Engine Cooling Revelation

swpilot3

swpilot3

4-Place Kit Builder
Hi Everyone. I've been doing some testing of various configurations to try and help lower my CHTs. I've recently had a breakthrough so I thought I'd share.

First, a little background. I'm out in Arizona and in the summer I've struggled to keep my CHTs at anything reasonable. I have an O-540 turning 2700 rpm making 260hp. After reading Mike Busch, I've adopted 415F as my "do something now to fix it" temp even though Lycoming lists 500F as the max temp. To be honest, I'd be elated if I could keep them below 415.

Up until now, I haven't done a great job documenting with concrete numbers. I've read forums, talked with people and implemented what seemed to make sense. I double-checked all the baffeling. I increased my outlet area, then increased it again, then increased it again! I placed a lip at the front of the outlet area, then made that lip bigger. I added a "fence" from the cowl lip to the tunnel. I added a small 1" aluminum round (don't know what to call it, if anyone can tell me, I'll edit this description.) at the bottom of the firewall, at the front of the tunnel. Then I made it bigger. With still unacceptably high CHTs I decided to add louvers. I decided to either go big or go home. So I made large louvers on the sides of the cowling based on lots of reading.

After all of that, I could sometimes manage the CHTs... Until the summer days came.

So I decided to get scientific. I had a homemade manometer, but it's hard to read and bounces around a lot. So I found a monometer on Amazon. https://www.amazon.com/gp/product/B07K7HT3XJ/ It's not too expensive and measures the same as my homemade one. However, this one has an average feature. Which is perfect for our use. Hold an airspeed for a few minutes and look at the average reading.

For my baseline, I used the current configuration with a 1.5:1 outlet to inlet ratio, a 2" lip, fences, large firewall round, and large louvers.
At 92 KIAS it was pulling 3.50 inches of water.
At 105 KIAS it was pulling 5.10 inches of water.

I found that the size of the firewall round didn't make any difference at all. However, what was interesting was that my louvers were making the differential pressure worse. With the louvers removed:
At 92 KIAS it was pulling 3.69 inches of water.
At 105 KIAS it was pulling 5.15 inches of water.

So now on to what I discovered. I decided to replace the cowl opening and lip with a one-piece fixed cowl flap. Same exact outlet area.
At 92 KIAS it was pulling 4.20 inches of water.
At 105 KIAS it was pulling 5.70 inches of water.

For my installation, a large cowl flap seems to do a substantially better job of cooling. Especially at lower airspeeds. At a cruise yesterday at 2500ft, 23 squared, with 108F OAT, this configuration lowered my CHTs from 430F to 380F. This substantial!!! On the descent, the CHTs were falling to around 310F.

Since this fixed cowl flap is proof of concept, now I'm going to build a proper cowl flap that I can control.

If you're dealing with high CHTs with a big engine, don't take this as my recommendation on what to do. Just take it as a datapoint while you research solutions. YMMV. Figuring out what the air is doing inside your cowl is difficult. However, with my particular installation, this seemed to work!

IMG_3602.jpeg For reference, you can see the louvers that I had before (that only hindered) and the fixed cowl flap. Please excuse the dirty airplane!
 
What a fantastic post Bobby! Thanks for the data and please keep us updated on the progress.

For what it's worth, here's what Jimmy Tubbs had to say in his very good article on the topic:

..Lycoming Engine Installation Manual does specify that the O-320 engine requires 5-1/2 inches of water while the O-360 engine should have 6-1/2 inches
pressure drop for good cooling...
 
Last edited:
Hey Bobby thanks for the post, in my opinion this is what EXPERIMENTAL aviation and all about. Solving problems and you knocked this one out of the park!!! I believe this info can be applied to any airplane with an ICE engine.
 
Thanks Davz! It was a fun process. Especially when you have an "Ah-ha!" moment!

Thanks, Jared. I'll go back and re-read that article. I do get around 6-1/2" at a fast cruise. I'm gonna do some homework and see if I can find that same chart for my engine.

