power and climbing flatter to keep the temps below 430.

Anyway, I'm going into my inspection this week and working with a local A&P and I wanted to hear what people think about these temps.

In terms of baffling, there isn't any obvious defects.

Really appreciate thoughts here- is this way warmer than most engines? Something to be concerned about?


Mike
 
A PMag could have more advanced timing than the engine needs, which could make it run hot. But, start with the basics. Remove cowl (upper and lower). In darkened hangar, shine flashlight up from bottom of engine. If you can see light inside the baffled area other than around the fins, you have leaks that need to be plugged. Silicone RTV is your friend. Next verify that the baffle seals are actually contacting the upper cowl, and are facing inward (opposing direction of airflow, so that plenum pressure holds them against the inner cowl surface). That may seem self-evident but I've encountered an overheating RV where all seals were facing away from the airflow. Flipping them inward solved that overheating problem.
 
Good advice so far, check baffles, check timing. If this does not get it done, if equipped, pull the carb and drill the jet. My rv6 with a 320 always ran a little hot. I pulled the carb and it had a #42 main jet. I drilled it to #37 and it runs perfect now. I went one drill bigger than recommended due to the port/polish job and 10-1 compression. Most 320’s will need a #38. My fuel flow went from 10gals/hr at takeoff to 14gals/hr. I also changed from a fixed pitch to constant speed prop. The fixed pitch prop will mask the problem due to the lower rpms at takeoff requiring less fuel. As it unloads in the climb, the fuel requirements will increase to more than is available through the jet hence the higher temps.

Here is the link to the document to support the change.

https://drive.google.com/file/d/0B-SOSxyKpCQoMFlmenI4WnY0cXc/view?usp=sharing
 
Good advice on the timing check, also on checking the baffles, and carb. Basics, do these and know definitively what they mean.

Timing, leaks and mixture are the the three huge factors and all can be easily managed/repaired to yield a well performing cooling profile.

Mike, regarding your baffles, it does not take much, and many of those add up quickly. The full speed at 8500 is the most revealing. Speed should provide all the cooling you could need, but it did not.

Leaks, take a 2" piece of soda straw (material does not matter :D) blow through as hard as you can, and feel the blast. Imagine a dozen of these.

Don't think about it, just seal all the cracks, gaps and holes that leak. NO gaps are acceptable. It will make a huge difference assuming the timing and mixtures are in range.

Good luck, do some research and post some pictures for comment. Comments will help tune your sight picture.

When you report CHT, do also report ambient temp too. The cat-ambient delta is key for comparing different conditions.
 
This is a Lycoming cooling chart for the 320. It assumes typical cylinder and head baffling (the wraps) and 25BTDC ignition timing.

I've plotted two examples. In red, the difference between the upper and lower plenum pressures is 4" of water, and air density is standard for 5000 feet. If OAT is (ballpark) 57F, the air at the engine face will be about 60F, so the chart tells us to expect 435F CHT at 75% power and best power mixture (0.51 BSFC). Mass flow is about 1.5 lbs per second.

In green, same altitude, but now 6" deltaP. CHT will be somewhat lower at the same OAT and power setting. Mass flow is 2 lbs per second.

Advanced timing and lean mixtures can push CHT upwards, but the majority of cooling issues are just failures to supply enough mass flow in contact with hot metal.

The Pmag unquestionably contributes to higher CHT. As for mixture, there is no point in drilling a jet until after establishing full rich EGT in relation to peak EGT.

COOL%20O-320%20Cooling%20Air%20Example.jpg
 
This is a Lycoming cooling chart for the 320. It assumes typical cylinder and head baffling (the wraps) and 25BTDC ignition timing.

I've plotted two examples. In red, the difference between the upper and lower plenum pressures is 4" of water, and air density is standard for 5000 feet. If OAT is (ballpark) 57F, the air at the engine face will be about 60F, so the chart tells us to expect 435F CHT at 75% power and best power mixture (0.51 BSFC). Mass flow is about 1.5 lbs per second.

In green, same altitude, but now 6" deltaP. CHT will be somewhat lower at the same OAT and power setting. Mass flow is 2 lbs per second.

Advanced timing and lean mixtures can push CHT upwards, but the majority of cooling issues are just failures to supply enough mass flow in contact with hot metal.

The Pmag unquestionably contributes to higher CHT. As for mixture, there is no point in drilling a jet until after establishing full rich EGT in relation to peak EGT.

COOL%20O-320%20Cooling%20Air%20Example.jpg

Dan is right. You should have 150-200 degs from full rich to peak available with the mixture knob. My engine, with a #42 jet was already at peak. After drilling to #37, i now have 170degs from full rich to peak available. Your engine will never run cool with the stock jet in a carb.
 
