Hight CHT - Solution found

walter · #1 06-04-2006, 05:31 PM

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.

vlittle · #2 06-04-2006, 07:03 PM

Quote:

Originally Posted by walter

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

wooody10 · #3 06-05-2006, 12:52 PM

Hey walter, How about some before and after photos?

Jim Streit
RV973MC

walter · #4 06-05-2006, 01:57 PM

Quote:

Originally Posted by wooody04

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.

I'll post some pics of the completed cowl this evening.

walter · #5 06-05-2006, 05:22 PM

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.

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

walter · #6 06-05-2006, 05:46 PM

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.

Davepar · #7 06-06-2006, 09:56 AM

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.

Chickenlips · #8 06-06-2006, 12:04 PM

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

osxuser · #9 06-07-2006, 10:53 PM

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...

Rick6a · #10 06-12-2006, 02:07 PM

Quote:

Originally Posted by walter

......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.