After I build a proper cowl flap, I'm planning to build a plenum for the baffling, just to rule out any issues with that.
 
Last edited:
Good stuff, I'm in the process of building a new lower cowl, hoping for better cooling along with some other mods I'm doing. Did you consider trying different size inlet openings? I've been thinking smaller openings in the nose bowl might be helpful. I see these tiny round inlets on fast planes, and maybe they only work well going fast, but they don't seem to overheat on climb out. We can only go so big on the outlet area so another way to get the pressure differential might be to go with smaller inlets. I'm going with the idea that air movement is more important than air volume.

Once I get mine back in the air if I don't see the results I'm looking for I'll try changing the inlet size and shape but I figured since you are actively testing this sort of stuff right now I would suggest it.
 
I've noticed the same thing. It's an interesting point. I read a pretty good AOPA article about cooling and inlet size. Here: https://www.aopa.org/news-and-media/all-news/2006/june/pilot/airframe-and-powerplant-(4) It says the smaller inlets were done on more modern aircraft to reduce cooling drag. They were able to do this because they've become better at optimizing engine cooling. So it doesn't seem that a smaller inlet actually aids in cooling. They were just able to get away with it because they were able to cool by other means... redesigned cowling, modified cylinder fins or placing the inlets farther away from the prop.

"Minnis says the biggest reason for the reduction in the size of cowling air inlets is an improved understanding of the effects of the propeller hub and propeller spinner on airflow. It's now known that the airflow in this area is very turbulent and greatly disturbs the orderly flow of cooling air. Current designs position the cooling-air inlets away from the propeller hub area."

Additionally, the first couple inches of the propeller doesn't create any lift, so it just blocks airflow. If the inlets were place farther outboard, then perhaps they would be behind the lift generating portion of the prop and that could be beneficial.

Oddly enough, the author seems to insinuate that cowl flaps aren't needed in modern designs. Maybe I need to rethink my directon.

The other concept that I keep reading about is that cooling is a function of time. It takes time for the heat to transfer from the cylinder to the cooling air. If a small volume of air is going by the fins very fast, it's not as effective as a larger volume of air going by slowly. Obviously, too slow and it's ineffective. There's a sweet spot. That's why I think the Lycoming installation manual calls out a specific volume at a specific pressure.

Tony Bingleis (old school but tried and true) recommends multiplying your HP by 0.35 to determine inlet size. So for my engine that comes out to 260 * 0.35 = 91sq in. The BH has about 84sq in of inlet.

So perhaps, if the nose bowl was going to be altered, the best thing to do is move the inlets outboard, away from the spinner. Then you can decide if you want to play with making the inlets bigger or smaller.
 
Thanks for that info Bobby. I'm just a little east of you in NM. Figures I will be doing my initial break-in and Phase 1 in August, I hope. I may need some of this. I think I have the same engine, Barrows carburated O-540. I might add one other little nuance that can have an affect that is counterintuitive. I retired from the petro chemical industry with some practical experience with heat exhangers. One thing that took me a long time to accept was there is an optimum velocity for heat exchange. Most of us believe more and faster is better. There is a point that too high a velocity actually has an opposite effect. I've seen first hand where moving the rear baffle wall back to create more clearance and slow the air past the rear cylinder helped a lot. That is only helpful if a particular rear cylinder is your temp problem.

Maybe we can visit in fall??
 
Hi John. That's good insight. Absolutely, I live at an airpark on the east side of Phoenix. Fly out when you get your plane going.

As a matter of analogy, I recently had a pool built in our backyard with a spa. After it was all done the pool builder was programming the app that controls the spa. He said one of the big advantages of a variable speed pool pump is that while the spa is heating, he can program the pump to run at a very low speed.

Counterintuitively, this actually heats the spa faster. This allows the water that is being pumped through the heater time to pick up more heat, then be delivered to the spa. If we ran the pump at full speed while the spa was being heated, it would take longer. Fascinating! This is all starting to make sense now!

That's also a good point about moving the back baffle wall. I hadn't thought of that. I might try that also. How far did they move the back baffle wall?
 
