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Low CHT/High EGT

A

algrajek

Well Known Member
I have an I/O 360. My last two flights i noticed my #4 cylinder is about 80 degrees lower that the rest, and the EGT is higher than normal. Normally when I lean at cruise, my #1 peaks first, but now the #4 peaks way before the other 3 and if I lean to where I normally do, the engine gets really rough.
Any ideas?
Thanks
Nordo
 
I am not an engine expert but it sounds like incomplete combustion in the cylinder. That may lead to lower CHT but a higher EGT if the unburned fuel is burned in the exhaust.

Or something is causing less fuel in that cylinder. Could a partially plugged injector cause these symptoms?
 
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Do a compression check.

It could be a bad exhaust valve or valve spring. If that is the case, you don't have the charge burning fully in the cylinder (lower CHTs) and fire going out the exhaust (high EGT).

Between and what Ron suggested, you should be able to find the problem.

(Like Ron, I'm not an engine expert either.)
 
next

After doing a compression check of that cylinder, clean the injector for #4 and the clean/check those spark plugs for #4.

Good luck
 
Make sure you don't have a cold plug in that cylinder. I was surprised to find out how smooth my motor ran on 3 cylinders at 1700 RPM on the ground. Cycle from the L to the R mag on the ground however leave them in each position for at least 30 seconds and watch your EGT's. It takes about 10 to 15 seconds to start to see the EGT drop when a cylinder goes cold. A normal quick mag check will not show the cold cylinder.

George
 
Even if I had a cold plug in that cylinder it would hgave shown up before 60 hours.
I guess ill look at the injevtor and try cleaning the plug.
I thought i would have had a good answer by now.
Nordo
 
I had similar problem.

Turned out to be a broken wire in the harness. With only one plug firing on the cylinder the temp were as you stated.

Kent
 
I had half a coil go out giving me a cold plug. As you mentioned it produces the results he posted as far as EGT's and CHT's. It might be something else but it sure seems like the same cold plug situation I had. The cold plug is probably a very cheap fix. If its something else then it might get expensive.

George
 
In an attempt to help future readers, I am reserecting this thread.

I have the exact same symptom as the OP ...

"Low CHT and high EGT on one cylinder" - in my case it is my engine is a carbureted O320. In flight this cylinder is 80 degrees colder than my hottest cylinder (#2).

I performed an in-flight mag test and the delta on all cylinders remained the same on both, left-only, and right-only.

Any new suggestions on cause or further tests?
 
The first question needing an answer is has it recently changed, or always been this way?
This much difference would not be unusual if you have a carburetor engine.
 
I can not say if it has always been this much. I am not the builder. However, for the first 100+ hours I flew it, CHT #2 was the hottest. The original instrumentation was a single analogue gauge with a selector switch. I could not say is the speed was 80 degrees but it was noticeable.

If we assume for a moment that the spread has been this way from the start, then I'd ask a follow up question.

"Why is #2 the hottest cylinder?"
 
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Same Issue - Unknown how to resolve

I own an RV-7A with a Lycoming IO360. It has a slick impulse mag on the left and a Pmag on the right. TT on engine and airframe is roughly 600 hours. I have a Dynon engine monitoring system with CHT and EGT probes on all four cylinders. I purchased the plane in April 2022. I am not a builder and not a mechanic. My expertise in mechanical/engine related stuff is very low. I am grateful for your help and patience!

Shortly after purchasing, I had an issue where the #1 CHT and #1 EGT spiked on climb out. I quicky landed and contacted a mechanic who assisted me with blowing out the fuel injector (which appeared clean). This resolved the issue.

A couple of times while flying over the next 6 months, I noticed the #1 CHT begin to drop while #1 EGT began to rise. Both times, I was LoP in cruise. The #1 CHT dropped from about 330 to 280. Both times, the issue seemed to resolve itself by the time I flew next.

When the issue did not correct itself, I took the plane to a mechanic for further inspection.

The mechanic did a boroscope and found the #1 cylinder to be badly "stepped" and the exhaust valve sticking. He sent the cylinder off for a complete overhaul.

I began flying again about 3 weeks ago and have flown 30+ hours since the cylinder overhaul. I completed the recommended break-in procedure. The mechanic also did a one hour run up on the ground per Lycoming prior to my flight break in.