The Pmag aggressive timing curve hurts, but since you only have one it softens the effect quite a bit. Is the jumper in or out? Assuming timing is correct, you have enough mixture control, and the intake tubes are not leaking (if they aren't, they will), then you are down to baffling. Many people have no idea what is required. They must be tight. This means more than the rubber against the cowl. This means the factory installed intercylinder baffles are bent to make contact with the case and cylinder bases and sealed with RTV. Cylinder 2 & 3 need the bypass ducts. The inlet ramps need to seal the cowl inlets perfectly. The baffle tin needs to be sealed with speed tape and RTV so there are no gaps. In short, any air that comes in the cowl MUST pass through cylinder fins before it can be allowed to escape. This takes a bunch of time but will pay off.
 
I agree that baffling is the likely issue here. I run aggressive timing and still have 360* CHTs at cruise in warm weather (2700 RPM cruise, LOP) and likely not the best baffling. If I were to advance my timing more at cruise, power would fall off and so would the CHT's (they may spike a small amount before declining). If the red knob is anywhere other than full in, your main jet is not the issue. Titanhawk is correct, that you will need a bigger jet to get good fuel flow at full rich in high power demands, such as low altitude climb. However, once you start pulling the red knob back, you are restricting flow before the jet, so jet size is not an issue. Your main jet only restricts flow at full rich. Generally speaking, once you start pulling the red knob, the mixture control valve is restricting flow, not the main jet. I do concur that you will likely need to drill the main jet to get your CHTs down in the t/o and climb, but it won't help your 8K cruise.

A well baffled 320 should deliver well less than 415* in cruise at 8K'

Many good posts here to search as you undergo efforts to improve your baffling.

Larry
 
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CHTs and leaks

I'm just 10 hours into Phase I with IO360 and CS Prop. My CHTs are running 380 - 390F. they will go 420+ on full RPM climb out. So I'm reading this thread with eye on sealing leaks. I have not used RTV to seal leaks because the few leaks I have are very small especially compared to the two holes I have for tubing that blows air on the mags and the large hole that sucks air for the heater. Are cooling tubes for Mags really needed? Would moving the heater inlet hole to the engine inlet ramp make much difference. I guess I'll tape over these and see.

What are "normal" CHTs on a 70F day at 25x25 square climb at 110kts 1000 - 3000' msl.?
 
Are cooling tubes for Mags really needed? Would moving the heater inlet hole to the engine inlet ramp make much difference.

Don't get me started on blast tube requirements.

As for results, Bobby Looper (then working at ECI-Titan) once posted that a 1" diameter hole would drop deltaP by 1" of water. I have not confirmed it independently, but I have no reason to doubt it. According to the Lycoming cooling charts, every inch counts. Two or three inches deltaP is a Big Deal.

Moving the inlet to the ramp would make no difference. It's still a leak, trying to equalize the upper and lower pressures.
 
Was thinking about this thread yesterday while flying around NorCal. Pic below was taken a couple of minutes or so after I leveled off, leaned out fully, etc. Eventually the highest CHT dropped to 330?F. This is a roller cam Superior IO-360, parallel valve, 8.5:1 compression ratio, running P/A Silver Hawk injection and dual LSE Plasma III ignitions timed per LSE specs, turning a Hartzell B/A prop. About 800 hours on the combo now. If my baffles were any better sealed they would be water tight. It took a lot of time. Big payoff though. I have to work to get CHTs to 400?F except on the hottest days, and they never get close to that in cruise. Actually my cooling airflow is probably too good. Could probably pick up a few knots with a little restriction. Point of the above being to add a data point.

i-TTQTppg-XL.jpg


I added this crude half-round shape to smooth exit flow over the engine mount and make the exit more "RV-8 like." Was worth a few degrees, though I don't have exact numbers.

i-m9PWZr9-XL.jpg
 
Don't get me started on blast tube requirements.

As for results, Bobby Looper (then working at ECI-Titan) once posted that a 1" diameter hole would drop deltaP by 1" of water. I have not confirmed it independently, but I have no reason to doubt it. According to the Lycoming cooling charts, every inch counts. Two or three inches deltaP is a Big Deal.

Moving the inlet to the ramp would make no difference. It's still a leak, trying to equalize the upper and lower pressures.
In your opinion; P-Mag blast tubes, Yea or Nay???

:cool:
 
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CHT normal Temps

Lars
Thanks for posting the photo. Looks like you are at 8,500 feet. I don't see AOT, but at that altitude, and MAP of only 21.5 and RPM at 24.5, I would expect your CHT temps to be below 350F. It is a good data point to compare with. But I don't think this is a data point that is pushing on temperatures.

I'd be very interested to see a low altitude high MAP/RPM combo that is a challenge for maintaining low temps. Here is an image at 1300 ft; OAT 53F;
25 squared and CHTs are 407, 401, 370, 383. Also no wheel pants. Is this high? Normal?

2019-04-15%2018.13.58.jpg


thanks
 
Lars
Thanks for posting the photo. Looks like you are at 8,500 feet. I don't see AOT, but at that altitude, and MAP of only 21.5 and RPM at 24.5, I would expect your CHT temps to be below 350F. It is a good data point to compare with. But I don't think this is a data point that is pushing on temperatures.

Remember, cooling is about mass flow. Given equal power and velocity, cooling at altitude is more difficult, due to reduced air density.