Last edited:
I'll double check. I've been hitting my head on the plane a lot lately and not sure about my memory. I'm pretty sure it was under a 1/2". Doesn't take much over 6 or 8 inches to slow things down.
 
swpilot3 said:
As a matter of analogy, I recently had a pool built in our backyard with a spa. After it was all done the pool builder was programming the app that controls the spa. He said one of the big advantages of a variable speed pool pump is that while the spa is heating, he can program the pump to run at a very low speed.

Counterintuitively, this actually heats the spa faster. This allows the water that is being pumped through the heater time to pick up more heat, then be delivered to the spa. If we ran the pump at full speed while the spa was being heated, it would take longer. Fascinating! This is all starting to make sense now!...
Click to expand...



This is the same principle that allows a liquid cooled engine to often cool better (run cooler) WITH a thermostat than without. The old farmer trick with an overheating tractor of removing the thermostat often doesn't work anymore.
 
Removing a thermostat works if you replace it with an orifice. This has worked well In race cars over the years. Start with a 3/8 - 1/2 hole and drill as needed.
 
yup, gotta slow down how fast the water moves through the radiator, orifice does that pretty well too, just takes more experimenting to get the right size.
 
Just checked with my mentor on the exact measurements of his rear baffle move. On an IO-390 in a Carbon Cub, he fought high CHT for two summers. He moved the rear baffle wall back 1/8" on that one and hasn't looked back. He has a Murphy Rebel with an O-320 and on that one moved it back 2" because he had room. Fixed high CHT's on #2, and #4.

That all the practical data I can muster. He mentioned using spacers with ALUM pull rivets till you get the results you desire, then go solid rivets. Hope this somewhat helpful. Back to swaging aileron cables for final rigging.
 
Awesome info and interesting discussion.

I think it’s very interesting that your louvers made things worse. While not the same, my cowl flaps are positioned on the cowl cheeks like your louvers and they significantly aid in cooling. Opening them can take my CHTs from 410 and rising to a steady 370ish.

While velocity (residence time) does play a role in heat exchangers I not sure doing things to alter the air velocity will make a significant difference in our situation. I know I’m stating the obvious but cooling or heating a liquid isn’t the same as air cooling an engine. But discussing similarities does help develop ideas.

I like having cowl flaps. Sure you can get along without but in some climates that means having a hot engine in the summer and/or a cold engine in the winter. I can’t flying my airplane in the summer without the cowl flaps open during some phases of flight. During the winter if I leave cowl flaps open my engine is way too cold.

Probly worth noting the Conti IO360 has a reputation for running hot and being hard to cool. I haven’t found that to be the case and think it probly gained that reputation in the turbo applications and the rear engine of a Cessna 337.
 
Quick update. While I'm waiting for materials to arrive from Aircraft Spruce to build my cowl flap, I decided to make my fixed one bigger. It now opens 7" rather 4". Add this to the tunnel and it now opens a full 10". It feels ridiculously huge! However, flight tests revealed that it improved things a little more.

Big Cowl Flap:
At 92 KIAS it was pulling 4.45 inches of water.
At 105 KIAS it was pulling 5.90 inches of water.

This is a big improvement seeing as how I started at:
At 92 KIAS it was pulling 3.50 inches of water.
At 105 KIAS it was pulling 5.10 inches of water.

I think there's more work to be done figuring out what helps to cool CHTs, but at least now I can fly my plane in the summer!
 
Hey Bobby, in regards to your louvres, they might work better you swapped the "aspect ratio". Yours are narrowand long with respect to the direction of airflow. You might have better luck if they were wide and short instead. Most of the louvres I have seen for this application are wider than they are long.

I also remember reading a while back about closing off part of the inboard part of the cowl opening. If it is too close to the spinner, it can function more as a leak than an intake. In other words, without the ram effect, air can more easily spill out of this area, reducing pressure under the cowl.
 
