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High CHT & Timing

N733JJ

Well Known Member
I year ago I upgraded my panel to a Garmin G-3X touch. Since then I have been battling high CHT?s with no explanation. On the way to OSH last year I had an extreme engine misfire and landed short. While at OSH we determined that my 16 year old Electroair Ignition was the culprit, causing a serious pre-ignition. I borrowed parts from Electroaire while at OSH, installed them and got the plane home. After installing a completely new system, I still have high temps. Work, weather and family issues have prevented me from flying much but I finally got out and got some hard data.

P.A. 7500? Temp 13C 22.2 MP, 2500 RPM showed 70% power on the G-3X though according to the power charts I took out of the C-177 the engine came from this would be 75%. Leaned to 25 ROP. Flying on the mag only, temps were 351-371. About what I used to experience with the Electroair in advance mode. Flying on the Electroair only (with automotive plugs), at 25 BTC they were 377-397, a 20 degree increase across the board. With the mag and electroair at 25, the temps went up further to 392-414. I then turned on the advance on the Electroair, it fired at 34.8 BTC and temps were 389-415. Adding the mag to this brought the temps up 398-426, a 10 degree increase across all cylinders.

I reduced power to MP 20.9 and 2300 RPM. 60% according to the G-3x and 65% according to the Cessna chart. The effects of the different firing situations were similar though not as dramatic. Still reached 400 with both systems on (Electroaire was firing at 36.4BTC).

I have a couple questions/theories. First, I suspect maybe the timing collar (crankshaft pickup) has slipped and the TDC reference is off. This could explain the higher temp with the advance turned off as compared to the mag only. This will be the first thing I check.

Questions for those of you with Electroair or other electronic ignition systems: Would you expect or have you noticed higher temps with both systems firing? Think maybe better burning causes that. How much advance are you experiencing? The Electroair manual shows that at 22 MP the advance should be about 4 degrees, 29BTDC while I am getting 34.8. Much earlier. At 20.9 MP the manual shows seven degrees advance for 32BTDC and I am getting 36.4.

Any other thougfhts/ideas?

Thanks
Scott A. Jordan
N33JJ
 
You have way too much timing running ROP at the lower MAP and probably too much well LOP as well. You'd be losing power as well as causing the high CHTs.

Not sure what you can do about it, but if you can, I'd pull it back so you don't have more than 32 degrees total in that 22-25 inch range as a starting point. Even that may be too much ROP.
 
You have way too much timing running ROP at the lower MAP and probably too much well LOP as well. You'd be losing power as well as causing the high CHTs.

Not sure what you can do about it, but if you can, I'd pull it back so you don't have more than 32 degrees total in that 22-25 inch range as a starting point. Even that may be too much ROP.

I agree 100%.
 
If the mag timed at 25 and the EI timed at 25 degrees, when run individually give different CHT results, it is highly likely they weren't timed correctly or you have an issue with the timing setting on the EI. It seems quite apparent from your reported tests that the EI is running quite a bit more advance than the mag.

Suggest you do base timing on the EI with a timing gun.

CHT generally increases with advance, once advanced to a certain point. Two ignitions running nets a more advanced timing than one, due to both plugs firing. Even with 2 mags, when you turn one off the effective timing retards and CHTs drop. With two sparks, you have two flame fronts that meet and peak pressure occurs before it would with one spark (combustion takes longer with one spark as the flame must travel twice the distance before reaching peak pressure.)

Frankly, given that you were advanced enough to cause pre-ignition I am quite surprised teh electroair folks did not suggest that you confirm base timing before replacing the unit.

Larry
 
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