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Lycoming Cooling Requirements

I must say there's a wealth of information on this forum! Wish I would have used it sooner! One last question/s re measuring pressure differential between upper and lower cowls: I'm guessing this has to be done in flight rather than static. Can it be accomplished by extending the manometer tubing from the cockpit to the engine compartment and take the readings from the inside? Do I have to take readings from both sides of the upper and lower cowls or is one reading from upper and lower cowls sufficient? I should get the manometer in a few days. This is not my favorite time to fly with 95+ degree heat, and I certainly don't want to damage the engine. Thanks for your help.
 
purchase enough tubing (hardware store, think it's 1/8" I'd) to operate the unit in the cabin. Run the tubing out the window vent into the cowl. Blow on the tubes to determine which will give a posative differential reading, that one goes on top of engine. other goes on bottom. I've tried different placements - top on engine lift ring, bottom near carb.
to avoid false reading, I've found it helpful to place a small piece of copper tubing (primer line works) in the end of tubes Pinch the end closed and cross drill several small holes in it. (To avoid ram air reading)
tape the unit someware in the cabin clear of controls. tape the hoses along the boot cowl, zip tie copper tipped ends in place in engine compartment.
i think anything over 4" h20 at 90 mph you're doing well.
also mentioned earlier, try measuring pressure differential between cabin interior and lower cowl to be sure you're not getting positive pressure buildup under engine.

please report your findings, good luck!
 
Many thanks to CanTom12, Oliver, and Steve Pierce. When I get the manometer and the tubes rigged, I'll report my findings. Thanks again!
 
Once you get some readings, move the tubing (at the meter, while you're flying), so that the low pressure side of the cowl is going into the + side of the meter, and the - side of the meter is open to the cabin. This will measure the suction side of the cowl with respect to ambient(check that assumption first as above)....you want to see as low a pressure as possible. Any positive pressure at all is not enough scavenging from the low side and/or too much leakage around the baffles.

*Remember that the air you bring in the front will be expanded in volume quite a bit by being heated. This places the emphasis on scavenging the hot air, rather than worrying about the pressure side air.
 
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New discovery to me: if you've got a hot #3 then you ought to look at how tight your rear baffle is to that cylinder. Turns out that on the back of #3 (and the front of #2, if you're having problems with that one) the fins have a flat spot in them where air can't pass through. If your baffles are too tight against the back of #3, air won't be able to flow to the fins on the lower aft section of that cylinder. Steve's dad suggested this to me after new rubber baffle seals didn't cure my heating problems, his is mounted away from the cylinder with a spacer and mine is not. His temps are very normal and my #3 is 50-60 hotter than #1. I'll be installing a spacer and will report back.
 
This back-of-the-cylinder issue is interesting. The clearance on the back of the #4 cylinder seems to be too large, and reducing it to 1/2 inch seemed to help temps about 5 degrees. When I did the same to #3, I didn't notice any change. I think that getting air around the back of the rear cylinders is a problem. I wanted to build an impedance-matching horn(or funnel) on the exhaust side of the #4, (to help suck air around the back of the cylinder) but there is so much exhaust hardware back there it would be problematic and labor intensive.
 
I rechecked the baffles and did find some minor leakage and subsequently sealed them with high heat silicone. However at cruise setting I didn't see much difference in temps. I've rigged up the plus and minus tubes like you suggested sans copper tubing...as it's hard to find at the local stores. After receiving the manometer I'm a bit confused how to use it...particularly the "diff" and "rec" buttons...and which one to use. Can you shed some light on that?
 
You want to use the "diff" function. That reads the pressure from each tube and reports the difference between them - which is what gives an aircooled aircraft engine its cooling potential. The air in the high pressure upper cowl wants to join the low pressure area of the lower cowl, and it rushes through the fins which cools the engine. The bigger pressure difference you can make between the upper and lower part of the cowling, the better cooling you'll have. Also remember to check the difference between the pressure cowl to ambient and between the lower cowl to ambient. To do this I just unplugged one of the tubes and read ambient pressure as cabin pressure, then plugged that tube back in and unplugged the other. Write down your numbers or take a picture of the meter - I was in a hurry and wished I had!
 
I have had a JP Instruments gauge for a long time. This measures EGT as well as CHT on the 0-320. Even though we've attempted to seal up the baffles (after a rebuild), I'm guessing there's still work to be done. I will use the light again to see if there are any more leaks, check the baffles for security and then run the manometer to see if this work makes any difference. EGT runs a bit high, but my main concern is CHT.