I’ve owned two 0-320s, a 150hp Model on a 172 and a 160hp on an RV-9. I fought high CHTs on both, particularly on the rear cylinders, especially the right rear. I tried a bunch of things: baffle seals, caulking all holes holes and cracks, forming better flow behind the spinner inside the RV9 cowl with foam, and nothing really worked. I understand the parallel-valve 0-360s are similar in terms of cooling needs, at a slightly larger scale.

I also have read of guys who moved the rear baffle back, giving 3/4 “ of space behind the cylinder and they swore it cured the issue. I went to local experts for help in doing this mod, and they all refused, saying it was non-sensical, that I needed to squirt cold intake air at high speed past those cylinders. I no longer own either plane, but I sure wish I had done the mod on the RV9, so I could better judge whether it was worth doing on my Patrol. I may go with a plenum, and if my building skills allow, I will try to find a way to make the rear baffle position adjustable without tearing the whole thing apart.

Some other thoughts I’ve been pondering: I wonder if vortex generators or other “turbulators” inside the cowling might be of use in mixing and slowing down the airflow inside the cowl. Bernoulli also suggests a plenum with more volume on top of the engine would provide higher pressure. Most cowls do the opposite: small volume on top and large volume below. I’m interested in any opinions or experience on this topic.
 
When Roy Lopresti took over as chief aerodynamicist for the Grumman AA-5 Traveler (developed into the AA-5A Cheetah and AA-5B Tiger), first thing he did was put a bunch of tufts on the cowl, including the inlet area. Pictures showed significant "reverse flow" where the air was flowing from inside the cowl toward the outside. Uh oh! Moving the cowl inlets outboard helped, and making them smaller helped as well. In the end, they also increased the size of the "exit area" at the aft lower cowl, and that was the final piece of the puzzle at the time. Nowadays, a lot of Grumman owners with "less than perfect" baffling use a "spoiler lip" in front of the lower cowl opening, which prevents upflow through the bottom of the cowl in high AOA situations. Seems to help a LOT! Perhaps applicable to the Bearhawk line, or perhaps not...
 
I thought I had pictures but I haven’t found them yet. If I do I’ll post them.
 
Great post.

I was surprised to see the louvers were making it worse!! Very surprising.

Our cowl flaps have proved very effective, as Whee said - we leave ours closed most of the time, but wide open on hot summer days. Our new cowl flaps are over 140 sq.in when fully open.
It's the opening area when the tunnel meets the firewall which matters most.
Having a smooth tunnel lip makes a big difference too.

We have the same engine as yours.
 
Haven't got to this point yet but have a question re use of cowl flaps. So instead of louvers or the weight of adjustable cowl flaps, would fixed and adjustable cowl vents like on Carbon Cubs be a viable option? The Carbon Cub has cowl cheek vents on both sides that have four fixed positions to choose (four season adjustments). I wont know till I put her on the scales but I've attempted to build light. If I encounter cooling issues I would want the lightest fix that delivers the needed results.
 
Excellent commentary everyone! Thank you for all the great additional information!

I’m not sure why my louvers didn’t work. Perhaps they were just poorly designed like svyolo pointed out. I’ve never seen louvers as large as mine, but I figured that louvers were usually smaller to minimize cooling drag. I researched the optimum angle for a louver and made them very large to penetrate the boundary layer and see if they made a difference or not. Perhaps I exceeded some diminishing return point.

Carl gave me the idea to put a scoop on top of the cowling that fit into the oil access door... just to see if it made any difference. It added 10.6” of additional inlet. It resulted in 0.23” less differential pressure and my CHTs seemed to be a few degrees hotter on average.

I’m beginning to feel a little like Thomas Edison. I’ve found 1,000 ways not to cool an engine! :-)

Without scoops or louvers, the only place that air can come from is the inlet to the tunnel outlet. My hypothesis is that air wasn’t going the direction I intended with the scoop or louvers. I did put tell-tale yarn around each. But it was so chaotic that it was really hard to tell what was happening.

Here’s the video: https://youtu.be/fcFGrSyavjY
Kinda makes you wonder how chaotic the air is as it enters the cowl through the normal inlets!