I'd be very interested to see a low altitude high MAP/RPM combo that is a challenge for maintaining low temps. Here is an image at 1300 ft; OAT 53F; 25 squared and CHTs are 407, 401, 370, 383. Also no wheel pants. Is this high? Normal?

High..but look at the speed! You're only going 139 TAS. Dynamic pressure rises with the square of velocity.

140 KTAS@1000 MSL Q=12.7"
165 KTAS@1000 MSL Q=17.75"

Difference is 5.75". Even at a so-so 70% conversion to increased static pressure above the cylinders, an 18% speed increase would boost cooling pressure by 32%.
 
High..but look at the speed! You're only going 139 TAS. Dynamic pressure rises with the square of velocity.

140 KTAS@1000 MSL Q=12.7"
165 KTAS@1000 MSL Q=17.75"

Thanks for the quick reply Dan. I don't have the wheel pants nor leg fairings on yet, so I suppose I'll get more speed when they are on.

thansk
 
Lars
Thanks for posting the photo. Looks like you are at 8,500 feet. I don't see AOT, but at that altitude, and MAP of only 21.5 and RPM at 24.5, I would expect your CHT temps to be below 350F. It is a good data point to compare with. But I don't think this is a data point that is pushing on temperatures.

I'd be very interested to see a low altitude high MAP/RPM combo that is a challenge for maintaining low temps. Here is an image at 1300 ft; OAT 53F;
25 squared and CHTs are 407, 401, 370, 383. Also no wheel pants. Is this high? Normal?

2019-04-15%2018.13.58.jpg


thanks

Steve, the OAT reading is on the left side of my display- PFD side- upper center. 53?F.

Here?s a photo I took a few minutes ago. 1500? MSL, ~25 squared, about 40? LOP, OAT 68?F. I had been in level flight for about 15 minutes when the photo was taken. I can?t share the photo directly using the app on my iPhone but here?s a link. Will fix later. (fixed).

i-kmbs6Qm-XL.jpg
 
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Normal CHT

Lars
Nice photo. Thanks for sending. Great data point for comparison. Those CHT temps look pretty low -- below 300F. I'll put my wheel pants and leg fairings on and do this again and see if I get such low CHTs under the same conditions. Nice to have this data for comparison. Thanks
 
normal chts

Carl
Thanks for that data and info about seemingly lean mixture. I'm at 10 hours after tonight's flight and need to change oil and inspect everything. I better re-check cable connection of mixture. Perhaps I'm not getting full rich on the cable. I did notice a slight backfire, pulling power at the numbers to land, which also indicate a lean mixture.

thanks
 
Scrap the Vans stock rubber cowl material and install McFarlane Cowl Saver baffles. One hour job. Make them continuous across each side and rear, three pieces total, formed to the top cowl.
 
Thanks for all of this feedback fellas, very helpful. I'm going to go over it with my A&P tomorrow before we start the work.

One thought is that I recently had a 2" diameter intake put into the left front of the cowl for a second heat muff last winter when I was living in the northeast. Perhaps that is robbing a good amount of ram air, and should be covered up in warmer months?
 
There are multiple posts from several individuals with good advice on baffling, etc. All worth investigating for the original poster. However, the original post does not provide info on what the mixture setting is while seeing the posted high CHTs. Yes, correct baffling and sealing leaks are very critical to keeping CHTs under control but so is running the engine with a full awareness of what the mixture is doing to the performance of the engine. Running full rich, 100 deg ROP, 50 ROP, PEAK, 10 deg LOP, 25 deg LOP will all produce different CHT temps.

Someone mentioned that it is important to provide OAT when discussing engine temps. It is also valuable to report mixture settings when reporting. Everyone posting advice is presenting the best info they can given what information is available. However, generalizations will be the best advice to give (i.e. checking baffling for leaks, etc.) without all the facts.
 
Thanks for all of this feedback fellas, very helpful. I'm going to go over it with my A&P tomorrow before we start the work.

One thought is that I recently had a 2" diameter intake put into the left front of the cowl for a second heat muff last winter when I was living in the northeast. Perhaps that is robbing a good amount of ram air, and should be covered up in warmer months?

I have one heat muff (with enhanced hx) and dropped the hole size to 1". It works pretty well at lean conditions and down to zero F. Two 2" holes is massive.Try blocking one and restrict the other.
 
Carl
Thanks for that data and info about seemingly lean mixture. I'm at 10 hours after tonight's flight and need to change oil and inspect everything. I better re-check cable connection of mixture. Perhaps I'm not getting full rich on the cable. I did notice a slight backfire, pulling power at the numbers to land, which also indicate a lean mixture.

thanks

I had to turn out my mixture adjustment on the linkage 3-4 full turns to stop the popping, but it was cold, 25-35 F ambient. This adjustment only affects to maybe 2000 rpm, though so not when flying, that is the purview of the red knob.

If you find a slight stumble on TO with throttle advance then an explosion of power, it is the linkage mixture adjustment.
 
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