Engine temps and leaning.

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UngaWunga

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UngaWunga
1973 172M. About 1300hours on the engine.
What I've noticed, with the engine monitor, is that the back two cylinders run much hotter than the front two. I've checked that the baffles are not folded back, but there are small gaps here and there in them.

Based on the engine monitor, I can't lean it out until the engine stumbles and push the mixture back in a notch or two. The engine temps shoot right up. I'm burning way too much gas, around 9 gph with the mixture I'm using to keep the engine temps down on the monitor.

So, is it normal to see the back two cylinders another 1/3rd hotter than the front two?

Should I replace the baffles if there's a small gap or hole?

What do others do for leaning of the Lycoming 320 E2D engine in a 172? My instructors, in the rental 172Ms, just used to lean until it stumbled, and then pushed the mixture in 2-3 clicks. But the plane didn't have an engine monitor, and we didn't know engine temps other than oil temp.
 
What temps are you seeing? 4 cylinder Lycomings usually act like you describe. Some guys will build ramps to channel air to the aft cylinders. That essentially raises the temp of 3 and 4 to lower 1 and 2. In most cases all a monitor shows is what's been normal for lots of years but now it gives the owner something to fix. Single point CHT-EGT instruments monitor the hot cylinder without regard to the cooler ones. Personally I don't find 9 gph excessive for an 0-320 and I wouldn't pay too much attention to temp spreads as you describe until after an hour or so of operation when the engine is warmed through and temps are stabilized. Another quick fix may be a cooling lip attached to the bottom cowl to increase cowl air flow.

In my current airplane I've struggled a bit with temps. In my case the best action was to enrich fuel supply at take-off/full power ops. Once that was done the temps were well managed from the start and maintenance of healthy temps and equalized temps after that became much easier. In other words I stopped the excessive heat from ever starting. Cruise temps had showed a symptom, they weren't the problem.
 
Its cruise where the temperature spread becomes apparent. Climbout is fine.
I'm also surprised that I'm not able to lean as much as I did in other 172s. Do other pilots with non-injected engines do the lean-to-stumble-then-back-in-a-bit?

I'll admit I need to read the manual to find out exactly what the monitor is telling me. It does have both EGT and CHT on all four cylinders.

How tight should the baffling be to the cowl? Should I aim for having it completely sealed, or are some small gaps acceptable? I'll try to get pictures this evening.
 
I have a 150 horse O-320 and 9 gallons seems high to me. Takeoff power for me runs 10-12 at full rich depending on density altitude.

On an 80 degree day, starting at 1,200 feet airport elevation I can usually climb at full power for about 15 minutes before #4 cylinder reaches 420 F. Sometimes I can keep the power on and it will start decreasing temps by itself on the way up as the mixture (presumably) gets richer and richer.
My climbs generally are at 65-70 mph to 800 feet AGL then at 90 mph from there up to cruise altitude.

At cruise around 5,500 feet or so there is no way in heck I'll see 9 gallons/hr. I shoot for 7 but can be anywhere from 6.5 to 8 depending how big of a load I have or how "fast" I want to go.

6.5 gph turning about 2,100 RPM with car gas which comes out to about $18 an hour for fuel.

I don't know what carb you have but I'm running an Avstar AV10-5217 which lines up with the application chart in the Lycoming Service Instruction 1523C http://www.lycoming.com/portals/0/t...carburetor_manufacturers_cross-references.pdf


Just for kicks: (Leanest flight I ever made)

 
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UngaWunga said:
Its cruise where the temperature spread becomes apparent. Climbout is fine.
I'm also surprised that I'm not able to lean as much as I did in other 172s. Do other pilots with non-injected engines do the lean-to-stumble-then-back-in-a-bit?

I'll admit I need to read the manual to find out exactly what the monitor is telling me. It does have both EGT and CHT on all four cylinders.

How tight should the baffling be to the cowl? Should I aim for having it completely sealed, or are some small gaps acceptable? I'll try to get pictures this evening.
Click to expand...

baffles should not have gaps and holes. always try to make as close to 100% seal as you can manage. I've sealed my suspect spots with silicone caulk. every little bit helps.
 