IMG_3614.jpeg
 
I think that would be fine as long as you're careful and plan power reductions well. Yesterday my wife and I flew to Sedona. I kept the CHTs at 380 in cruise but from the downwind through final with the power reduced the CHTs cooled to 340. I don't like them cooling that fast. In that particular instance, I would have wanted to close the cowl flaps from the cockpit. Mine is still fixed at the moment.
 
Last edited:
Thanks Bobby. That is a good point to consider. I'm a student of Mike Busch too. I need to go back and review if there was mention of rapid cooling of CHT's.
 
swpilot3 said:
Excellent commentary everyone! Thank you for all the great additional information!

I’m not sure why my louvers didn’t work. Perhaps they were just poorly designed like svyolo pointed out. I’ve never seen louvers as large as mine, but I figured that louvers were usually smaller to minimize cooling drag. I researched the optimum angle for a louver and made them very large to penetrate the boundary layer and see if they made a difference or not. Perhaps I exceeded some diminishing return point.

Carl gave me the idea to put a scoop on top of the cowling that fit into the oil access door... just to see if it made any difference. It added 10.6” of additional inlet. It resulted in 0.23” less differential pressure and my CHTs seemed to be a few degrees hotter on average.

I’m beginning to feel a little like Thomas Edison. I’ve found 1,000 ways not to cool an engine! :-)

Without scoops or louvers, the only place that air can come from is the inlet to the tunnel outlet. My hypothesis is that air wasn’t going the direction I intended with the scoop or louvers. I did put tell-tale yarn around each. But it was so chaotic that it was really hard to tell what was happening.

Here’s the video: https://youtu.be/fcFGrSyavjY
Kinda makes you wonder how chaotic the air is as it enters the cowl through the normal inlets!

Click to expand...

I think your louvers being so large were doing a lot of what you saw that scoop do, create a lot of turbulent flow. You want laminar flow over the louvers in one direction at high speed to create lower pressure outside them. If the flow over the louvers is turbulent it's being slowed and won't have the lower pressure. It's possible it could have been spilling over those and going in! Given your data, I'd say that is likely. Look at the louvers on the 206s and 207s. Sould give you a good idea of what it should look like. With the cowl flaps closed, the louvers are almost the only outlet, so they are very efficient.

At the inlet, ideally the flow would also be laminar. That's why you've seen modern cowls go to more round and gradually receding inlets. No sharp corners or edges. They all did wind tunnel testing with tufts I'm sure to get a laminar flow into the cowl.

If you really want to see what's going on, put tufts on the inlet and then inside the cowl and on the fins, place a gopro inside.
 
Last edited:
That makes sense. Originally before I built those louvers I used some that I purchased from Aircraft Spruce. That was before I was keeping good records of numbers, but they didn't seem to help my high CHT problem.

I looked at some pictures of 206/207. They have, for lack of a better term, inverted louvers. They go in rather than out. Does anyone have good insight into if perhaps that's better?

image_7394.jpg
Here's the one I purchased from Spruce.
 
Last edited:
I have noticed a few pictures with inverted louvers as well. I have no idea which is better. Even inverted they might provide some negative pressure due to "motive flow", if that is what they call it.
Try 'em both ways and see which works better.
 
I've seen them both ways. But all the Cessnas and Beeches have them as you describe. I just went and looked real quick and even the King air has them "inverted" vs what you did.

Here's a photo of a Bonanza cowl.

Click image for larger version Name: page7.jpg Views: 0 Size: 282.8 KB ID: 55563

Cessna 205
440px-Cessna205C-GHOR03.jpg
Cessna 180
ff7f6cd18c7122a9423dfd2a.jpg
 
Last edited:
Carl gave me the idea to put a scoop on top of the cowling that fit into the oil access door... just to see if it made any difference. It added 10.6” of additional inlet. It resulted in 0.23” less differential pressure and my CHTs seemed to be a few degrees hotter on average.
Click to expand...

The scoop might be reducing the dynamic pressure from the normal air intake by feeding air to the rear of the cowl area.

