What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

More high CHT's etc. Some test results

hlangebro

Well Known Member
So, I have had high CHT's in climb since day 1 (410-425f). I was pondering about the fuel nozzle and wanted to check my egt's, fuel flow etc. So here are my test data from today. Cht's corrected for 100F day.

Engine is a O-320 D3G, 160hp, Whirlwind GA200, RV7A. Cowl flap.

On ground 1200rpm -60rpm increase from full rich to just before rough. Just about right.

Static; 2160rpm, 130hp, 81%, 12.8gal/hr. A little low for fuel flow...

Climb, cowl flap full open, full rich, 115kias:
cht: #1 393, #2 408, #3 391, #4 379
137hp, 85%, fuel flow 12.3gal/hr. Fuel flow seems low....

Cruise, 8500ft DA, WOT, full rich, cowl flap closed, no increase in rpm during leaning, 2730RPM, 77%, 124hp, fuel flow 13.2gal/hr
EGT's (from full rich to peak, delta F):
#1 88F, #2 96F, #3 127F, #4 151F

CHT (as indicated)
#1 352 #2 365 #3 354 #4 361

Conclusions....
Seems like it is running a bit lean, but not by much. I would say that an increase in fuel flow would definitely keep the cylinders cooler and I could potentially climb at a lower speed without hitting 400F. I'd like to stay below 380F.
I am pondering about re-jetting. One thing that I also need to double check is that the mixture lever actually opens fully. I have the James cowl with pressure plenum. I have sealed everything and checked for induction leaks, I have been modifying for about a year now, with some success, but I still haven't found the smoking gun... my guess is that the mixture is a bit too lean...
 
Last edited:
i almost did the jet drill. I had the carb off, ready to drill, and i was told not to by a guy on this forum and i also found more reading that said it isn't a good idea. But you know what? kinda thinking i should have. it has worked for others. Let us know your results if you do it. I started going full rich at take off instead of leaning to peak and then two turns rich- it helped a little but not sure. Again I was told to try to achieve 1250 egt and i can't get it that low.. my temps are pretty similar to yours except i have louvers not a cowl flap... cooler weather is almost here and i'm exhausted from fighting this problem this year. i've made good improvements but still not satisfied. good luck
 
I think that if, when you lean the mixture, you can get a 100 degree rise in EGT, you are not likely to be running too lean and I don't think I'd be inclined to re-jet. Also, the ground test at 1200 is likely still on the idle circuit and is probably not going to reveal much in terms of the jetting.

I think I'd look at #2 & #4 with an eye toward balancing the temps on climb - if you could get #2 down a little and #4 up (adjusting the dam at the front of #2), then the spread would be more uniform and the numbers don't look to be bad at all. You'd be at or just below 400 all the way around on a WOT climb.

I think if I could get the balance of those cylinders I'd declare victory.

Dan
 
So, I have had high CHT's in climb since day 1 (410-425f). I was pondering about the fuel nozzle and wanted to check my egt's, fuel flow etc. So here are my test data from today. Cht's corrected for 100F day.

<snip>

Climb, cowl flap full open, full rich, 115kias:
cht: #1 393, #2 408, #3 391, #4 379

<snip>

CHT (as indicated)
#1 352 #2 365 #3 354 #4 361

<snip>

Your temperatures look fine to me. There is nothing wrong with cooler temperatures, but there is nothing wrong with where you are today.
 
Temps and speeds

I would like to be able to climb better, around 100kias, but there is no way I can do that without, in my opinion, running to hot on Cht's...
I've been trying to balance the temps, but some reason the left bank runs hotter...
 
....Climb, cowl flap full open, full rich, 115kias:
cht: #1 393, #2 408, #3 391, #4 379
137hp, 85%, fuel flow 12.3gal/hr. Fuel flow seems low....

Cruise, 8500ft DA, WOT, full rich, cowl flap closed, no increase in rpm during leaning, 2730RPM, 77%, 124hp, fuel flow 13.2gal/hr
EGT's (from full rich to peak, delta F):
#1 88F, #2 96F, #3 127F, #4 151F

CHT (as indicated)
#1 352 #2 365 #3 354 #4 361

... I have sealed everything and checked for induction leaks, I have been modifying for about a year now, with some success, but I still haven't found the smoking gun... my guess is that the mixture is a bit too lean...