Unga,

Here's an article to read about baffles. Pressure cowls require high pressure on top and low pressure on the bottom. All the air transfer should go through the cylinder fins. With that accomplished you may need to build dams to block some of the impact air from hitting the front cylinders. Many guys do so by making ramps to let that air pass smoothly over the fro t cylinders and into the cowl where it'll increase pressure and subsequently the flow through the rear cylinders. Always start by making sure your inner cylinder baffles are there and correct. Those are little sheet metal pieces that fit closely in between the front and rear cylinders fin-to-fin. Good hunting!

http://www.avweb.com/news/savvyavia...-14-Engine-Cooling-Less-Is-More-188945-1.html
 
I had a C172n with the 180 conversion and your complaint sounds like what I complained about for years.

I got all the usual responses about checking baffles and leaning.

I got a lot of responses that said don't worry about, they all do that, but people with out an engine monitor just don't realize it.

My A&P got so frustrated with me constantly complaining about it that he advised pulling out the engine monitor.

But 440 CHT on climb is "not normal" and it is not good for the engine.
Neither is 400 CHT during cruise.
the only way I could keep the temps down were to fly around 100kts and run rich.

So I sold that damn thing and bought a bonanza. Now my temps run between 290 an 320 during cruise. Yesterday with the OAT around 95 the CHTs reached a blistering 370 during take-off climb, but settled down to 320 when we leveled off.
 
Thanks guys. I'll see if I can take it up this evening and get some actual numbers.
 
Stewartb said:
baffles should not have gaps and holes. always try to make as close to 100% seal as you can manage. I've sealed my suspect spots with silicone caulk. every little bit helps.
Click to expand...

yup....get the baffling all gue'd up. :nono::yikes::eek::yikes: btw....I wouldn't do that to my plane. :no:

brew-004.jpg
 
There is a huge hole on the 150/180 powered Cessna 177 that is about the size of a pop can right above #4 that dumps air overboard no matter what position the cabin air is set to. I've plugged that hole and saw very little change in CHT which leads me to believe there is more to these problems (real or not) than meets the eye.

 
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Most Cubs that have had major re-powers move the oil cooler to the aft left baffle. Most of us manage our CHTs just fine, and some of the guys are running very juiced-up 400+ cubic inch engines. It's still about differential pressure and routing the air flow to achieve your desired results.
 
Judging by some of the 172s I've seen, baffles are most often ignored and they start cracking and/or coming loose and letting the air escape without taking any heat with it. Baffle seals get hard and don't fit the cowl anymore. And there's a plastic baffle around the induction filter that stops ram air from getting into the lower cowl and spoiling the low pressure there; that thing breaks and some guys take it off, not realizing its importance, or just reluctant to spend the significant chunk of an AMU for a new one. There is an aftermarket fiberglass unit available that is far cheaper and works as well and is repairable.

Dan
 
UngaWunga said:
So, is it normal to see the back two cylinders another 1/3rd hotter than the front two?
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Yes, It's simply a inherent bad design of the intake system.
 
How so? A 4-cyl Lycoming's intakes are pretty darned equal as far as I can see.
 
Stewartb said:
How so? A 4-cyl Lycoming's intakes are pretty darned equal as far as I can see.
Click to expand...

The Lycoming 4 bangers are notorious for #3 cylinder failing first. Simply caused by poor fuel distribution.
 
the fuel injected engines have cooling issues there too.....it's a baffling problem.
 
I disagree. I think it gets hot but that has nothing to do with fuel distribution and everything to do with poor cooling air distribution, as is well established by modified Cub builders everywhere. And why do I use Cubs as the example? High power, slow airframe. Cooling solutions are well defined for the most part, even with over-sized Niagara oil coolers hanging on the rear baffle.
 
Lycoming says cylinder operating cylinder temperature should be between 150-435 F, Max 500F

My O-320 runs 325 F on the front cylinders and 375/400 F on the rear cylinders
 
Never worked on a Cessna, so take that for what it's worth.