Neat what you’re doing with the experimentation. Keep posting results !
 
I tested moving the rear baffle wall back. I moved it back 1/2". It certainly has a very quantifiable effect! Cylinders 5 and 6 used to be the hottest, now are my coolest! They dropped by about 30 degrees! However, the other cylinders suffered a bit. Number 2 is now the hottest and 20 degrees hotter than it was before. Numbers 2 & 3 are also 10 degrees hotter than before. Number 1 is about the same.

The differential pressure went way down with this setup.
92kias = 3.82" (0.63" less than before)
105kias = 4.97" (0.93" less than before)

So, it appears that moving the baffle wall really does help cool the back cylinders better but at the expense of the other cylinders. Makes sense. I just overshot a bit. I'm going to try a 1/8" gap now. I read on a few RV forums that they just put a washer between the cylinder and the baffle and that did the trick for them.
 
  • Like
Reactions: Nev
Something from my non-aviation distant past. If I remember right for a manifold to function as a equal pressure manifold the manifold needs( at an absolute minimum) to have double the volume of the sum of the outlets. If not, you end up with each outlet having different pressures/flow rates. The bigger the manifold compared to the outlets, the better. Without that you end up having to control the flow to each outlet to equalize the flow rates, if that is a requirement. It was for what I was trying to fix.

I am not sure that helps you fix it, but it might help to understand the problem at hand.
 
Ran across this today with the Van's louvers. They are also completely opposite of what you have.

You should flip those louvers around so they are oriented like all the other stuff out there and take a measurement.

Click image for larger version Name: cat-med_cowl-louver.jpg Views: 0 Size: 112.5 KB ID: 55615

Click image for larger version Name: FP19082019A0001F.jpg Views: 0 Size: 38.7 KB ID: 55616


Here's some on a bearhawk mounted what appears to be the correct way.

100_8944.jpg
 

Attachments

  • cat-med_cowl-louver.jpg
    cat-med_cowl-louver.jpg
    112.5 KB · Views: 0
Last edited:
This is pretty interesting. So I would assume that the cowl under the engine would be the "pressure manifold" in this case? Did I understand that right?
 
Thank-you! I do plan to flip the louvers to see what they do. I've already patched and riveted where they used to be, but I think it's worth investigating further.
 
Follow up to this test:

I reduced the gap to 1/8". Cylinders 5 & 6 are still the coolest. So, this gap behind the cylinders absolutely makes a difference for them! The smaller gap increased differential pressure and as you would imagine, it evened out the temps on the other cylinders.

92kias = 4.20"
105kias = 5.40"
 
Cylinder #2 is still the hottest... however, I noticed it flucuating about 20 degrees after landing. I think that thermocouple has gone bad (I've had this before). I ordered a new one. It might not be as hot as its indicating.
 
Yeah too much exit (flow between cylinders) volume are compared to manifold volume. We are stuck with that ratio as we are not going to build a bigger cowling. I know RV-10s and Rockets have a couple of little tricks to equalize cylinder cooling but I can't remember what they are at the moment.
 
I just saw a couple of other aircraft that had "inverted" louvres. My guess to try what works better (right side up or upside down) might not be just a bit of jest.
 
My plans now are to re-build the fixed cowl flap into a controllable one. Then, in no particular order, I'm going to try inverted louvers, build a carbon fiber plenum for the baffling, and putting a go-pro inside the top cowl to watch the inlet tell tales.
 
I was looking at various pics to see how different GA companies dealt with engine intakes and cooling. I looked hard at the Mooneys as they are fast not because they are overpowered, but they are low drag. Most of the newer Mooney's look like they intake all or most of their air from the two cowl cooling intakes. There are a couple that have a large NACA duct on one side. I am guessing that is either for an oil cooler, or possibly a turbo intercooler. I am talking specifically the 201-231-252 models. I haven't looked at the really new ones. The 252 in particular is supposed to cool really well, including the turbo intercooler.

