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Hight CHT - Solution found

If you have high CHT, do you have the sides of the inlet ramps glassed in?

  • No

    Votes: 27 71.1%
  • Yes

    Votes: 11 28.9%

  • Total voters
    38

walter

Well Known Member
After examining, again, thoroughly, my baffles to make sure they were not
the cause of my high CHTs I found part of the problem, yep, the baffles,
well sort of.

Last weekend, after I pulled my p-mags (that's another story for a different
thread), I gave the top cowl a good lookover. I noticed something strange.

Picture in your mind the cowl inlet "ramp" which gets epoxied to the top cowl.
On the outboard section of the ramp, it's quite easy to get a nice tight fit
with the baffling material (I use the red silicone stuff).

On the inboard side of the ramp it's a bit more difficult as you have to
transition the baffling material from vertical to horizontal as it comes from the
baffle behind the spinner to the inlet.

I stuck my hand way up in there and found that I had a section which was
not covered by the baffles. The hole was big enough to stick 3 of my fat
fingers in, on each ramp. The real problem is that the air passing
through that hole was going right under the ramp, between the ramp and
the top cowl, out to the other side of the ramp and dumping into the low
pressure area of the cowl outboard of the nice sealing outboard baffle.

I brought the top cowl home and glassed in both sides of each ramp so that
there was no way for air to pass under the ramp. Doing this also made the
inboard baffle fit much better.

Prior to this modification I could easily see 450+ in a 300fpm climb, way to hot
and it was seriously limiting my flying.

Today I flew with the modified top cowl. I could not get the CHT hotter
than 404 degrees, and this was during a very very hot day (90+ degrees)
climbing at 1500 fpm, full power.

I'll call that success. Now I'm going to play a bit with exit area and see what
effect that has. I suspect there may be more go gain, especially on the
nose draggers.

If you have high CHT temps, do you have the sides of the ramps glassed in?
It would be interesting to know if that is a common issue. I don't recall
seeing in the plans whether they should be glassed in or not, and I don't have
the plans handy.

Anyway, I'm happy. My hours are flown off (finally) and the plane is now at
her new home at KCPM.
 
walter said:
After examining, again, thoroughly, my baffles to make sure they were not
the cause of my high CHTs I found part of the problem, yep, the baffles,
well sort of.

Last weekend, after I pulled my p-mags (that's another story for a different
thread), I gave the top cowl a good lookover. I noticed something strange.

Picture in your mind the cowl inlet "ramp" which gets epoxied to the top cowl.
On the outboard section of the ramp, it's quite easy to get a nice tight fit
with the baffling material (I use the red silicone stuff).

On the inboard side of the ramp it's a bit more difficult as you have to
transition the baffling material from vertical to horizontal as it comes from the
baffle behind the spinner to the inlet.

I stuck my hand way up in there and found that I had a section which was
not covered by the baffles. The hole was big enough to stick 3 of my fat
fingers in, on each ramp. The real problem is that the air passing
through that hole was going right under the ramp, between the ramp and
the top cowl, out to the other side of the ramp and dumping into the low
pressure area of the cowl outboard of the nice sealing outboard baffle.

I brought the top cowl home and glassed in both sides of each ramp so that
there was no way for air to pass under the ramp. Doing this also made the
inboard baffle fit much better.

Prior to this modification I could easily see 450+ in a 300fpm climb, way to hot
and it was seriously limiting my flying.

Today I flew with the modified top cowl. I could not get the CHT hotter
than 404 degrees, and this was during a very very hot day (90+ degrees)
climbing at 1500 fpm, full power.

I'll call that success. Now I'm going to play a bit with exit area and see what
effect that has. I suspect there may be more go gain, especially on the
nose draggers.

If you have high CHT temps, do you have the sides of the ramps glassed in?
It would be interesting to know if that is a common issue. I don't recall
seeing in the plans whether they should be glassed in or not, and I don't have
the plans handy.

Anyway, I'm happy. My hours are flown off (finally) and the plane is now at
her new home at KCPM.

Hi Walter... you did the right thing. I took a different approach (see below). I glass wings onto the ramps so I could have a smooth transition for the baffle seal.