From the very first flight, the #1 CHT was lower than it has ever been and the #1 EGT has been higher. This remains true whether I am running full rich like on takeoff or lean of peak. The cylinder still peaks at the same time as all of the other cylinders (within 0.1 gallon fuel flow) but the temperatures are vastly different than before the overhaul.

Also, at the time of overhaul, the mechanic flushed the fuel lines and blew out and cleaned the number one injector.

Prior to the overhaul, the typical CHT and EGT temps for my 4 cylinders while LoP in cruise were.

#1 CHT: 315
#1 EGT: 1360

#2 CHT: 320
#2 EGT: 1360

#3 CHT: 325
#3 EGT: 1235

#4 CHT: 335
#4 EGT: 1300

After the overhaul, the temperatures I'm seeing while LoP in cruise are:

#1 CHT: 285
#1 EGT: 1440

#2 CHT: 325
#2 EGT: 1215

#3 CHT: 325
#3 EGT: 1260

#4 CHT: 335
#4 EGT: 1300

I read several forums about possible issues that could lead to seeing high EGT and low CHT on one cylinder. I have run a variety of tests and I cannot find the issue.

I did an extended mag check to make sure none of the cylinders went cold. All of the CHTs and EGTs responded in unison during the mag checks. Everything seemed to run smooth on the mag check at 1700 RPM.

The mechanic did a compression check immediately after replacing the cylinder and after 30 hours. The first was 78. The second was 77. He also completed a boroscope after 30 hours and everything looked great on the new cylinder.

The mechanic checked the fuel flow spider and confirmed it is delivering equal fuel to all four cylinders.

All four cylinders are peaking within 0.1 gallon fuel flow.

The mechanic checked that there are no intake or exhaust leaks in the cylinder.

Is it normal to see a significantly lower CHT and higher EGT after overhauling a cylinder? Is there anything else I should be checking?
 
Everything described in the OP’s problem statement is consistent with a blocked injector nozzle.

It’s peaking first because the fuel is restricted but the air is the same so it’s running leaner and reaches peak earlier. Leaner also yields higher EGT. If it’s lean enough you’ll also get less HP out of that cylinder, which will make its CHT lower. You can’t lean to where you usually do because the leaner-than-usual cylinder can’t sustain combustion and turns your 4 cylinder engine into a rough running 3 cylinder engine.

Unscrew the fuel line, remove the injector restrictor, squirt it out with parts cleaner to remove any contamination, and out it all back together.

Pity it’s #4. If it was #1 or #2 you’d be able to get a wrench onto the fuel line through the nose and could do the whole job in 5 minutes without taking the cowls off :)


- mark
 
I agree with you in diagnosing OPs issue. However, for my issue, my cylinders are all still peaking at the same time based on the data I have available from the EGT probes on each cylinder. Any idea what might be causing my issue?
 
I'd try an in-flight magneto check and an in-flight induction leak test. No cost and you don't even need a screwdriver.

Like you though, I'm a buyer and not a builder. Hopefully the engine experts will be along shortly.
 
When the mechanic reinstalled the reworked cylinder, did he check the dry tappet clearance? Grinding valves and seats will change the height of the end of the valve stem in the closed position. Were the valve springs checked for proper tension?
 
That's a great question. I don't know if he checked those things. The plane is currently at his shop. I will ask him those questions on Monday and get back to you. Thank you for taking the time to share your wisdom.
 
Could a weak/failing/clogged spark plug cause this issue? I'm thinking it is unlikely since I did an extended mag check on the ground and saw a rise in EGTs on all four cylinders regardless of which mag was in operation. However, I read in another forum that might happen with a weak plug up to the point of complete failure of the plug?
 
Jeffhaagenson said:
I own an RV-7A with a Lycoming IO360. It has a slick impulse mag on the left and a Pmag on the right. TT on engine and airframe is roughly 600 hours. I have a Dynon engine monitoring system with CHT and EGT probes on all four cylinders. I purchased the plane in April 2022. I am not a builder and not a mechanic. My expertise in mechanical/engine related stuff is very low. I am grateful for your help and patience!