Funny - I'm working cooling issues as well, but what you consider a "problem" is my "goal". Barely above 400 in the climb and mid 300's in cruise is the promised land for me... I don't think you have a problem at all. I'm not saying you should stop looking at the details- air cooled engines need a lot more attention to detail for localized airflow management, but as a gross figure you're in great shape.
 
More data...

Did another test today. I heard from Sam James that wrapping the inlet tubes with exhaust wrapping would cool the inlet air and thus lower CHT's. Don't think it did anything...
This time I climbed out at 110kias, so 5 kias slower than before. Temps compensated for a 100F day:

#1 403, #2 411, #3 399, #4 385

Ok I guess, but not great.

I also did the induction test. EGT deltas:

#1 38, #2 44, #3 50, #4 111

What does this really tell me? cyl 1-3 are reasonably close, but #4 is way more... Seems like 1-3 are running lean and #4 is rich...? Would this mean I have induction leaks on 1-3?
 
You can't diagnose induction leaks by doing a climb test versus temps. Run the test I linked earlier.

Also, what does 'Temps compensated for a 100F day' mean? Are you saying the temps listed are not your actual temps? 100F at what altitude?
 
Let me clarify.

Climb temps are compensated by the OAT and 100f delta, as it should be done. So, actual temps were lower since it was not a 100F day. The temps stabilize fairly quickly, so I would say about 1500ft or so. You should always normalize the cht temps to 100F.

As far as the induction leak test. I was a 8500ft DA and noted the full rich EGT, WOT. I then pulled back 10" and noted the EGT temps. Deltas as shown.
 
Last edited:
Let me clarify.

Climb temps are compensated by the OAT and 100f delta, as it should be done. So, actual temps were lower since it was not a 100F day. The temps stabilize fairly quickly, so I would say about 1500ft or so. You should always normalize the cht temps to 100F.

.....

I'm not sure that really works for our engines.

Some cooling is created by the oil system, and that has a thermostat in it which makes it non-linear with the OAT.
 
It is a standard flight test standard to compensate for 100F day. Any flight test pilot will agree. I have worked with quite a few. I am an aerospace engineer. Places I have worked for, Piper for instance, would agree.

You can not use just actual temps. That doesn't mean anything. The standard is to normalize to a 100F day to get data you can compare to.

The 100F was chosen as a standard for a "hot" day. Anytime you do your climb test, this is the temp you normalize to, to get accurate data.
 
Last edited:
How

How do you normalize then?

I'd be happy with your temps before "normalized". I was curious how your #4 was so cool but looking at your build log, your oil cooler is mounted sooooo low that it appears all the air cooling your oil also has to pass through the #4 cylinder fins first. Vans instructions says to mount it as high as possible. But considering #4 is my hottest, maybe I should have gone lower.....
 
It is a standard flight test standard to compensate for 100F day. Any flight test pilot will agree. I have work with quite a few. I am an aerospace engineer. Places I have worked for, Piper for instance, would agree.

You can not use just actual temps. That doesn't mean anything. The standard is to normalize to a 100F day to get data you can compare to.

The 100F was chosen as a standard for a "hot" day. Anytime you do your climb test, this is the temp you normalize to, to get accurate data.

You can normalize for atmospheric conditions and density altitude, but I know my Tiger does not normalize CHTs with temp.

I do fly on actual 100F days here in AZ, and I know my CHTs are not running 50 F lower when I fly on a cool winter day. Measured with a temp. compensated GEM 610 system
Has anyone else seen this effect?
 
How do you normalize then?

I'd be happy with your temps before "normalized". I was curious how your #4 was so cool but looking at your build log, your oil cooler is mounted sooooo low that it appears all the air cooling your oil also has to pass through the #4 cylinder fins first. Vans instructions says to mount it as high as possible. But considering #4 is my hottest, maybe I should have gone lower.....

The oil cooler location is what Sam James recommends for his baffles and plenum. Yes it is a bit cold, but rather a bit cold than too hot :)
 
Mine seem to show a reasonable correlation between OAT and CHT. I am sure there can be minor differences, but that is in general the way to do it...
 
Last edited:
Drilled the Power jet from a #42 to a #40. This really helped both CHT's and EGT's.
I average the CHT's dropped 10-12F.
Here are the normalized climb temps at 100kias, which is 15kias slower than the first post:

1 390
2 402
3 385
4 365

Cruise CHT's, 75%, cowl flap closed

1 347
2 355
3 353
4 370

Leaned at cruise, 75%, 8500ft DA, EGT's Delta F, rpm increased 100rpm during leaning.