Grumman AA5 uses similar 150HP Lyc engine, and they often have serious CHT issues as well. I had one with serious overheating issues. After disassembling the whole thing and plugging every air leak on the TOP side, I heard that there was an option to open up the air outlet on the bottom of the cowl. Once I changed that air exit from the bottom of the cowl where the hot air exhausts, the temps went down dramatically.

While the mass of air exiting the engine bay through the cooling fins is pretty close to the intake, the volume is quite a bit higher due to adiabatic heat extraction from the engine. Something a lot of folks don't really get. The air may be thinner being hot, but the number of molecules transiting the choke point is basically the same.

Just my 2 cents for a plane I've never worked on, but I would start investigating the air exit below the baffles as well. I think there are also some air deflectors down there that may be damaged or missing. If they are required, and not in place, the air is not 'sticking' to the fins of the cyl/head, and just blowing by the space without extracting the heat. I think they're called shroud ducts, or cyl ducts. They are critical on the Conti 6 cyl, but I don't recall if the Lyc 4 cyl has them.
 
Stewartb said:
I disagree. .
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as you normally would.

This has been a known problem for 50 years that I know of.

It happens in Pipers, Cessnas, and Mooneys, all have different baffling, all have the same intake system.

And Yes, it happens in the IO- series too, until you put Balanced nozzles in the engine.
 
Checkout_my_Six said:
the fuel injected engines have cooling issues there too.....it's a baffling problem.
Click to expand...

Yes and no. If the fuel control is on the lean side at full power they will run hot too. A lot of 210 owners complain about it with fresh engines until someone tweaks FF at WOT.
 
Maybe that's part of my problem. I'll decrease rpms a bit to see if that affects temps and my ability to lean.
 
while you're trying stuff.....try a touch of carb heat when leaning. Also, being high enough to have full throttle can setup nice turbulent flow for better fuel atomization from the carb....but back off just a touch so the butterfly (throttle plate) is slightly canted to enhance that turbulence.

Backing off too much on the throttle will actually make the engine run leaner....cause it will be out of the enrichment range.
 
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docmirror said:
Just my 2 cents for a plane I've never worked on, but I would start investigating the air exit below the baffles as well. I think there are also some air deflectors down there that may be damaged or missing. If they are required, and not in place, the air is not 'sticking' to the fins of the cyl/head, and just blowing by the space without extracting the heat. I think they're called shroud ducts, or cyl ducts. They are critical on the Conti 6 cyl, but I don't recall if the Lyc 4 cyl has them.
Click to expand...

Those are intercylinder baffles, and the Lyc has them too. Often find them loose or beat up so they let a lot of air escape. Another bad spot is the aft side of the aft cylinders; the baffle is supposed to be held tightly against the fins as it curves down and under the cylinders, yet I often find the wires missing. When the airplane is in flight and there's a lot of air coming in, those baffles flex away from the cylinder fins if they're not tied properly.

Dan
 
Do you have a picture of what you're talking about? Just curious.
 
Checkout_my_Six said:
while you're trying stuff.....try a touch of carb heat when leaning. Also, being high enough to have full throttle can setup nice turbulent flow for better fuel atomization from the carb....but back off just a touch so the butterfly (throttle plate) is slightly canted to enhance that turbulence.

Backing off too much on the throttle will actually make the engine run leaner....cause it will be out of the enrichment range.
Click to expand...

The 172M's carb doesn't have an enrichment circuit. But I have found, in one airplane, that the RPM in climb rose about 50 RPM if I pulled the throttle back just a hair. That throttle plate atomizes the fuel better, and helps even out the fore/aft mixture distribution. The MSA carbs have the nozzle in the aft wall of the carb barrel and they tend to send too much fuel up one side of the intake plenum, depending on which nozzle is installed and how its crossdrilled holes are oriented. It's a poor setup, and my 67-year-old Stromberg carb offers better performance than that.