Shortly after purchasing, I had an issue where the #1 CHT and #1 EGT spiked on climb out. I quicky landed and contacted a mechanic who assisted me with blowing out the fuel injector (which appeared clean). This resolved the issue.

A couple of times while flying over the next 6 months, I noticed the #1 CHT begin to drop while #1 EGT began to rise. Both times, I was LoP in cruise. The #1 CHT dropped from about 330 to 280. Both times, the issue seemed to resolve itself by the time I flew next.

When the issue did not correct itself, I took the plane to a mechanic for further inspection.

The mechanic did a boroscope and found the #1 cylinder to be badly "stepped" and the exhaust valve sticking. He sent the cylinder off for a complete overhaul.

I began flying again about 3 weeks ago and have flown 30+ hours since the cylinder overhaul. I completed the recommended break-in procedure. The mechanic also did a one hour run up on the ground per Lycoming prior to my flight break in.

From the very first flight, the #1 CHT was lower than it has ever been and the #1 EGT has been higher. This remains true whether I am running full rich like on takeoff or lean of peak. The cylinder still peaks at the same time as all of the other cylinders (within 0.1 gallon fuel flow) but the temperatures are vastly different than before the overhaul.

Also, at the time of overhaul, the mechanic flushed the fuel lines and blew out and cleaned the number one injector.

Prior to the overhaul, the typical CHT and EGT temps for my 4 cylinders while LoP in cruise were.

#1 CHT: 315
#1 EGT: 1360

#2 CHT: 320
#2 EGT: 1360

#3 CHT: 325
#3 EGT: 1235

#4 CHT: 335
#4 EGT: 1300

After the overhaul, the temperatures I'm seeing while LoP in cruise are:

#1 CHT: 285
#1 EGT: 1440

#2 CHT: 325
#2 EGT: 1215

#3 CHT: 325
#3 EGT: 1260

#4 CHT: 335
#4 EGT: 1300

I read several forums about possible issues that could lead to seeing high EGT and low CHT on one cylinder. I have run a variety of tests and I cannot find the issue.

I did an extended mag check to make sure none of the cylinders went cold. All of the CHTs and EGTs responded in unison during the mag checks. Everything seemed to run smooth on the mag check at 1700 RPM.

The mechanic did a compression check immediately after replacing the cylinder and after 30 hours. The first was 78. The second was 77. He also completed a boroscope after 30 hours and everything looked great on the new cylinder.

The mechanic checked the fuel flow spider and confirmed it is delivering equal fuel to all four cylinders.

All four cylinders are peaking within 0.1 gallon fuel flow.

The mechanic checked that there are no intake or exhaust leaks in the cylinder.

Is it normal to see a significantly lower CHT and higher EGT after overhauling a cylinder? Is there anything else I should be checking?
Click to expand...

The most common cause of high EGT/low CHT is a cyl firing on one plug. When going from 2 plugs to 1, usually the EGT rises around 150* and the CHT lowers a much smaller amount (in neighborhood of 20*). This is where I would start. Clearly one of the #1 plugs is not firing. Could be the plug, the wire or the coil (Pmag) or erosion of the post associated with the problem cyl inside the cap (mag). This is where your mechanic should have started IMHO, as sticking exh valves drop both EGT & CHT.

Also check that the wires are fully seated on the Pmag. I believe they use low quality wires and several folks have had them back off a bit and stop firing.

A quick diagnostic test - Find a buddy with a good old fashion timing light ($20 at HF). Start and idle the engine with the pick up clamp attached to both of the #1 plug wires (one at a time). The wire that does not produce light strobing is the problem circuit. Problem may only exist at leaner mixtures or higher RPMs, so some effort may be required to get the no spark to show itself.

Larry
 
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StephenC said:
Just had exactly the same issue and it was due to a leaking inlet gasket.
Click to expand...

A leaking intake gasket WILL NOT raise EGT when operating LOP and the OP specifically posted LOP reference numbers. That requires retarded ignition (what happens when going from 2 -> 1 spark events) or much more involved issues in the cyl or valve train. When LOP, further leaning, as an intake leak does, can only LOWER EGT. ROP is a completely different story.
 
