Q: I have a Mattituck TMX O-360 in my Glastar. During a longer climb phase, my CHTs differ up to 100°F between Cylinder No. 1 and No. 4. Leveling off and reducing to cruise power (65%) at top of climb, CHTs stabilize all within max 30°F.
What could be the reason for this difference (airstream entering the cowl at a steeper climb angle?) and do I have to be concerned about this?
Christian Stuessi, Switzerland
A: Christian, thank you for submitting your question. I’m certain others who have engines equipped with a carburetor may have noticed similar cylinder head temperature differences, providing their engine has all cylinders equipped with CHT probes.
For a normally aspirated engine with a carburetor installed, it is not unusual to see this occur at different flight attitudes.
Most importantly, if the cylinder head temperatures are not reaching or exceeding the maximum allowed in accordance with the Lycoming Operator’s Manual, which is 500° Fahrenheit for your engine, or the specific limitations established by Mattituck in your case, I wouldn’t be too concerned.
There are a couple of things that may have an impact causing you to see a difference in CHTs.
First, I’d be looking very closely at the front cowl openings and what lies directly behind them. The location of the engine baffling and its condition have a direct impact as to how much ram air is allowed in and where it goes. It’s important to make certain nothing is allowing the incoming air to be mis-directed, resulting in higher CHTs.
Climb attitude certainly can be a factor, which might require a more shallow climb.
If the aircraft is equipped with cowl flaps, opening them may help in controlling the CHT difference.
I’m almost certain the Glastar does not have cowl flaps, so maybe making the mixture a bit richer would help also.
There is another factor that may be coming into the equation, which is the different lengths of the intake pipes on the engine. As I’ve mentioned many times before, the fuel distribution on an engine equipped with a carburetor is poor at best.
With the required fuel being metered at the carburetor on the bottom of the engine sump and with the different lengths of the intake pipes, the fuel/air mixture at each cylinder may not be identical and may cause a difference in cylinder head and exhaust gas temperatures readings.
You wouldn’t expect this on an engine that is fuel injected because each cylinder receives an identical amount of fuel. Should a difference in CHTs occur on that engine, I’d be thinking along the lines of a cowling/baffling problem causing the CHT to rise.
Christian, I don’t think you’ve got anything serious to be worried about, but I’d suggest a close visual inspection of the cowling/baffling on the right side of the engine looking at it from the pilot’s seat. It may be something as simple as a piece of baffling out of place, allowing the inlet air to be directed away from where it should be going.
One last thing that I would suggest, and that is to check with other Glastar owners operating an aircraft configured exactly like yours. It would be interesting to learn if anyone else, flying the same envelope as you, is experiencing the same condition.
Here’s the ECI paper on cooling referenced above by skypilot: http://eci.aero/pdf/EngineCoolingProblems-SAE.pdf
Hi Paul,
The Glastar actually does have a cowl flap (if using the factory default cowling). It is ground adjustable if built to plans.
I do see differences in climb out with my Glastar (O-320 with a Rotec TBI (Throttle Body Injector)), although not as great as 100F, then leveling to about 30F in cruise. I concur with your assessment that Christian does not have a serious issue, presuming the temps aren’t exceeding the recommended values.
A contributing factor may be that a typical Glastar install has the air feed to the oil cooler coming off the rear right side of the baffling.
The factory supplied Glastar cowling also has a sharp “lip” around the air inlets. One mod that some have done is to build a ramp on the bottom side of the top cowling to eliminate the lip and make a smooth transition for the incoming air. I believe there are details on the glastarnet.org member site (A must for any Glasair/Glastar/Sportsman owner).
Hello, it seems to me that your main concern at this time for the cht temps being out of sink w/the others is the name ” Mattituck”. Do a compression test on the engine, both hot and cold and if those two cylinders are lower than the others, you may have a more serious problem. Does your engine have over 400 hrs. since tbo? does the oil turn black in just a few hours of flying more often than before? You may be a victum of foul play, meaning the engine was assembled incorrectly..nuff said..
Is the temp delta a new thing or has it always been this way? Maintenance induced mistakes could be the cause if the problem is recent. Sometimes a review of past maintenance will disclose possible causes.
Modern planes are designed to cool with pressure, not ram air. The cowl creates a pressure differential between the top and bottom of the engine and do not (in theory) rely on ram air to cool the heads. (It’s the aluminum heads that get hot and fail as they heat up. The steel cylinder barrels are relatively resistant to heat related failures.)
Ram air is drag and provides uneven cooling. Modern design is intended to slow the air. Cutting the speed in half doubles the pressure, so the inlets should expand at about 7 degrees or so to get the pressure rise. Many cowls use the ‘great wall’ method of allowing incoming air to slam into the front cys, swirl and hit the rear engine cooling baffle with great chaos. Small, well designed inlets and ramps cool better than big inlets.
