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Thread: Best Cylinders for CHT & EGT Probes?

  1. #11
    J Ryd's Avatar
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    Default Re: CHT/EGT Probes

    My CHT is on #3. EGT on #4. I have the oil cooler mounted behind #4. It's the way I got it.

    Jan
    Last edited by J Ryd; 10-09-2020 at 08:34 PM.
    ​Jan

  2. #12
    andya's Avatar
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    Default Re: CHT/EGT Probes

    I agree with J Ryd's install. In what I have and have flown the #4 is usually the warmest cylinder.
    for whatever reason the #3 seems to be the best place for the EGT. My PA-30 had the alcor
    EGT (I think from Piper or installed when new) was using the #3 cylinder.
    "Progress is our most important problem"

  3. #13
    Administrator Steve Pierce's Avatar
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    Default Re: Best Cylinders for CHT & EGT Probes?

    I merged the two threads so this information is all toh=gether and easy to find next time.

  4. #14
    Kyle Duncan's Avatar
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    Default Re: Best Cylinders for CHT & EGT Probes?

    Thanks for the replies. That will give me a place to start. Iím learning towards #3 cylinder. Thanks for merging Steve. I searched but for some reason I didnít see the other thread. Maybe Iím just getting old.


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  5. #15

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    Default Re: Best Cylinders for CHT & EGT Probes?

    Number 3 has the warmest CHT on my O-320 (and I have the oil cooler behind #4). The difference however is usually only 6 to 8 degrees.

    I don't think choosing one over the other would be a bad choice. Lycoming allows 425 degrees max CHT, but the consensus is that running in the 350-375 range is preferred with a max of 400 degrees. So if you have the cylinder with the CHT running at 374 and other is actually hotter at 382 degrees, things will still be just fine. However, if you have the one with the probe running at 400 degrees or more, it *might* be a concern.

    For a CHT with a bayonet probe, it's pretty simple to move the probe from one socket to the next. With similar flight profiles and cruise settings on the same day at the same temp (or at least different days with the same temp at the same altitudes) you can figure out which cylinder runs hotter and how much. You just need to leave the wire on the probe long enough to reach both 3 and 4.


    EGT wise, my highest EGT is almost always #2 but occasionally #1. The spread in EGTs varies from about 50 degrees at wide open throttle and full rich mixture on initial climb to as high as 100 degrees on some days leaned and at lower power (2100-2200 rpm). It normally runs around 70-80 degrees difference at normal cruise.

    That is problematic however, and more so in a PA-22 or PA20 where 65% to 75% cruise power settings are common. If you lean 100 degrees rich of peak but the EGT probe is on the "cool" cylinder, then the EGT on 2 or 3 others could be in the 40-50 degrees rich of peak range which is right in the area of maximum stress at higher cruise power settings (google EGT and "red fin" or "red box"). On the other hand, if are leaning to peak EGT or 10-25 degrees lean of peak, and the probe is on the "hot" cylinder then you can again be operating some of the cylinders in the 40-50 degree rich of peak range.

    My advice is to go with a 4 probe EGT if you are going to actually use the EGT to lean.

    If you do have a single probe EGT and are using it for leaning purposes, then lean for 150 degrees ROP, to reduce the potential to have any cylinders running 40 degrees ROP at high cruise power settings. The downside is cooler EGTs, greater fuel burn and potentially more lead fouling issues with the lead containing gaseous combustion products potentially condensing on the exhaust valve seats and stems, which is what leads to burned valves down the road.

    If you are running 100LL, then you may want to check the plugs every 25 hours for lead fouling, and bore scope the cylinder to check for deposits on the valve seats and on the valve stems.

    If you are running mo gas, then that's a non issue.
    Last edited by LarryV; 10-11-2020 at 09:37 AM.

  6. #16
    Kyle Duncan's Avatar
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    Default Re: Best Cylinders for CHT & EGT Probes?

    Quote Originally Posted by LarryV View Post
    Number 3 has the warmest CHT on my O-320 (and I have the oil cooler behind #4). The difference however is usually only 6 to 8 degrees.

    I don't think choosing one over the other would be a bad choice. Lycoming allows 425 degrees max CHT, but the consensus is that running in the 350-375 range is preferred with a max of 400 degrees. So if you have the cylinder with the CHT running at 374 and other is actually hotter at 382 degrees, things will still be just fine. However, if you have the one with the probe running at 400 degrees or more, it *might* be a concern.

    For a CHT with a bayonet probe, it's pretty simple to move the probe from one socket to the next. With similar flight profiles and cruise settings on the same day at the same temp (or at least different days with the same temp at the same altitudes) you can figure out which cylinder runs hotter and how much. You just need to leave the wire on the probe long enough to reach both 3 and 4.


    EGT wise, my highest EGT is almost always #2 but occasionally #1. The spread in EGTs varies from about 50 degrees at wide open throttle and full rich mixture on initial climb to as high as 100 degrees on some days leaned and at lower power (2100-2200 rpm). It normally runs around 70-80 degrees difference at normal cruise.

    That is problematic however, and more so in a PA-22 or PA20 where 65% to 75% cruise power settings are common. If you lean 100 degrees rich of peak but the EGT probe is on the "cool" cylinder, then the EGT on 2 or 3 others could be in the 40-50 degrees rich of peak range which is right in the area of maximum stress at higher cruise power settings (google EGT and "red fin" or "red box"). On the other hand, if are leaning to peak EGT or 10-25 degrees lean of peak, and the probe is on the "hot" cylinder then you can again be operating some of the cylinders in the 40-50 degree rich of peak range.

    My advice is to go with a 4 probe EGT if you are going to actually use the EGT to lean.