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lr172 said:
A leaking intake gasket WILL NOT raise EGT when operating LOP and the OP specifically posted LOP reference numbers. That requires retarded ignition (what happens when going from 2 -> 1 spark events) or much more involved issues in the cyl or valve train. When LOP, further leaning, as an intake leak does, can only LOWER EGT. ROP is a completely different story.
Click to expand...

algrajek said:
I have an I/O 360. My last two flights i noticed my #4 cylinder is about 80 degrees lower that the rest, and the EGT is higher than normal. Normally when I lean at cruise, my #1 peaks first, but now the #4 peaks way before the other 3 and if I lean to where I normally do, the engine gets really rough.
Any ideas?
Thanks
Nordo
Click to expand...

Sorry don’t see the LOP reference here but I will stand corrected.
 
Larry and Stephen,

Thank you both for your thoughtful responses.

Larry, I've ordered new spark plugs for the Emag side and I'm going to start with replacing those and seeing if the issue resolves. If it does not, I'll move on to checking the other spark related issues you mentioned.

I'll report back when I have an update. Thank you!
 
Update

I got my plane back from the mechanic a few days ago.

The mechanic did the following:

-Compression check and bore scope on cylinder 1: Compression was 78 and bore scope looked excellent.

-Replaced all spark plugs on the electronic ignition side.

-Fuel flow test from the spider (the flow appeared even between the four cylinders)

-Replaced the injector on cylinder 1 with a new injector. I have standard injectors and all four are the same. The replacement is identical to what was in there before.

-Replaced EGT probes 1 and 2 due to those probes appearing burned.

The results:

First off, ignore EGT probe 4 which has gone bad. The replacement probe is on order.

I flew the plane and performed a lean test. Things seem to have improved when on climb out in terms of CHTs and EGTs being more balanced than before. However, when I run lean of peak in cruise, CHT 1 drops significantly below the others. (See attached photo of data in Savvy).

I am attaching two screenshots of my Savvy data. The first is prior to this most recent work (changing spark plugs, changing injector, etc). The second shot shows the data after the work. You can tell the two sets of data apart because EGT probe 4 went bad for the second set of data.

Thank you again for looking at the data and offering advice.
 

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It is hard to tell from your graph, but as per the Savy fuel flow you should be leaning in 0.1 gal/hr increments, reducing every 10 seconds (or longer, I like 30 seconds). And then repeating the test 3 times. This should be at your typical cruise altitude. My suggestion is to do another lean test, graph the FF in addition to the EGT and then see if all cylinders are consistently peaking at the same time.


Jeffhaagenson said:
I got my plane back from the mechanic a few days ago.

The mechanic did the following:

-Compression check and bore scope on cylinder 1: Compression was 78 and bore scope looked excellent.

-Replaced all spark plugs on the electronic ignition side.

-Fuel flow test from the spider (the flow appeared even between the four cylinders)

-Replaced the injector on cylinder 1 with a new injector. I have standard injectors and all four are the same. The replacement is identical to what was in there before.

-Replaced EGT probes 1 and 2 due to those probes appearing burned.

The results:

First off, ignore EGT probe 4 which has gone bad. The replacement probe is on order.

I flew the plane and performed a lean test. Things seem to have improved when on climb out in terms of CHTs and EGTs being more balanced than before. However, when I run lean of peak in cruise, CHT 1 drops significantly below the others. (See attached photo of data in Savvy).

I am attaching two screenshots of my Savvy data. The first is prior to this most recent work (changing spark plugs, changing injector, etc). The second shot shows the data after the work. You can tell the two sets of data apart because EGT probe 4 went bad for the second set of data.

Thank you again for looking at the data and offering advice.
Click to expand...
 
Hey Chris,

I did the most recent lean test in .1 gal/hr increments. I did not repeat the test 3 times. I will do that this week and post the data. Thank you.
 
Jeffhaagenson said:
I got my plane back from the mechanic a few days ago.

The mechanic did the following:

-Compression check and bore scope on cylinder 1: Compression was 78 and bore scope looked excellent.

-Replaced all spark plugs on the electronic ignition side.

-Fuel flow test from the spider (the flow appeared even between the four cylinders)

-Replaced the injector on cylinder 1 with a new injector. I have standard injectors and all four are the same. The replacement is identical to what was in there before.