GAMI put tufts of yarn and cameras under the hood of their Bonanza to record the flow and designed new baffles and ducts to control the chaos. GAMI and ECI have a free paper available for testing the pressure differential with a home-made manometer.
The lower cowl area is also important. The best designs have a ramp that allows hot, buoyant air to exit smoothly into the free stream without drag. Indeed, the better designs create a negative pressure to pull out the hot air. The problem is that at high AOA the airframe pressure is higher under the cowl exactly when you need it least. Cessna did such a poor design job on early Cardinals that their air flow reversed at high AOA and the engines cooked. We added lower cowl and exhaust pipe fairings to get better cooling.
Air entering the cowl often exits at the prop hub. Most modern designs add a baffle behind the prop to stop the reverse air flow in that area.
Check how the cowl fits. The front of the windshield is a high pressure area and there should be a gasket to prevent pressurized air from entering the upper cowl.
OTOH the sides of the cowl are low pressure areas where a loose fit of the lower cowl allows hot air to escape regardless of AOA . Some designs have exit ramps built there because it is so effective. But it’s rare. Because it is ugly.
Peter Garrison wrote a great article about pressure mapping planes in the july 2007 issue of Flying. It includes color diagrams of positive and negative pressure regions, and the results are not intuitive. But they are useful: http://tinyurl.com/jp7um3t
The respondent didn’t tell us the numbers. There is a huge difference between a max of 400 and 500F so a spread of 100F might be meaningful it might not. If it is 200-300 go play. If it is 400 to 500 then get it fixed. The guys at GAMI/APS have data suggesting one should not exceed 400F, and 500F redlines are from the people eager to sell new cylinders.
She also didn’t tell us if the TMX O360 is a six or 4 cyl engine, a Lyc or Conti knockoff. That matters because the Lyc design with the carb bolted to hot oil sumps has an effect.
We don’t know how she is managing the engine: Is the problem at WOT, balls to the wall or something else? What is the climb speed, OAT and weight?
We also don’t know how the readings are derived: A digital engine monitor with recording capability or analog gauges with color coding. If it records, send the data dump to Mike Busch, who offers a service analyzing thermal data.
Where the CHT thermocouples attach affects readings: Those on top sparkplugs read cooler than those in the CHT well, which read cooler than those on the bottom plug. Top to bottom plug can have a 100F spread.
What are the EGTs? It usually doesn’t matter, but for troubleshooting purposes they could be diagnostic.
Are the exhaust pipes of different length? Longer pipes retain heat longer. A powerflow exhaust will bring them closer together.
Is the delta between front and rear cylinders? There are rear cylinder baffle mods that help a lot.
Are the engine cooling baffles tight against the fins and flexible seals properly oriented? The seal between the top cowl and engine is easy to install wrong so it leaks badly. Seals on the front lower part of the engine prevent cool air from pressurizing the area under the engine. If they are bad or installed wrong then cooling suffers.
If equipped with a fuel flow transducer, determine takeoff fuel flow. Your engine has a minimum at WOT. Make sure it does that. Marvel has normal and rich carbs certified for installation on Lycs. Which do you have.
Lycs are famous for lousy oil flow to the rocker boxes, and that oil carries away a lot of heat. Most owners set oil pressure to redline with hot oil to get max flow. There are articles on the web about this. The Restart Cessnas increased redline from 99 to 125psid to make up for this.
Induction leaks can also make a cylinder run hot, if that is indeed the case. Leaks are usually not apparent at WOT, but have an effect when the throttle is reduced. There are flight tests one can conduct to diagnose leaks.
If the engine is a Lyc knockoff, the front and rear cylinders share one intake cam lobe each. If the lobe is worn those cylinders make less power and run cooler. One has to dial the rocker arms to see the lost lift.
Do an in-flight mag check: Get the engine knobs parked at the power you like and stabilized then run the check. Let it run that way for a minute or two to stabilize at the new setting and note the EGT/CHTs. Then the other mag. It can tell you a lot about the ignition system. Temps should cool considerably.
Finally, I recommend the Advanced pilot seminar online course. It is designed for big six FI operators but you’ll learn a lot about your carbed engine and how to diagnose problems like this. https://www.advancedpilot.com/
Another thought, the factory Glastar cowling forms a rather sharp lip at the front where the air enters. Some builders have built a short “ramp” on the inside of the top cowl to smooth out the airflow around the openings, which has helped cooling in some cases. I think there are details on the glastarnet.org web site (membership required). If you are flying a Glasair/Glastar/Sportsman aircraft, the Glastarnet is an incredibly good resource.
Hi Paul,
The Glastar actually does have a cowl flap (if using the factory default cowling). It is ground adjustable if built to plans.
I do see differences in climb out with my Glastar (O-320 with a Rotec TBI (Throttle Body Injector)), although not as great as 100F, then leveling to about 30F in cruise. I concur with your assessment that Christian does not have a serious issue.
A contributing factor may be that a typical install has the air feed to the oil cooler coming off the rear right side of the baffling.