1 116
2 100
3 241
4 250

I am happy with this. I am glad a drilled the jet :)
I could go to a #39, but I think this is good.

I have an afterbody fairing for the cowl exit that I will put back on. Before this I couldn't use it. I will do some mods to it and then I will try it again.
 
You can normalize for atmospheric conditions and density altitude, but I know my Tiger does not normalize CHTs with temp.

I do fly on actual 100F days here in AZ, and I know my CHTs are not running 50 F lower when I fly on a cool winter day. Measured with a temp. compensated GEM 610 system
Has anyone else seen this effect?

I am still wondering about this as well. It would be lovely if it turns out to be standard flight test technique, but I've never heard of it. That could simply be my own ignorance though.

My own experience is that a climb at 60 degrees F OAT is NOT going to end in the same CHT plus 40 degrees F for the identical climb at 100 degrees F. Now, is it standard technique? I don't know, but maybe this explains some of the differences in "cooling" capabilities of different airplanes...significantly different measurement techniques and extrapolations.

It would be great to get more feedback on this.
 
It is a standard. Sure you might see some differences, but that is how you normalize, otherwise there is no way to compare data. 100F is considered a "hot day".
Today, for me, I had a 92F day so it was close.
 
I think you may want to reconsider your full rich fuel flow as part of your solution.

I had a -4 for 10 yrs, O-320-D1A with Ellison TBI and Catto prop and I never came remotely close to running the high CHTs you indicated. I'm down in central Georgia where high temps and humidity are the norm this time of year.

At full rich on takeoff I'd get FF of 14+GPH and could climb at best rate with full throttle and never get beyond 350 CHT(if that high) to whatever altitude I chose.

I know there were several factors in my favor, like the greater efficiency of the TBI, and could easily adjust to any fuel flow I needed. There was also much more even fuel distribution and better atomization. Also the -4s have a well earned reputation for very good cooling.

Also check out Mike Busch's articles on the value of higher FF.

Good Luck,
Glenn Wilkinson
 
The standard correction for CHT testing is found in FAA Advisory Circular 23-8C, section 23.1047 Cooling Test Procedures for Reciprocating Engine-Powered Airplanes.

The test data is corrected to determine predicted CHTs on a day with 100deg F at sea level, and normal lapse rate from sea level upwards, as follows:

Predicted CHT = Test CHT + (100 - 0.0036 * test pressure altitude - test OAT).

Example:
Test CHT = 400 deg F
Test pressure altitude = 4000 ft
Test OAT = 75 deg F

Predicted CHT on a day where the temperature is 100 deg F at sea level is 400 + (100 - 0.0036 * 4000 - 75) = 411 deg F.

In other words, the standard correction assumes that if the OAT increases 10 deg, the CHT would also increase 10 deg. This AC suggests the same correction would apply to oil temperature. For cylinder barrels, they apply a 0.7 factor to the correction - i.e. the cylinder barrel temperature is predicted to increase 7 deg F for a 10 deg F increase in OAT.
 
Last edited:
Thank you H?kan and Kevin...I love the expertise available on this site! This is very timely and useful information, as I'm in the process of finishing up the EZ Cool cowl flap installation and it will be great to have more valid numbers.
 
.......
In other words, the standard correction assumes that if the OAT increases 10 deg, the CHT would also increase 10 deg. This AC suggests the same correction would apply to oil temperature. For cylinder barrels, they apply a 0.7 factor to the correction - i.e. the cylinder barrel temperature is predicted to increase 7 deg F for a 10 deg F increase in OAT.

That is for calculation, not for measurements, correct?

If a cylinder barrel is less affected by OAT (30% less by those numbers) than a cylinder head, doesn't it then imply that in real life the measured CHT would probably lie somewhere between the two numbers?

Are our Lycoming cylinder barrels that much better cooled, and thermally isolated, than the aluminum cylinder heads?

Is this what I see when I notice that my measured CHT is not quite following the OAT in real life?

On a hot day here in AZ that 30% difference between barrel and head could easily be 20 F to 25 F
 
Last edited:
Drilled the Power jet from a #42 to a #40. This really helped both CHT's and EGT's.
I average the CHT's dropped 10-12F

Sure. You reduced power.

I am an aerospace engineer.

Good. Measure upper plenum pressure. Measure the same in a stock Vans cowl. Compare, then consider the shape of your right inlet in the context of converting dynamic pressure to static pressure without separation losses. Right side view, from your build log:

29p2lp2.jpg
 
Dan, how does more fuel reduce power? Have Ly-Con ported heads and they want to open up the jets too - why bad?
Thanks!

