Q: I bought a 1972 Cessna 177B with an O-360 a few months ago. It had 900 hours on the engine. My first 20 hours I would use 23 squared and pull the mixture back to 10 gph or less, then watch the cylinder head temperatures, monitoring to keep the hottest (#2) around 385°.
When the ambient temps started rising, I noticed #2 and #4 were spiking into the 420° CHT level on climb out. When I level off and set to 23 squared, I can only keep #2 below 400° by going full rich (12.5 gph) and occasionally opening the cowl flaps.
If I use 10 gph, #2 will stay around 415°. #4 will run in the 380s, and #1 and #3 will run in the 350s.
Swapping probes did not go as planned, as the problem didn’t change. I am growing concerned and looking for advice.
Thank you for your time.
Greg McKeever, via email
A: Here’s a situation that may have several contributing factors that could result in the numbers you’re seeing.
My first inclination is, for some reason, we’ve had a change in air flow through the cowling, especially in the front where the #2 cylinder is located.
I’d recommend you take a very close look in that area and you may find there are some gaps that may have been a result of cowling removal and re-installation or a piece of baffling that has gotten out of place.
Sometimes it doesn’t take much to allow incoming air to sneak past where it’s supposed to go and change temperatures in that area.
As ambient temperatures increase, it becomes even more important that the air coming into the cowl be directed properly to assure best cylinder cooling.
With the difference between the #2 and #4 cylinders when you are at 23 squared confirms we need to focus on the baffling and cowl fit at the #2 cylinder area.
If the aircraft is a 1972 year model and this is the original engine with 900 hours on it, I’d be curious to learn the overall condition of the engine baffling.
Don’t forget, the baffling lives in a very hostile environment from all of the engine heat, so make certain it’s properly located and not drooping, allowing air to escape rather than being directed for proper cooling.
If the baffling does require replacement, don’t go with the cheap stuff!
Another thing that I’d like you to check is to be absolutely certain that you don’t have an induction leak on the #2 cylinder intake pipe either at the cylinder head end (leaking gasket) or at the intake pipe connection point at the oil sump.
If there is an induction leak, it may be indicated by a high manifold pressure reading on your manifold pressure gauge at engine idle of 650 to 700 RPM.
With no leaks in the induction system I’d expect to see a reading of right around 10″. If you see a reading higher than that, say 11 or 12, this could indicate there may be an induction leak.
I appreciate the fact that you provided some actual temperatures you recorded during flights. Using those numbers as references, I think you’re still going to be okay, but let’s see what else we might want to check.
First of all, the maximum CHT for your O-360 series engine is 500°F, and for maximum service life, we’d like to keep it at 400°F or below.
There is no doubt that as ambient temperatures increase we know this will have an impact on engine CHTs. Let’s assume nothing has changed except for the increase in ambient temperatures. That gives us two choices when it comes to cooling the engine.
You can either cool it with more fuel, which you already know, or by using the cowl flaps, which you also know.
If during very hot days you can keep the #2 cylinder in that 415° area, I wouldn’t be too concerned because I don’t have a great deal of confidence in the accuracy of the CHT gauge as being dead on.
The other thing that I get concerned about is the accuracy of the tachometer. It’s not uncommon for engine tachs to read a little low, so maybe the 2,300 RPM you are seeing on the tach may actually be 2,400 RPM or a bit more.
That means the fuel flow you are putting into the engine isn’t quite enough for the additional horsepower you’re taking out of the engine. This may be the reason that when you went to 12.5 gph at 2,300 RPM you were able to keep the #2 cylinder below 400°.
I’ll wrap this up by saying you are very close to being where we want you to be, and that a little tweaking here and there should get you in the ballpark.
One last comment with regard to using more fuel to keep the engine cool: If you really stop and think about it, one of the least expensive things you can put in your engine is fuel.
Did Greg McKeever, recently have alternator or magneto work done?
The aft baffle often folds aft during cowl installation, creating air leaks. Inspect for gaps with a flashlight.
#3 is typically the hottest cylinder, so Greg’s experience is atypical, but not unexpected.
If opening the cowl flaps fixes the problem, there is no problem. That’s why they are there.