-Replaced EGT probes 1 and 2 due to those probes appearing burned.

The results:

First off, ignore EGT probe 4 which has gone bad. The replacement probe is on order.

I flew the plane and performed a lean test. Things seem to have improved when on climb out in terms of CHTs and EGTs being more balanced than before. However, when I run lean of peak in cruise, CHT 1 drops significantly below the others. (See attached photo of data in Savvy).

I am attaching two screenshots of my Savvy data. The first is prior to this most recent work (changing spark plugs, changing injector, etc). The second shot shows the data after the work. You can tell the two sets of data apart because EGT probe 4 went bad for the second set of data.

Thank you again for looking at the data and offering advice.
Click to expand...

In the first pic, it still seems that #1 if off. If this was done while LOP, then still expect running on one plug. Simple test - go up to altitude and set up the same way as for that chart and do a mag check (i.e. turn off one ignition at a time). The non firing plug will be obvious. Sometimes plugs only fail to spark in lean condition and/or lower MAP conditions and obviously it will not be the plug, but some other ignition component in the chain. If this happened ROP, then I would be looking for lower fuel flow on #1.

EDIT sorry, just noticed that the first pic is from before. No longer sure it is ignition, but SOMETHING is happening when you lean that is causing #1 CHT to drop far too much. Notice how well all 4 track together prior to leaning and then the long slow drop on #1 after. That implies that #1 is losing power, relative to the others, when you lean. Still recommend the in flight mag check to rule out a weak ignition component. Generally a partially plugged injector or spider is going to drop the EGT with the CHT when LOP and the ROP data looks correct and would not with a clogged injector.
 
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New Lean Tests and Mag Checks

I completed several additional lean tests. The results seem to be staying consistent. See photos.

I also completed a mag check while in cruise. The engine runs noticeably rougher while running on only the left mag. I also see a much more significant spike in EGTs while running on only the left mag. The left mag is the slick impulse mag. I completed the in flight mag checks at 6500' an LoP in cruise.

When the engine is running on only the electronic ignition, the EGTs still climb but much less so. See photos.
 

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If your electronic ignition advances the timing, then larger spike when running on a single slick mag makes sense, since by turning off the electronic ignition you basically retard the timing. This lowers your CHTs and raises EGTs.

Could you overlay airspeed and fuel flow on these plots? Or just share links to these flights on Savvy?
 
Jeffhaagenson said:
I completed several additional lean tests. The results seem to be staying consistent. See photos.

I also completed a mag check while in cruise. The engine runs noticeably rougher while running on only the left mag. I also see a much more significant spike in EGTs while running on only the left mag. The left mag is the slick impulse mag. I completed the in flight mag checks at 6500' an LoP in cruise.

When the engine is running on only the electronic ignition, the EGTs still climb but much less so. See photos.
Click to expand...

Something going on with EGT on #4. Possibly a failing probe, but could be something else. In some regimes, it is stable and apparently accurate and in others it is bouncing around way too much. The pattern seems to point more to an engine issue over the probe, but just a guess.

#1 CHT tracks with others when richer, but falls off when Leaner. Struggling to understand what that might be based upon the limited data set.

Total timing retard is much greater when running on just the mag, so EGTs will climb higher than the other way. This is normal. For the same reason, runnning on the EI will be a bit smoother than just the mag, with less power drop. When everything is working as it should, It is quite noticeable in sound, but not really rougher. If yours is truly rough when running on the mag, that is pointing you to an issue with the mag or more likely one of the plugs and / or it's associated connection points to the coil inside the mag. EGT rise should be similar for each of the 4 cylinders when running on the mag. Any noticable variations point to that plug. Weak ignition will sometimes work fine when rich but have issues when lean. Might be wise to do the in flight mag checks both rich AND lean to see if the roughness occurs in both.
 
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Jeffhaagenson said:
https://apps.savvyaviation.com/beta/shared/flight/6902707/fb565a23-ea87-499c-8256-305d14a48053

https://apps.savvyaviation.com/beta/shared/flight/6902694/1da56d99-3ebf-479d-a558-14509a6595f9

https://apps.savvyaviation.com/beta/shared/flight/6902702/5f8c2e0e-b2db-4b78-bd67-45ebb613c764

Savvy Flight Links. I didn't know that I could share these. Hopefully these help.
Click to expand...

