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Thread: My efforts to reduce high CHTs

  1. #1

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    My efforts to reduce high CHTs

    A month or so ago I installed a CGR30P in my cub and during my inaugural flight with an OAT of 50 degrees, I was a little dismayed that my #4 CHT was 400 degrees in cruise at 2400 rpm, 8.6gpm (50 ROP), at around 700 msl. This engine has around 1150 smoh and all compressions a few weeks prior at annual were all 77 and better.

    I ran down the rabbit hole on this and wanted to share what I found and what I ended up doing. After talking to my A&P and a few others, I decided to do the following in chronological order with the supervision and assistance of an A&P.

    1. I tightened up my baffling, installed new baffle material where needed, silicone caulked everything, and made all baffling as tight as possible. I also changed out my intake gaskets and hoses to make sure I didn’t have any minor intake leaks. Both gaskets and hoses were old anyway. By doing this, I reduced my #4 cht by 10 to 12 degrees.

    2. I then began down the ramp path and built multiple iterations (about 10 altogether) from what I found on here and from talking to others. I found the best ramp design was at an angle that met the midway point of the cylinder of #1 and #2. #1 ramp is only half the cylinder and #2 is the entire length of the cylinder with a “fence” on the inside. I drilled holes for front cylinder cooling. After adding ramps, I gained an additional 10 to 15 degrees of cooling on #4. So I’m now down to about 25 degrees cooler.

    3. I then decided to add a lower cowl lip. I bought one from Charlie Center and installed. I found zero decrease in CHTs by adding a lower cowl lip.

    4. Charlie Center suggested that I tape off around the front oil cooler to remove air flow from lower cowl. I did some prelim testing by just taping off around the oil cooler with metal tape. I found this gave me another 5 degrees of cooling. I will be fabricating up a permanent baffle for around the oil cooler in the future.

    As a whole, I reduced my #4 CHT from 400 down to 367 to 372 degrees at cruise running at 2400 rpm at 8.3 to 8.6gpm (ROP).

    I will post some pics here shortly. Hopefully this helps others, as the previous threads on this site really helped me visualize the ramps.
    Last edited by AKaaron; 05-16-2021 at 10:35 PM.
    Thanks JeffP, Steve Pierce, OLDCROWE, Colorado-Cub thanked for this post
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  2. #2

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  3. #3

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    Timely thread. I noticed a similar issue after installing a JPI EDM700. #4 is 400ish during cruise. I sealed off the hinges and made sure the baffles were making contact all around. No change. Then I put some aluminum tape on the lower front cylinders and still no noticeable change. Sucks the cowl lip didn’t work for you, as calling Charlie was my next step. I think I’m going to replace my baffle/cowl seals and seal everything with Sealant. That’ll be better than the worn baffles and aluminum tape I have “sealing” it up now.

    I also have a rear mount oil cooler and my oil temps run well under 200 all the time.

  4. #4
    akavidflyer's Avatar
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    I went down this hole with my pacer. Use that tape and seal up around the alternator bracket and any other leak path from upper to lower if possible. The big hole for the starter can be sealed up as well. My biggest CHT reduction came from sealing up around the airfilter as it had a substantial gap around it.

    My IA told me from day one that the easiest way to fly and enjoy it was to remove the 4 cyl analyzer and just fly the damn thing like it has since 57...
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  5. #5

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    Quote Originally Posted by akavidflyer View Post
    I went down this hole with my pacer. Use that tape and seal up around the alternator bracket and any other leak path from upper to lower if possible. The big hole for the starter can be sealed up as well. My biggest CHT reduction came from sealing up around the airfilter as it had a substantial gap around it.

    My IA told me from day one that the easiest way to fly and enjoy it was to remove the 4 cyl analyzer and just fly the damn thing like it has since 57...
    We must have the same IA, cause I’ve heard that same comment from quite a few people! ����

  6. #6

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    I was told that as well, albeit mostly joking. I’m still flying the crap out of mine, just as I would’ve done before, but I will be working on cooling it down as much as I can as I go. I see no reason to just ignore a potential issue, just because “it’s flown like that for 60 years.”
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  7. #7
    JimParker256's Avatar
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    Ignorance is bliss... until it isn't.

    Personally, just as with weight and balance situations, I'd rather know what's going on.

    I think part of the challenge with "good instrumentation" is that we see that we're operating close to the "yellow" range (versus nearing redline) temps, and sometimes overreact. Lycoming set those limits for a reason. If you're one degree below the yellow line, you're operating within "normal" limits.

    I'm not an A&P, much less an IA, but what I'm watching for is trends... If I'm flying along and suddenly see escalating CHTs, it's time to take immediate action: enrich the mixture, lower the nose/increase airspeed, or both, but do something to stop the rapid CHT rise... Fast-rising temps can mean lots of things, but NONE of them are good, and there is little time to resolve the issue. If you can't resolve it, expect to lose that engine sooner than later, and fly accordingly. For me, that's a "land as soon as possible" situation...

    I think Mike Busch is partially responsible for this "fixation" on CHTs, with his oft-repeated advice to maintain cruise CHTs at 380ºF or less... That could well be excellent advice for Continentals (which is what he flies and mostly what he supports), but it's probably unreasonable for most Lycoming engines in most airframes...
    Jim Parker
    2007 Rans S-6ES

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