What style of CHT probes do you have? Are they screwed into the well on the cylinder or do they fit under the spark plug? Are they all the same style or mixed? I bring this up as the spark plug style tends to read hotter.

Web
 
As mentioned earlier, I struggled with temps on a new engine.
part of the issue was fuel mixture.
At 2500 rpm 1500 msl, mixture full rich, I was average 40* ROP EGTs all cylinders. seeing cht's Well in the 400s 60*oat
(measured with JPI).
lycoming recommends at least 150* (EGT) leaning authority at all power settings
Since rejetting to recommended mixture things cooled off to 400 cht's.
now at 50 hrs, seeing 380* 2500 level cruise 80*oat

Since you have a JPI installed, simple quick check will verify you have adequate mixture authority.
level out at 2500 rpm. (preferably below 3000 msl) mixture full rich, pick hottest cyl on jpi, record egt, slowly lean mixture till egt peaks, record that number. there should be at least 150 deg difference, ideally 200 deg difference between full rich and peak.
Though Lycoming also recommends you dont lean to peak beyond 75% power settings, may result in detonation
and engine damage.
 
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Post #25
When running a J-3 type open cowl, remove the inter-cylinder baffles.

Do you guys have the Lycoming Technical Installation Manuals for your specific engines? (the manuals that provide the model specific mass flow vs horsepower and mass flow vs pressure drop and also specify the maximum waste heat dumped to the oil cooler)

As an aside, I prefer the P&W equation for calculating face rise as a function of horsepower, rather than Lycoming's assumption of a constant 150°F face rise.

Re cooling drag, it can be expressed as either pounds of drag, as horsepower, or as mph lost. I haven't calculated it for a Cub, but for a Piper Cherokee, depending on model and power setting, about 8 to 13 mph can be recovered by cowl redesign.
 
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What range of dynamic ram recovery efficiency are you guys seeing in the upper plenum?

I would guess about 40-50% due to the Cub's crappy inlet velocity ratio, but that is just a hunch.
 
Somewhat related to ram, I had the chance to measure pressures on two (pretty much identical) cubs, one with a sensenich 54 in prop(climb) and one with a 56 in prop. I got a cowl pressure increase with application of full throttle on the 56 in cub, but nothing with the 54 in cub. Later I tufted the prop (on the 54 in cub), and noticed that the inboard section of the blades out to the edge of the cowl was pretty much stalled at idle rpm.....so the prop isn't adding much of anything to the cooling, but the higher pitch props are, at least to some extent...
 
Props do make a difference in CHT cooling. My Catto and old Warnke props will run 15 deg. cooler CHTs than my 80 X 44 sench, wood prop
 
Think it has to do more with the load on the engine vs how much air the prop is pushing the cowl.
climb prop flatter, engine not working so hard at same rpm = lower CHTs

JimC, post#45 completely lost me after the first paragraph..��
 
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Sorry, Oliver. Didn't mean to do that.

I'm assuming everyone knows what the Lycoming Technical Installation Manuals are, and has them in hand for the particular engines they work with (BTW, if you call Lycoming, the folks who answer the phone probably aren't going to be familiar with them, and may tell you there is no such thing). There is. Talk to the Tech guys. What you are after contains the required mass flow curves and the massflow vs pressure drop relationship for your engine model. You use this information to size your inlets and outlet for climb and cruise. The inlet velocity ratio should be between 0.3 and 0.8 to maximise dynamic ram recovery efficiency. I usually design for 0.4 as a compromise.

When doing engine cooling calculations, Lycoming makes the simplistic but conservative assumption that the face rise (the temperature difference between the upper and lower plenums) is a constant 150°F. Pratt and Whitney calculates that difference based upon the horsepower being produced by the engine. When calculating engine cooling, I use the Pratt and Whitney equation for face rise instead of the Lycoming assumption (it usually allows smaller inlets snd outlet to be used). I do use the Lycoming methodology for calculating upper and lower plenum temperatures, pressures, densities, and mass air flow for cruise and for climb. This is how you size the inlets and outlet. The system is almost always in outlet control.

Re cooling drag, it can be expressed as pounds of drag, or as the horsepower required to move the cooling air through the cowl. Six of one, half a dozen of the other.....
 
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