Fixed gear cardinals have a forward CG bias. Shift it aft and mine runs both faster and cooler.
Engine baffling is horrible on the 1968-1974 FG cardinals. It results in chronic high CHT and oil temps. Part of the cause is a gap on the front of the engine and around the alternator that allows ram air to pressurize the underside of the engine. In the off position, cabin heat also dumps hot air into the lower cowl, which is unhelpful. From 1975 and on, cooling improved considerably when Cessna revised the baffles to create a funnel of metal that extends from the crank, behind the alternator to the engine cowl. AOPA’s catch a cardinal has good photos of the engine with the baffle installed for study. That is the model to emulate.
Take a tour of other Cessna’s to see how the cooling baffles are installed. The 2005 C-182 runs very cool. Note how they added large gaps between the aft cylinders and baffle to smooth air turbulence. They also include a prop baffle to block air escape at the prop hub. The air inlets follow the seven degree expansion rule to slow the air and build pressure without turbulence. Sonja Englert’s book on engine installations dwells on these details (Sonja figured out how to cool the rear engine on the C-337T ‘Huff & Puff, no small task).
I have a 1968 Cardinal upgraded to the O-360-A1A and constant speed prop. I added the JPI EDM830 with fuel flow and data recording, a rather useful device since Cessna saw fit to include no CHT with the original O-320 engine. Installing the EDM scared me because the CHTs were in the 450 and higher range. Bringing that down to 400F climb/380F cruise CHTs has been a life’s work.
Owners with a penchant for data have reported on this many times at the cardinalflyers.com website. Membership will allow you to search the digest for past discussions plus the tech pages for CHT mods. It is the best money you will ever spend on your Cardinal because it represents a rather large brain trust of owners. Searching the virtual digest for ‘connor’ will take you to my detailed posts. Send photos of your engine baffles to the moderator for an opinion.
Mike Busch’s Savvy analysis web site will analyze your engine monitor data for free. Of course, he wants to sell books and other stuff, but some find it saves their bacon if exhaust valves are involved.
https://savvyanalysis.com/home
Here’s the short list of things to do in addition to Paul McBride’s suggestions:
The ‘Braley mod’, which is a double-strip of engine baffle seal inserted between baffle and cylinder to improve airflow.
The “doghouse.’ a mod added for the same reason.
Prop baffle: From http://www.aircraftspeedmods.ca/ (the baffle isn’t listed on the site. It is the founder’s project and we think highly of it, along with his exhaust pipe and lower cowl fairings as cooling mods.)
Peter Garrison wrote a splendid article in the july 2007 article of Flying titled ‘What keeps them up’ that describes his pressure mapping of the cardinal. Find it on the web. Essentially, there is a high-pressure area in front of the windshield and low-pressure area at the lower side of the engine cowl/fuse interface. Cessna added a seal at the top of the fuse to prevent air intrusion at the top of the cowl. If it is sagging, reinforce it by gluing some engine baffle seal to the underside. Or test by applying duct tape to the top cowl and fly to see how it affects CHT and oil temps. If it improves, fix the seal.
Take advantage of the side low pressure by shimming the lower cowl Lord mounts out as far as possible to allow hot air to escape. Some have flared the aft edge of the lower cowl to add to that. Others have added c-182 -style ‘gill slits’ with good results.
The side shims plus top seals are very effective, good for a 20F drop in cruise CHT.
Ensure the carb flows at least 18 gph at sea level.
Lycoming approved two carbs for my engine, one richer than the other. I can provide part numbers if needed. It isn’t as effective as I wanted, and at high DA it makes the engine quit, probably not a great feature, but leaning fixes it.
We have in-flight tests for induction leaks and a way to determine the desired EGT for takeoff and climb for your installation (which I have never been able to achieve with my setup).
Do the GAMI lean test. http://www.gami.com/gamijectors/leantest.php While it is designed for injected engines, the data is useful for carbed engine management.
With a carb temp gauge you can run with partial carb heat. Setting it to give 42F carb temp allows me to run slightly lean of peak on some days – and the CHT drops like a stone.
Adding carb heat gives lots of headroom’ for mixture control. It quenches the fire and cools the engine. Re-leaning brings power back. The ’68 Cardinal carb heat is via a 2.0″ duct, which really makes the engine choke, but the later year cardinals use 2.5″ duct, which has the desired effect and range of control.