Even so, I had persistant hight CHTs. 3 cylinders were within 2 degrees, and Cylinder #3 (yes 3!) is the coolest about 10 deg cooler. In climb, I could get #2 and #4 up over 435F in climb if not careful.

The solution was to rejet the carb for a richer mixture. This brought my CHT temps in climb down by about 20F.

Vern Little

122_2290_1.jpg
 
wooody04 said:
Hey walter, How about some before and after photos?

Jim Streit
RV973MC
Here's a before picture. Where you see the black sharpie lines is where
I glass in the openings. You really only need to do the inboard side.

145_4552.jpg
http://rv7-a.com/145_4552.jpg

I'll post some pics of the completed cowl this evening.
 
Last edited:
Here's the cowl fix. The fiberglass is laid up vertically. Orange line indicates
where the front baffle is and you can see the scuff marks to the left of the
orange line where the baffle is positively touching the top of the cowl.

Where the orange line intersects the inlet ramp is where the problem area is.
Once it gets under the inlet ramp, if it's not glassed in, it's lost cooling air.
With the glass in place the vertical part of the baffle material has something
positive to push against.

cowl1.jpg



cowl3.jpg


And a picture with a few arrows and such. (ignore spelling errors pls) :)

cowl2.jpg
 
Last edited:
I'm starting to see a trend in the poll.....

If you do have High CHTs, perhaps you'll note how high they get.
As I mentioned I could easily get to 460. Now I cannot get past 404
on the hottest cylinder, and that on a very very hot day.
 
Not flying yet, but I noticed the same thing when I was working on the baffles. I covered just the aft portion of the sides. Hopefull that does the trick.
IMG_0658.jpg

IMG_0659.jpg
 
In a roundabout way, you should be happy you have high CHT's!! the reasons for low CHT are not ones you may want to face.

My experience has shown that low CHT's are most often the result of an engine producing low output horsepower. This translates into a "worn out" engine. Often in association with this is low vibration. A healthy new engine produces a lot of vibration and higher CHT's while the old "familiar" engine with worn rings and leaky valves is often described as "quite smooth" in operation with low or moderate CHT's. It seems more output per cylinder equals more heat and vibration! A good thing.

A worrisome problem is high oil temp along with high CHT's. This can happen when a new enigne is not properly breaking in and has a lot of exhaust blowby past the rings. This superheats the oil around the piston skirt. A good tip-off being that it tends to give the oil a "burnt" or dark color very early in the oil change cycle.

Generally as the engine breaks in (first 25-50 hours) the rings will seat sufficiently to stop the blowby and the overall friction will decline at the same time. The break-in effects will lower oil temperatures with better sealing and the CHT's will lower slightly with less friction. The related vibration seems to remain relatively constant during the "healthy" years of the engine's life.

cl
 
That is a very over-simplified view. Yes CHT and vibes are a biproduct of power output, especially in the 4-cylinders we fly, but they are NOT an indication of the health of the engine directly. Our 35+ year old O-360A1A was very vibe-y but it still was weak, no data on the CHT's, but we just installed a JPI with the overhaul, so we'll see what they run like, might be doing some baffle work...
 
Cool idea!

walter said:
......cause of my high CHTs I found part of the problem.....
I brought the top cowl home and glassed in both sides of each ramp so that
there was no way for air to pass under the ramp. Doing this also made the
inboard baffle fit much better.

Prior to this modification I could easily see 450+ in a 300fpm climb, way to hot
and it was seriously limiting my flying........
Today I flew with the modified top cowl. I could not get the CHT hotter
than 404 degrees, and this was during a very very hot day (90+ degrees)
climbing at 1500 fpm, full power. I'll call that success...... .
Walter,
From day one, I've been concerned about those CHT's. I tweaked the baffling as much as I knew how. I plugged every small hole I could find with RTV. In the aggregate those steps helped, but I'm always looking to improve those CHT numbers even more. I took your advice and did the same modification. Took the upper cowl home and completed the mod in little over 24 hours. Today I flew for the first time after glassing in the openings and can happily report the following preliminary observations. Monitoring the VM1000 engine monitor, all four CHT's are markedly lower in CRUISE. I still find I have to use caution and limit steep climbouts as the CHT's can quickly exceed 430 degrees even with the mod. Still, I found the temps come down quickly and it seems that instead of usually seeing around 400 degrees of normal CHT in cruise, today the numbers floated between 370 and 390 degrees. It was not a particularily hot day today, but I do know that from 139 hours of operational experience, the CHT temps would have been predictably higher had I not done the mod. On balance, I'd have to say this quick and easy mod was well worth the small amount of effort it took to accomplish. BTW, that small lip you see bonded to the cowl was an early and lame attempt to deflect more cooling air onto #3.....the hottest running cylinder. Did it work? Not really.
img0002209zo.jpg
img0002209fp.jpg
 
