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High CHT's On Climb

John Deakin from AVWeb has a great column that I've read and learned a lot from. I cut a pasted parts here, but go read the entire article for the full effect on the subject. I know this info is not about gallons per hours flow, but for those of us with a flow meter, it is good info.

Normal climb CHT in a well-baffled normally aspirated engine is around 330?F at full power and sea level, at any decent climb airspeed. CHT might be higher if you insist on low climb airspeeds. If you see higher CHTs on your engine monitor, your fuel flow is too low. A lot of these fuel flow indicators are not very accurate at all, and most are not even true fuel flow gauges at all. They're pressure gauges, marked in flow. We really like digital fuel flow systems that have been calibrated by actual tests for this reason. If your redline is 27.0 GPH, and you get only 26.5, have it set higher.

I don't want to be a curmudgeon and dispute a highly regarded published guru, but there are more factors to CHT than fuel flow and baffling - perhaps as great a factor as anything - outside air temperature. A CHT of 330 can not be in boiler plate and I don't think there is a need to be concerned if it is not.

I believe it is a generally accepted engineering premise that all things being equal, if you see 330 CHT flying in 50 degree weather, you will see 370 when flying in 90 degree weather. Yes, fuel flow is a factor but it is not the total answer.
 
I believe it is a generally accepted engineering premise that all things being equal, if you see 330 CHT flying in 50 degree weather, you will see 370 when flying in 90 degree weather.

David, ambient temps certainly make a difference. My numbers were based on ~85 deg F ambient. Climb angles have to be adjusted to keep the CHT's correct in higher ambients, but if the sea level EGT's are not right you will have a VERY hard time controlling CHT's in high ambient conditions.
 
How many of you are out there with O-360's with the 3878 that have good CHT's? Was there a problem and if yes what did you do? Also how many of you have drilled main jet or conversion of the 3878?

This is an excellent question that no one has answered. Who is operating a stock 10-3878 in an RV series aircraft and is able to keep EGT and CHT temps under control? For the purpose of this post, let's assume 'under control' is defined as CHT's under 400 deg F in a reasonable climb (not too shallow, not too steep - around 110 - 120 kts) and sea level EGT's of 1300 or so? My money is on there are very few.....
 
Fixed my high CHT problem

after struggling for the first 12 hours or so of Phase I. OAT 90F and #1 CHT was hard to keep under 430 during initial climbout even with pulling back the power and RPM. Earlier I had added a couple 2" x 5" scoops to the bottom of the cowl with only minor effect.

The big improvement came when I started trimming the front cylinder deflectors. I installed them full size (Vans Baffles and Vans Cowl) during the build. Especially on the #1 cylinder this leaves very little room for air inflow between the deflector and the eyebrow. I'm now down to ~5/8" on #1 and #2 deflectors and able to keep the power in with only minor RPM adjustment (to 2600) as long as I want. Max CHT with 90 OAT is 405. Cruise CHTs are ~375 at 6000' @ 100 ROP and 2500 X 22" or so.

My advice is to leave the front cylinder deflectors OFFfor first flights. Then add aluminum tape and trim to balance CHTs. Only then should you make and mount the aluminum deflectors.

26 hours Phase I and counting...
 
Very long thread.

Try the simple items first.
On climbout I would see ~410.
On perf cruise, ~415.

Glassed in inlet deflector. No change.
Added spacer on rear #3 baffle. Now 380. Dropped CHT 25-30 on climbout.
Perf cruise now ~360-370.

Added forward cylinder deflectors to achieve balance.
Used the tape method for test cases as others have suggested.
 
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after struggling for the first 12 hours or so of Phase I. OAT 90F and #1 CHT was hard to keep under 430 during initial climbout even with pulling back the power and RPM. Earlier I had added a couple 2" x 5" scoops to the bottom of the cowl with only minor effect.

The big improvement came when I started trimming the front cylinder deflectors. I installed them full size (Vans Baffles and Vans Cowl) during the build. Especially on the #1 cylinder this leaves very little room for air inflow between the deflector and the eyebrow. I'm now down to ~5/8" on #1 and #2 deflectors and able to keep the power in with only minor RPM adjustment (to 2600) as long as I want. Max CHT with 90 OAT is 405. Cruise CHTs are ~375 at 6000' @ 100 ROP and 2500 X 22" or so.