If CHT gets out of control during climb, go to single-mag operation. There is about a 15% drop in power and a rapid drop in CHT.
Finally, consider a Power Flow exhaust (CFO members get a discount). The aft cylinder exhaust pipes are longer on the stock exhaust than on the front, which makes them run hotter. The PFS pipes are all the same length. It both dropped and evened up the CHTs on my engine by 20F. Many find the ‘short stack’ desirable, but the long stack gives better LOP ops.
While we are focusing on CHT, look for patterns of a worn intake cam lobe. Lobes are shared between 1&2 and another lobe shared between 3&4. If either pair runs cooler than the other pair it could indicate a worn cam lobe. The engine probably also idles super smooth. A dial indicator on the rockers will tell the story: Look for about 0.5″ of lift (yeah I know, the hydraulic lifters are collapsing as you do this. It’s a quick check to see if the correct test is warranted).
That’s all that comes to mind at the moment. Search the CFO virtual digest for more.
I am asked this same question frequently, and the steps that I recommend are:
a) review baffles since any air that escapes through gaping baffles is less engine cooling air;
b) replace or swap to other cylinders the probes that are indicating high heat. After all, how do you know that those cylinders really are excessively hot? Commonly, the engine monitor gauge is inaccurate and is showing an erroneous reading so if you change the wiring from a hot cylinder to a cool one and then observe the reading if the monitor’s reading doesn’t change then you know the monitor needs calibration;
c) if the above don’t help then it possible that you are making heat, and that comes from lean cylinders or worn bearings (however worn bearings provide high overall heat, not just in a couple of cylinders).
The most likely culprit is the monitor giving erroneous readings. People become obsessive over monitor readings, and that’s the issue. What is real and what is erroneous? Before we had engine monitors we all flew happily with a single probe EGT and life was simple – then we had a probe in each cylinder and the pilot becomes worried. What changed? Installing an all-cylinder monitor is what changed.
Change wires to different cylinders and see if the problem travels with the wires and you will know.
Good comment PB, but I disagree with your ignorance is bliss argument. Been there, done that, got the roasted set of cylinders for show and tell. Chronic bottom plug fouling on 3&4 was the symptom, and we found a cracked exhaust valve in #4, so I spooked and replaced 1&2 as well.
I had been leaning for high DA takeoff and climb with no CHT gage in my ’68 Cardinal, relying on oil temp as a proxy, a really bad idea. Installing the EDM led to denial as you suggest. But it was the engine, not the monitor. And of course the loose nut behind the yoke.
I agree that for older planes where CHT redline was just a number and cylinders were cheap, nobody cared. After the EDM documented what I was doing it was apparent to me that if inspected, there would be head cracks, so I went for stud kits.
The guys at Advanced pilot seminars and the creators of GAMI LOP ops have data that supports honoring 380/400F operational redlines vs those of Lyc and Conti. Heavy detonation is one: Their test cell data suggests it can’t happen below 380F and probably won’t if the engine is conforming at 400F. Above that, it isn’t if, it’s when. They and Mike Bush have EDM data from engines that went into detonation and pre-ignition. Mike presents his data for free at the EAA, Avweb and his own web site, https://savvyanalysis.com/home Good stuff.
Another reason is head cracks, often at the barrel to head threaded area. Contis are famous for this, and they just pop off, pour expensive oil all over the cowl and if the intake falls off, completely shut down the engine.
There’s also exhaust valve failure. Lycoming clings to an Allis Chalmers exhaust valve design that heat sinks into the valve guide using molten sodium in a hollow valve stem. High CHT and lousy rocker-box oil cooling causes either rapid valve guide wear and valve wobble resulting in burned valves. Rich ops and high temps result in guide coking and a stuck valve that allows the valve train to hammer on the cam lobe, cracking the nitride coating. Lobe failure results later of the valve doesn’t fail first. Hence the so-called Lycoming Valve wobble SB every 400 hrs. Those who respect 380/400F CHT limits and lean aggressively or run LOP report no valve sticking and no wear during the wobble test. It could be a coincidence. Might not.