Hi Rick, glad it helped you a bit. About your #3 cylinder, have you added
the spacer between the rear of the cylinder and the aluminum baffling?

That little mod seems to help most folks.
 
Well, I have to admit to being skeptical about the effect of blocking the side of the cowl ramp. Saturday lunchtime Hecilopter stopped by my hangar and told me to stop complaining about high CHTs and do something about it. As the temp was forecast to be reasonable (for Texas) at about 85 I decided that it was going to be a glass fiber afternoon. To cut a long story short. I'm amazed at the difference. In a couple of short test flights today, with temperatures in the low 90s, it looks like climb CHTs are down 20 to 25 deg with hottest cylinder shifting from 3 to 1 (if I remove some of the block ahead of #1 then I might get better balance). The hottest I got in a 2000' climb (restricted by airspace) at 110 kts was 390F, that is down from 415 to 420F. Previously I was throttling back at about 2000' to keep the CHTs under control frustrating in a long climb). In a later climb, at 95 kts, CHTs topped out at just over 400F. Cruise CHTs are also way down. I need to fly for longer, and go higher to get some good compartive data, but overall it looks like this simple mod has a significant effect. I think I have also picked up 2 or 3 kts top speed - difficult to tell, prop kept overspeeding (O-320, fixed pitch sensenich), and the air was very bumpy.

If any one is suffering from high CHTs and has yet to do this mod, then my experience is that it doesn't take very long and is very worthwhile.

Yours, Pete
 
Thank You VAF

Hi,

After 26 hours of proving flights (in 1 week!!!!) I have been troubled with the no3 cylinder that would not 'break-in'. All other cylinders were running around 380 and no3 would reach 430 before the end of the take off run. All flying done high speed with WOT.

I have checked the baffles twice. No difference.

Out of desperation yesterday I searched the web and came across this link.

I immediately did the mod and flew yesterday afternoon.

WOW!!!!!!

OAT was 96 deg, full power climb to 7000' and the highest temp I saw on no3 was 410. In cruise 1,2 and 4 were at around 360 and no3 at 395.

Thanks guys.

Chris Spencer-Scarr
South Africa
RV-7A ZU-RVZ
 
I took the opportunity to glass those inlets per Walter's suggestion prior to first flight. Therefore I can't tell you what a before/after effect is BUT... here is what I am currently experiencing with my CHTs.

On climbout I can expect to see about 406 max on the hottest cylinder. We haven't had really hot temps yet but I did see that 406 the other day when it was about 80F out. In cruise I normally see about 375-385F with a 20F spread between the hottest and coldest.

Hope this helps.
 
I guess I missed this thread last year. I think the question needs a third answer - no, the sides are not closed off but it doesn't matter, because the baffle seal rides up both sides of the ramp, sealing off the void between the ramps and the cowl.
 
Another satisfied customer

I changed cylinders this winter and #1 cylinder was still running hot after 10 hours. Friday night I added foam and some fiberglass cloth to seal the ramp and the CHT on #1 dropped about 10 degrees. It still was running a little hot on climb out (415F) so I still was not happy. I cut back the baffle that was blocking the front of #1 cylinder about 1". Now #1 is coolest so I will add a little back and try to balance the cylinders a little.

Just wanted to let people know that even with good baffle seal along the inlet ramps closing off the "tunnels" does help.

Here is a picture looking at #1 cylinder before I cut the baffle down. It's amazing what 1" did.

562853996_jNZmj-M.jpg
 
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