Zenith Aircraft Builders and Flyers
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CH 701 - 912 UL with high EGT
I have recently finished my 701 and started flying, but am worried that the EGT’s. They are running around 1600 degrees Fahrenheit. The CHT’s and oil temp’s are 200 degrees Fahrenheit. I raise the carb needles with little improvement. Does anyone have a suggestion as to what I should do next?
Replies to This Discussion
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Permalink Reply by Matt Pope on
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1600 F is below the maximum (which has a significant safety factor built in), and it sounds like everything else is fine, so personally I wouldn't worry about it, but late ignition timing or running on only one ignition would cause high EGTs.
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Permalink Reply by Jim Kuntz on
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I have have checked and it is operating on both ignitions. I guess I will just continue to fly it, but will continue to look for ways to reduce it.
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Permalink Reply by James Ballenger on
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Jim
I also have a 701 with the 912ULS and warp prop. Depending on the season, I see between 180 and 200 f on my CHTs and EGTs around 1450 - 1525 F. I have 3 of my EGTs that read around 1450 and one that always reads higher around 1500. I think the distance you drill the hole from the head is critical on the temp read. The manual states the required distance. I would call Lockwood in Sebring and see what they have to say. I can regulate my oil temp with a flap like the one Roger Dubbert has on his 701. It works great. My oil temp is usually right in line with the CHT but once a fight I close the flap to get it above 212 F. I am hangared at X35, Dunnellon, so if you want to compare notes give me a call.
Jim
C# 757 724 0415
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I’ve talked to Lockwood, they told ma to raise the needles. I have friends with 100 HP 912’s and they are running with 1200 EGT’s.
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I think around 1475 F is typical for the EGTs but it really depends on the installation and airframe. I have heard that CHT temps are a better indicator for engine health. What is your static rpm ? Mine is around 5300 on the ground with the tail tied down. That gives me 5800 WOT in the air. I usually fly around 5200 with an IAS of 90 mph. I just switched to the streamline struts and love them.
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I attached an independent EGT gage to one of my existing EGT probes giving me three outputs and went flying yesterday. The two readings on the engine monitor were 1560 and 1610 and the independent instrument was registering 1510. Today I flew the 701 with the cowling off, and the readings were still high, almost the same as yesterday. To clarify the engine is equipped with 4 EGT probes, even though only two are connected to the monitor.
regards,
jim
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Hi Jim, I don't think cooling the engine from the outside will affect EGT significantly - as an analogy, the EGT is like measuring the temperature right above a candle flame - the room temperature won't affect it much.
Forgive me if you know this stuff - I'm just trying to think through it myself:
To change the EGTs you can change the fuel/air ratio (the actual temperature of the combustion gases) or you can change the ignition timing (when the combustion happens relative to when the exhaust valves open), or you can ensure the exhaust valves are closing properly during combustion, or you can measure the EGTs in a different spot, or with a different type of sensor. There might be other reasons for high EGTs, but those are the ones I can think of.
If you graph EGT against the fuel/air mixture, EGT reaches a peak at a specific fuel/air ratio, so if the engine is running rich of peak then richening further will reduce the EGTs, and if you're lean of peak then leaning further will reduce the EGTs. I don't remember if the carburetted 912 is designed to run rich of peak or lean of peak, but because of the layout of the intake manifolds on the carburetted 912 engines, the rear two cylinders receive a slightly different mixture than the front two cylinders (I believe the rear cylinders are richer), so if that's the case and the front EGTs are hotter than the rear EGTs then the engine is running rich of peak and adding more fuel to the mixture will lower the EGTs. (If it's the other way around, then leaning will lower the EGTs.)
If the ignition timing is delayed too long then combustion will still be occurring when the exhaust valves start to open, so you'll be measuring the temperature of the escaping flames (high EGTs), so check to make sure the ignition stator thingies (I forget what they're called) aren't loose and they're in the right places around the flywheel on the back of the engine (just remove the black plastic cover and you can see them). The measurements are in either the Line maintenance manual or the Heavy maintenance manual - I can't remember which one, but you can download both here: https://www.flyrotax.com/p/service/technical-documentation
Also check to make sure the spark plug wires are connected to the correct spark plugs.
If the exhaust valves aren't sealing properly and they're leaking hot gases during combustion, that would raise the EGTs. Leaking valves could be caused by lead deposits if the engine has been run for a significant amount of time on leaded avgas. Is it a new engine? There might be other reasons why valves don't seat properly that I'm not aware of. You can buy a borescope that links to your phone for about $30 on Amazon - pull out a spark plug and stick the camera in the hole and have a look around (rotate the prop so the exhaust valve is open. A borescope is also a great tool for inspecting enclosed areas.
Check to make sure the EGT probes are mounted in the recommended spots - if they're too close to the cylinders then they'll read hot. You could also test them on something of a known temperature to make sure they're working properly.
I'm sure there are other possibilities I'm not aware of...
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Another possibility is the propeller pitch being too fine, in which case unused combustion energy (heat) is dumped out the exhaust. Check that the full throttle RPM is within the recommended range in level flight.
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I did change the prop pitch. I don’t do it to lower the EGT but rather to keep the engine from over speeding. It didn’t make any difference in the EGT though.
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I agree with you, it is fuel/ air ratio issue. I’ve raised the jet needle, I’ve gone from a 158 to a 160 on the main jet size and I even raised the float level slightly. None of these changes made any noticeable difference to the EGT’s. I also flew with a handheld EGT in addition to the two EIS EGT read outs; all are reading in the low 1,600’s
Compression feels good and I looked at one exhaust valves with a friends bore scope. Even though this cylinders EGT is 1,600 + the valve looks normal
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