Best power mixture is ballpark 0.45~0.52 BSFC, maybe a little more. Richer or leaner results in less power.

From Haken's first post:
Climb, cowl flap full open, full rich, 115kias:
cht: #1 393, #2 408, #3 391, #4 379
137hp, 85%, fuel flow 12.3gal/hr. Fuel flow seems low....


Actually, if those numbers are accurate, BSFC is already at the upper limits for best power. A Lycoming at 25 degrees and WOT seems to ring in at 0.51~0.52:

(12.3 x 6)/137 = 0.53

Consider this Continental chart, used here because of its clarity. Peak power (at this MP and RPM) is 250 at about 108 lbs/hr, or 0.44 BSFC. This chart only goes to 120 lbs.hr, at which HP is down to 245 or so, about 0.49 BSFC. Of course CHT is dropping, here about 18 degrees between 0.44 and 0.49, and since it is dropping faster than Hp, some might consider it a good deal.

2qs1nhy.jpg


Of course, one might also argue that a better approach is to learn how to cool the engine at best power. No additional fuel cost, and you get to use all the engine you already bought. Failing that, retarding the ignition timing will also drop CHT, and again, it's free.
 
Last edited:
My results

I just finished finally resolving my high CHTs this summer. I reamed my main jet up from #42 to #40, AND I added a pair of lower cowl louvers (the ones sold by Spruce). I did both mods and then did some experimenting flying with the louvers covered and uncovered on the same day (to get meaningful comparisons). The results confirmed that the louvers made a much more significant difference in CHTs than enlarging the jet. Enlarging the jet alone only seemed to make a marginal difference in CHT (it did increase WOT fuel flow from 11.2GPH to 12.4GPH) but the louvers made a very significant difference.
I haven't done careful flight testing to determine the resulting loss in speed, but it doesn't seem to be more than 2 or 3 knots.
With the louvers, on a hot day (ground OAT 30 C, or 86 F) I was able to take off and climb at 1000ft/min continuously (to 8000ft) with hottest cylinder not getting above 395 F. In typical cruise all CHTs are sitting between 330 and 350. I'm very happy with these results. Before the mods on a hot day like that I would be up over 400 within the first 1000 feet of climb.
I luckily had some leftover blue paint from when my plane was painted, so I was able to paint the louvers to match the lower cowl (not that anyone looks under the plane on the ground anyway).
RV-9A, O-320, carb.
 
That is for calculation, not for measurements, correct?
Type certificated light aircraft are supposed to have enough cooling to operate at up to at least 100 deg F at sea level. It can be tough to find a day at exactly that temperature to conduct the test though. The intent of the correction described in AC 23-8C is to take test data at a day somewhat cooler than 100 deg F, and use the results to determine if the cooling would be adequate at 100 deg F. The correction is probably somewhat conservative - i.e. it probably predicts a CHT at 100 deg F that is slightly warmer than what you would actually find if you could do the test on a 100 deg F day.

If a cylinder barrel is less affected by OAT (30% less by those numbers) than a cylinder head, doesn't it then imply that in real life the measured CHT would probably lie somewhere between the two numbers?
The CHT wells are embedded in the cylinder heads, so I'd hope that the observed CHT was a good measure of actual CHT.

Are our Lycoming cylinder barrels that much better cooled, and thermally isolated, than the aluminum cylinder heads?
I suspect there are two factors at play:
  1. the bottom of the cylinder barrel is bolted to the crankcase, which is a lot cooler than the cylinder head. This must provide some conductive cooling to the lower portion of the cylinder barrel.
  2. the highest temperatures occur during the combustion when the piston is near the top of the stroke. The gas temperatures will be falling due to expansion as the piston moves toward the bottom of the stroke. Thus the cylinder barrels are subjected to average internal gas temperatures that are lower than seen by the cylinder heads.

Is this what I see when I notice that my measured CHT is not quite following the OAT in real life?

On a hot day here in AZ that 30% difference between barrel and head could easily be 20 F to 25 F
I think you are seeing that the standard correction is a bit conservative, as described above. I.e. it overpredicts hot day CHTs, to ensure that testing at cooler day does not allow a manufacturer to claim his design had adequate cooling, when in fact there would be a problem if you could find the correct temperatures for the testing.
 
Back
Top