I looked at the fist flight. I noticed that RPM goes straight from 900 to 2600+ on the initial take off roll. Unless I missed something on the data chart, this is the result of an aggressively under pitched prop. Most FP props will only make 2200-2300 RPM with the initial application of full throttle from a stand still.
 
Yeah, this is really weird. It does look like EGT4 is a probe or a connection issue given how it bounces sample-to-sample.

As for CHT1, this is strange, because EGT1 raises in line with the rest of the cylinders. This should indicate that all of the cylinders are leaning in a similar manner?

I wonder if another signal is being coupled into CHT1 somewhere. Are your probes wired using thermocouple wire all the way from the probe to the instrument? What kind of splices do you use?
 
kaa said:
Yeah, this is really weird. It does look like EGT4 is a probe or a connection issue given how it bounces sample-to-sample.

As for CHT1, this is strange, because EGT1 raises in line with the rest of the cylinders. This should indicate that all of the cylinders are leaning in a similar manner?

?
Click to expand...

Just not enough data here. It could be that #1 EGT is not normally in line. The drop off in CHT as the leaning happens is pointing to something off with #1 but just not enough data to pin point it. Given the low MAP he is running due to the over pitched prop, it is also possibly and intake leak leaning things out. These leaks are no issue at high MAPs, but the low MAP due to the pitch would bring it into question as he leans for cruise.
 
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EGT 4 and CS Prop

I'm aware of the issue with EGT probe 4. I have a new probe on order and will install when it arrives. Please disregard any data from EGT probe 4 right now.

As to the question about my prop, it is a constant speed prop rather than a fixed pitch prop.

I am happy to send more data if it helps with diagnosing why CHT 1 is dropping off so significantly when I lean.
 
New Data - Mag Check

I can't get the flights uploaded to Savvy for some reason.

I performed a mag check at altitude (5500') today. I did the test both lean of peak and rich of peak.

Rich of Peak:

EGT 1 (Both Mags): 1350

EGT 1: Left mag (slick impulse mag): 1410

EGT 1: Right mag (electronic ignition): 1360


Lean of Peak:

EGT 1 (Both Mags): 1410

EGT 1: Left mag (slick impulse mag): 1600

EGT 1: Right mag (electronic ignition): 1480

I'm not sure what this means but I'm hoping this data may mean something to all you engine experts!

My next thought is to replace the plug associated with the left mag and possibly adjust the timing on that plug? Any thoughts about whether I am on the right track?
 
kaa said:
Why? It seems to be tracking #2 and #3 very well?
Click to expand...

But that doesn't mean it is accurately measuring the temp. It is possible that the CHT is falling off due to excessive leaning. You can get tricked into thinking that EGT has a number similar to others and therefore not that lean when it is not accurately reflecting the real temp.
 
Jeffhaagenson said:
I can't get the flights uploaded to Savvy for some reason.

I performed a mag check at altitude (5500') today. I did the test both lean of peak and rich of peak.

Rich of Peak:

EGT 1 (Both Mags): 1350

EGT 1: Left mag (slick impulse mag): 1410

EGT 1: Right mag (electronic ignition): 1360


Lean of Peak:

EGT 1 (Both Mags): 1410

EGT 1: Left mag (slick impulse mag): 1600

EGT 1: Right mag (electronic ignition): 1480

I'm not sure what this means but I'm hoping this data may mean something to all you engine experts!

My next thought is to replace the plug associated with the left mag and possibly adjust the timing on that plug? Any thoughts about whether I am on the right track?
Click to expand...

Not good there. Something is off with your ignition and/or timing advance. At around 2500 RPM and 23" of MAP, your slick has 25* of advance and the EI probably has around 30*. So, with both plugs firing, your effective advance is somewhere around 40*. Let's just assume that for the moment. Up to a point, more advance = more power, lower EGTs and higher CHTs. Less advance is opposite. So, when you drop the slick, adv goes from 40 -> 30 and should see EGTs rise a good amount (lets say 100* for this exercise). When you drop the EI, adv goes from 40 -> 25, a bigger drop, and should see EGTs rise even more (lets say 150*). Your ROP data above is nowhere close to that and the LOP data doesn't make a lot of sense. Unfortunately the guidelines I listed do not universally apply. Once you go beyond the optimum adv range, the data trends change, so cannot exclude the possibility that there is too much advance as opposed to too little.