    If you do have a single probe EGT and are using it for leaning purposes, then lean for 150 degrees ROP, to reduce the potential to have any cylinders running 40 degrees ROP at high cruise power settings. The downside is cooler EGTs, greater fuel burn and potentially more lead fouling issues with the lead containing gaseous combustion products potentially condensing on the exhaust valve seats and stems, which is what leads to burned valves down the road.

    If you are running 100LL, then you may want to check the plugs every 25 hours for lead fouling, and bore scope the cylinder to check for deposits on the valve seats and on the valve stems.

    If you are running mo gas, then that's a non issue.
    Thanks for that. It gives me some more to think about. Right now I mostly wanted to monitor CHT but thought it would be beneficial to also have the ability to monitor EGT, although only one cylinder. I do intend on adding the switch and probes for four cylinder monitoring in the future. Right now Iím still doing the student pilot thing so I probably donít need to lean too often as Iím usually below 3000í.



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  7. #17
    andya's Avatar
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    Default Re: Best Cylinders for CHT & EGT Probes?

    when I had the O-320 150 hp i used to adjust cruise EGT on #4 cylinder (single probe) to about 75-100 rich of peak.
    as I leaned after climb out the increase in RPM was very hard to see on the tach.
    Possibly the tach was worse than I thought but it seem to respond with throttle movement.
    setting EGT for almost 1000 hours on that engine worked fine.
    The O-360 reacts differently. When I begin to lean I will see a 50-60 rise in RPM.
    I can lean further than that but RPM does not rise anymore and while still 50 rich of peak the rpm is past its peak
    so I noted the egt where the RPM peaked and use that as my set point for cruise.
    Leaning the O-320 it is relatively easy to watch the EGT and determine where the carb leaves the enriching mode,
    with the model carb on the O-360 that change in enriching point is not so definitive.

    Got rambling and probably more than anyone wanted to here but must have happy fingers on the keyboard today
    "Progress is our most important problem"

  8. #18

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    Default Re: Best Cylinders for CHT & EGT Probes?

    Quote Originally Posted by andya View Post
    when I had the O-320 150 hp i used to adjust cruise EGT on #4 cylinder (single probe) to about 75-100 rich of peak.
    as I leaned after climb out the increase in RPM was very hard to see on the tach.
    Possibly the tach was worse than I thought but it seem to respond with throttle movement.
    setting EGT for almost 1000 hours on that engine worked fine.
    The O-360 reacts differently. When I begin to lean I will see a 50-60 rise in RPM.
    I can lean further than that but RPM does not rise anymore and while still 50 rich of peak the rpm is past its peak
    so I noted the egt where the RPM peaked and use that as my set point for cruise.
    Leaning the O-320 it is relatively easy to watch the EGT and determine where the carb leaves the enriching mode,
    with the model carb on the O-360 that change in enriching point is not so definitive.

    Got rambling and probably more than anyone wanted to here but must have happy fingers on the keyboard today
    If you look at a power chart for an O-320 on page 3-14 you'll get a pretty good picture of what is going on.

    Best economy (i.e. lowest specific fuel consumption per horse power) occurs between about 75 degrees Lean Of Peak (LOP) EGT and peak EGT.

    Best power occurs at about 100-150 degrees Rich Of Peak (ROP) EGT.

    In between you'll find the curve for CHT peaks at about 50 degrees ROP. That's also the area where the engine experiences maximum cylinder pressure, and where the engine is most stressed.

    When you are using the tach and leaning to peak RPM you are literally leaning for best power and that's not a bad way to lean as it'll keep you in that fairly safe 100-150 degrees ROP range and as you note you are leaned enough that you are no longer activating the enrichment circuit.

    When you lean the engine by leaning until "stumbles" and then enrich it enough to get the engine to run smoothly, you'll be at peak EGT or slightly lean of peak on most carbureted engines. That's also not a bad way to lean, particularly at lower power settings, where the red box or red fin are smaller. With a climb prop (say a 74/56) 2500 rpm will only be producing about 60% power at 2500 ft and 2500 rpm, so you won't hurt the engine by over leaning, as long as it runs smooth. With a 60 or 61 pitch prop you'll be pulling around 65% power at 2500 rpm and 2500 ft, but again you are not likely to harm the engine by leaning.



    Digging a little deeper, the range where the engine is getting abused by high cylinder pressure and high CHTs varies with the power setting. At settings over 75% power it's a very wide range and thus pretty much everyone advises to take off and climb with a full rich mixture to 5000'. The downside is that wide open throttle (WOT) and 2700 rpm will give you fuel burns in the 13 to 15 gph range at low altitude with an O-320.

    That changes as you climb. When you get up around 5000', WOT and 2700 rpm is only going to produce about 75% power under standard temperature conditions and under 75% you can start leaning the mixture. That's where the "no need to lean until you are over 5000 ft" rule came from. However, if you are flying along at 2400-2500 rpm cruise settings at full rich, you'll still be burning a lot of extra gas and getting no benefit at all from it.

    As the power setting decreases, the range where the engine experiences potentially excessive stress and CHTs gets smaller. Down around 60% power the engine won't develop enough pressure to abuse the engine at any combustible mixture and the range of mixture settings that may damage the engine disappears.

    In that case, leaning to 100-150 degree ROP, leaning to 50 degrees ROP, leaning to peak EGT or leaning 50-100 degrees LOP are all fine. Just make sure the engine is running smoothly.

    ----

    However, the other side of the coin is that very rich mixtures at lower power settings (2000-2300 rpm) will result in cooler CHTs than desirable and EGTs, which can potentially cause lead deposits on the valve seats, valve stems, plugs, etc. That's because while 100LL has only about half the lead 100 octane "green" gas had, 100LL still has 4 times as much lead as the 80/87 octane fuel the 150 hp O-320s were designed for.

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