I am kind of wondering if the problem sometimes is leaking air from the air intake system into the lower cowl. It seems like we always concentrate on what is going on above the engine for leaks, but It seems to me leaking air into the lower cowling from outside is also a problem.

Bobby's louvre dilemma might add credence to this. For some reason his louvre design was drawing air in and reducing the pressure differential from the top. Wouldn't a leaky carb air intake scoop do the same?
 
I've had that same concern in the back of my mind. On mine, I have silicone baffling material sealing against the filter frame. I will take a good hard look at it and see how well it's sealing.

As a very interesting side note, during my cowl flap design modifications, I ended up with a 2"x6" gap at the leading edge of the cowl flap on the bottom of the cowl. I was going to seal it but decided to put some yarn tell tales around it and see what the air was doing.

What would you think? Would air be drawn out of the cowling? Or would it be sucking air in? Take a look at the video for the answer, not what I expected. https://youtu.be/Yw0g1niN9lo
 
Last edited:
I would say too many variables for me to take a guess. I would go the same path as you and experiment.
 
Which way does your the baffle material on your air filter bend? I would think it needs to bend forward so the pressure seals it. Maybe some Gorilla tape (or a pop rivet or two) to make sure it can't move?
 
It's sandwiched between the filter bracket and the cowl. So the pressure does seal it. Taping is a good idea. I'll take some differential pressure measurements before and after.
 
Could you not just install the cowl flap so that the material that is used to rivet it to the lower cowl is inside the cowl instead of outside?
 
I read this thread a while back and was impressed with Bobby's work, and only today got around to checking the pressure differential on my cowling. I didn't have any cooling issues as such, those were ironed out during the phase one.

The main take away for my results are that my pressure differential is less than Bobby's, and probably suits our more temperate climate whereas Bobby was dealing with very warm ambient temps. But I have two cabin heaters and barely use one in the middle of winter, so I wondered what would happen if I removed one. Would it increase the pressure differential enough that the CHT's would run lower, and I could then run closer to peak EGT (from LOP), thereby allowing an increase in speed? It appears that it might work. It'll also save a couple of pounds in weight.

Removing one heater increased the differential by 0.5", and already I could notice a small reduction in CHT. Closing the oil cooler butterfly valve also increased the pressure differential by another 0.5". I also checked that my cowl vents are contributing to an increase in pressure differential as they should. They are. The vents all lie in the low pressure side, and are lowering that pressure further by approximately 0.5" each. When I taped over them, all the temps got hotter. But the oil cooler and cabin heaters come from the high pressure side, and those had the greatest effect.

I might look at reducing the oil cooler inlet for both settings, or even repurposing the now unused spare heater control to the oil cooler valve, because it has a pronounced effect not just on the oil temperature but also a 20% effect on cowling pressure differential.

My Manometer didn't have a restriction in it, so some data smoothing was required. I tested two power settings, 22"/2200rpm/38lph and 26"/2200rpm/42lph.

Click image for larger version Name: 9B1CECA2-3275-4949-8A1F-A607F0182015.jpg Views: 0 Size: 195.3 KB ID: 79771

Click image for larger version Name: 9B1CECA2-3275-4949-8A1F-A607F0182015.jpg Views: 0 Size: 195.3 KB ID: 79771

Click image for larger version Name: 9B1CECA2-3275-4949-8A1F-A607F0182015.jpg Views: 0 Size: 195.3 KB ID: 79771

Click image for larger version Name: 9B1CECA2-3275-4949-8A1F-A607F0182015.jpg Views: 0 Size: 195.3 KB ID: 79771
 
Last edited:
Thread drift.......

Just to add to this in regard to the heater muff. I rang Clint at Vetterman exhaust this morning. My question was if I remove the shroud from the heat muff, can I then run it or do I need to modify the underlying exhaust. Clint advised that it can now run it as it is with the shroud removed. He did mention that the stainless shroud does have to be removed otherwise with the airflow disconnected the heat muff can overheat. The photo below shows what is underneath the shroud.