Step one is to re confirm the slick timing with a buzz box and competent operator. Step two is to reset the EI's TDC reference and check the advance table. Then put an automotive timing light on the engine to confirm what it is ACTUALLY doing. I would applying varying levels of vacuum to the MAP feed into the EI to confirm that it's MAP sensor is working and that the advance follows the vacuum changes. Most A&Ps know little about the EI systems, so it may be wise to start a dialogue with the EI manufacturer for troubleshooting support.

Did you observe all four EGT rises when doing the mag check? DId all have similar outcomes?

I have one mag and one EI. If I drop the EI, my egts rise by 150-200 Can't remember, but think that the rise is 100-150 when I drop the mag. These are at lower MAP ranges where the EI is advancing beyond 25. All of this is somewhat general, as many factors are in play and affect the EGT rise.

Larry
 
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So, with both plugs firing, your effective advance is somewhere around 40*
Click to expand...
Could you explain this? How can effective advance be more than the maximum advance of both systems?
 
kaa said:
Could you explain this? How can effective advance be more than the maximum advance of both systems?
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I stated that one ign was 25* and the other 30* and the effective timing (relative to what is understood on single spark engines) on both is about 40*. 25+30=55, so I DID NOT state the effective timing was more than both individual systems, though I did say it was more than either individual system. When I say more I don't mean ACTUAL timing, I mean more EFFECTIVE or theoretical timing, which take flame front movement into effect. DUal plug setups generally run less timing than their single plug counterparts.

Generally you want max cyl pressure around 5-8* ATDC. It takes time for the flame front created by the spark to move far enough to reach max pressure (somewhat simplified explanation) This why we light the flame well before we want max pressure. Several things impact flame travel time to max pressure, such as RPM MAP, mixture ratio, etc. With a single plug engine, there is one spark at the center and one flame front that radiates out to the edges of the cyl. Very simple. Could also be designed with a plug off to the side with a quench area. With a dual spark setup, you have two sparks and two discrete flame fronts emminating from the edges of the cyl towards the center. If you light both, the flame fronts travel 2.5" and meet, completing the full burn. If you light only one plug that flame front must travel 5" to reach the other side to complete the full burn. Therefore the effect of timing is different when using one plug vs two on a two plug setup. If you light both plugs, you get to max pressure MUCH faster than lighting either of the individual plugs alone. This is hard to explain in one paragraph and don't have time for a white paper on it. Hopefully it is enough to make sense.
 
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I see what you mean, thanks. I just never heard the term "effective timing" before.

It still seems hard to explain why CHT on a single cylinder would go down without an abnormal rise in EGT from an ignition system perspective, I think? It does kind of look like there may be an indication problem of some sort.
 
kaa said:
I see what you mean, thanks. I just never heard the term "effective timing" before.

It still seems hard to explain why CHT on a single cylinder would go down without an abnormal rise in EGT from an ignition system perspective,
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If thinking in the way that most do about advance within the ranges we are used to seeing, that is correct. Once you get to excessive advance or excessive retarding, the familiar patterns are no longer there. Excessive advance will drop both EGT and CHT, as will many other things. I have no real confidence that this is an ignition issue, but the ignition is highly suspect based upon the in flight mag drop results.
 
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I'm going to replace the aviation spark plug for cylinder 1 and have a mechanic check the timing. I'll report back next week. Thank you all.
 
Update

The timing of the slick impulse mag was off by 4°. The timing has been corrected.

The spark plug (slick impulse side) for cylinder 1 was replaced.

The EGT is now much more closely aligned with the other cylinders.

Prior to this, I had also replaced the fuel injector on cylinder 1 with a slightly upsized fuel injector. This brought the CHT for cylinder 1 in line with the other cylinders while running lean of peak in cruise.

Overall, I'm happy with where everything settled out. I'll upload some flight data soon showing the changes. Thank you to everyone for the thoughtful and helpful suggestions.
 
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