Dan
 
Dan Thomas said:
The 172M's carb doesn't have an enrichment circuit. But I have found, in one airplane, that the RPM in climb rose about 50 RPM if I pulled the throttle back just a hair. That throttle plate atomizes the fuel better, and helps even out the fore/aft mixture distribution. The MSA carbs have the nozzle in the aft wall of the carb barrel and they tend to send too much fuel up one side of the intake plenum, depending on which nozzle is installed and how its crossdrilled holes are oriented. It's a poor setup, and my 67-year-old Stromberg carb offers better performance than that.

Dan
Click to expand...
old MA3's will allow the throttle plate to go past the vertical, pulling back on the throttle will bring it back to vertical.
 
Dan Thomas said:
The 172M's carb doesn't have an enrichment circuit. But I have found, in one airplane, that the RPM in climb rose about 50 RPM if I pulled the throttle back just a hair. That throttle plate atomizes the fuel better, and helps even out the fore/aft mixture distribution. The MSA carbs have the nozzle in the aft wall of the carb barrel and they tend to send too much fuel up one side of the intake plenum, depending on which nozzle is installed and how its crossdrilled holes are oriented. It's a poor setup, and my 67-year-old Stromberg carb offers better performance than that.

Dan
Click to expand...

How do you rationalize the same syndrome happens to the fuel injected engines until you install balanced nozzles?
 
Dan Thomas said:
Those are intercylinder baffles, and the Lyc has them too. Often find them loose or beat up so they let a lot of air escape. Another bad spot is the aft side of the aft cylinders; the baffle is supposed to be held tightly against the fins as it curves down and under the cylinders, yet I often find the wires missing. When the airplane is in flight and there's a lot of air coming in, those baffles flex away from the cylinder fins if they're not tied properly.

Dan
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Thanks for confirming. Most people pay a lot of attention to the cool side of the air, and forget about the exhaust side. I had one of those lower baffles with a lot of cracks in it, and replaced it. Making some other modest changes down there on my Grumman made serious changes to my cooling on hot TX days flying down to Houston. Before I checked it, and changed the exit flap size and shape my CHT ran about 400F pretty constant. After I did that, it went down to a steady 375 on my hottest jug. Lucky I called Fletch-air when I was working on it, they had the special sauce to resolve it.
 
Tom-D said:
How do you rationalize the same syndrome happens to the fuel injected engines until you install balanced nozzles?
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Because of the unequal amounts of air reaching the cylinders through that 1920's style of manifolding.

Is this another test? A TomTest?

Dan
 
UngaWunga said:
So, is it normal to see the back two cylinders another 1/3rd hotter than the front two?
Click to expand...
Define "1/3rd hotter" in more quantitative terms.

Should I replace the baffles if there's a small gap or hole?
Click to expand...
Plugging the hole should be sufficient.

What do others do for leaning of the Lycoming 320 E2D engine in a 172?
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Lean to the first sign of roughness, then enrich until it runs smooth.
 
TS Alpin makes or made some cowling louvers that are popular on skywagons. I don't know if they're STC'd for a skyhawk...

You might try that... :dunno:

And exhaust pipe fairing is supposed to decrease positive pressure through the hole ...

LowerCowlVentsCopy_zpse9d1f0de.jpg~c200
 
Gas is cheaper than a engine....
I have a 172M and run it at 9gph all the time. I mainly run 2500 rpm and just lean it a tad if above 3000'. If below 3000 I leave it full rich. Not sure what you are trying to do? 9GPH is nothing in my book for a 4 seat airplane. I never lean mine until it stumbles.
Good thing you don't fly my Pitts. I burn 46 GPH at full power.
 
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Unga,

Here's another article to help you understand the big picture a little better. Please do get some warmed-up cruise temps for EGT and CHTs. Another value you need to establish is leaning authority. Once good and warmed run full rich in level cruise and note your EGTs. Slowly lean until the stumble and enrich just enough to smooth it out. How do your EGT temps differ from full rich? And last, how are you measuring your fuel flow?

http://www.avweb.com/news/savvyaviator/savvy_aviator_59_egt_cht_and_leaning-198162-1.html
 
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