0271F75D-AB09-46B4-A6A0-DE5FA2C4F84C.jpg
 
My CHTs were pretty high so far (420+ in high Pwr cruise on #6, #1 got to the same in a climb). Slow flight was short lived due to engine overtemping, etc. on and this was all at below freezing temperatures outside. Even one flight at zero F. My last flight showed a decrease of 40 degrees after several mods. I added a 2” lip, sealed the cowl hinge lines with baffling, added a round fairing at the firewall to tunnel transition, and replaced my oil inspection door with one that seals better. I also secured the baffling rubber in a couple places where I wasn’t sure it was sealing. I can now do the test flights that require sustained high power. Victory!
 
This has been a great thread posting.

Mother nature figured it out. While not for cooling, certainly efficient flow. This Great White has been around and seen some things.
9F44A7D1-3AA7-4AEB-9F37-448CEDA298CA.jpg
 
I have been fighting a hot cylinder (#6). I have tried everything but moving the baffling on that cylinder back, and it is bumping up against the cylinder with no clearance. I bought a digital manometer, $35, off amazon and flew with it today. I get a bit over 5" in 110 KIAS cruise, and 3 at 80 knot climb with cowl flaps closed. The cowl flaps give me an exter .7-.8" of pressure diff.

Temps are moderate, 70-75 F.

Cruise temps have always been fine, most are 330-340. #1 has always been a bit too cold. The whole left bank of cylinders runs warmer on climbout than the left, with #6 being the hottest. A bit <420 F today with cowl flaps open. Flaps are 20" Sq each.
 
For what it’s worth the pattern is similar on my engine. Over 400 on #6 and low-mid 300s on #1. I’m blaming the big 4” hole to my oil cooler behind #6 for much of it and am planning a significant cowl overhaul this winter to improve the situation.
 
Hey John, can you see any difference on your manometer if you position the pickup over the specific cylinders? Like I wonder if the airflow just isn't making it back to the corner, or like in Ken's case, if it isn't moving down between the fins.
 
I measured it only above the hot cylinder. Easy to move. I have the RV-10 oil cooler setup taking the air from behind #6 as well. #1 is always 70 F colder than the rest. I tried blocking as much air as I could, but the air really, really wants to go there.
 
My whole setup is pretty "stock" except EFI. Vans baffles, oil cooler mount, Vett exhaust. 1.5" lip at 30 degrees. Cruise is fine all around. I think I have enough air all around even in climb, it just isn't going to the cylinders evenly. I can easily add mixture, and take out ignition advance which had no affect. it is just uneven air distribution. Oil temps 210 in the climb, 185 in cruise.

The cowl flaps help. Maybe I should try just actuating the one on the hot side, and leaving the other one closed. They are on the cheeks like a couple of others have done.
I used red silicone baffle material. Interestingly when it rubs the cowl doors, it turns black. You can verify it is sealing along the whole length. The aluminum mating surface gets polished clean.
 
Last edited:
I am curious about something. The baffle cylinder wraps get tied together under each cylinder. Two halves meet leaving a gap for air to escape as it exits each cylinders cooling fins. That gap is like a throttle. If the gap is opened up on the hot cylinder it ought to cool a bit better of course. And that part of the baffle is so hidden from view it does not get our attention. I wonder if you have measured them ....if each gap is the same....and I ask because I went back and adjusted mine to help even out the CHT's.

I also am curious what your pressure pickups look like. I suspect four Piccolo Tubes - the advice I recieved was 1/4" OD tube with four very tiny holes drilled every inch. They are long enough to span the entire bank of cylinders....one tube for the left bank, one for the right. The two top piccolo tubes are connected via a tee, then the same on the bottom of the cylinders.
 
Last edited:
Someone mentioned on one of these cooling threads that silicone is not that great for baffles seals. Is the neoprene impregnated fiberglass better?
 
McFarlane Aviation has a baffle seal product called Cowl Saver. It expensive, but its not neoprene and its stiff, yet scored so it flexes where it needs to conform. IT works really well.
 
You must log in or register to reply here.
Back
Top