My advice is to leave the front cylinder deflectors OFFfor first flights. Then add aluminum tape and trim to balance CHTs. Only then should you make and mount the aluminum deflectors.

26 hours Phase I and counting...

Not at all bad advice but I think their is another factor that needs to be considered...
Unless you are running a previously broken in engine, your engine has also been breaking in during these initial flight hours. A newly overhauled engine in an RV makes it difficult to keep CHT's anywhere below 400 F unless the break-in flights are done in very cool temps.

I thing the fact that you now have 26 flight hours has a big part to play in your CHT's being lower.
 
No doubt

I am chasing a moving target with a brand new engine. However, the dramatic drops I have seen directly correlated with cutting down the front cylinder deflector baffles. On the very next flight. I still advise folks with new engines and Van's baffles to leave off the deflectors, and use aluminum tape to find the best configuration before installing the final aluminum deflector baffles...;)
 
Added forward cylinder deflectors to achieve balance.
Used the tape method for test cases as others have suggested.

Dan, what exactly is the 'forward cylinder deflector'? I am still fighting somewhat high CHT's on climb only. A 110kt climb will have 1 & 3 415 or so (and 1 & 3 are about the same temp), but 2 & 4 are 20 deg cooler,and they are matched. I have tweaked and tweaked my baffles and can see no way to improve them. BTW, cruise CHT's are all balanced, and 335 - 370 depending on where the mixture lever is set. So, would the magic 'deflector' help my climb CHT's?

Thanks
 
Jim, the deflectors are placed in front of one or more of the front cylinders when they run colder. Are #1 and #3 your front cylinders and not reversed with #2 and #4?
 
Ron, thanks. I understand what Dan meant now. I already have the deflectors which has balanced the cruise CHT's. My 1&3 are about the same....my 2&4 are about the same. 1&3 are about 20 deg hotter than 2&4, in climb only. All 4 cylinders are about equal in cruise.
 
I'm a little late to this discussion, but I think that's a misprint from Deakin. Everything I've heard him say, both online and through the Advanced Pilot Seminars, says 380 is the magic number for climb. I can't see 330 as a realistic target, and it's one I can't meet even with an over-cooled angle-valve 390.
 
Just out of curiosity. The fellow that says 330 is a "realistic target" and 380 the "magic number" for climb, does he sell injectors?
 
Normal ops Jim. I think you meant, you need to climb faster than 110 knots to lower the CHT's. If that is the case, thats exactly what I do. My RV8 has a 3878 carb, and I accept the Lycoming recommendation of 500 deg. redline and 400 degrees continuous as my limit. I lower the nose and climb at a faster speed to regulate my temps to stay under 425 for climb( still over 1000fpm). I do not view 5-10 minutes in climb as continuous! A 4164 carb. might help slightly, but if you expect to be able to achieve Van's rate of climb #'s and remain within your CHT limitations as opposed to Lycoming's, well good luck!
Best regards,


RV-8 with 0-360- A1A. I live in AZ. Its already getting hot. I flew this morning and right after take off in Climb I saw #1 CHT hit 425. I pulled back on power and leveled the nose and it quickly cooled below 400. Is there something else I can do to control this? It isn't even AZ HOT yet. The OAT was 92 Temp and climb speed was accelerating to 125. Thoughts , Suggestions? TIA
 
RV-8 with 0-360- A1A. I live in AZ. Its already getting hot. I flew this morning and right after take off in Climb I saw #1 CHT hit 425. I pulled back on power and leveled the nose and it quickly cooled below 400. Is there something else I can do to control this? It isn't even AZ HOT yet. The OAT was 92 Temp and climb speed was accelerating to 125. Thoughts , Suggestions? TIA

I have determined, that with most (but not all) electronic ignitions, when you pull back on power, the timing advances. This may not be ok in all flight regimes. I have an advance disable switch on my Rocket (using Electroair) and will be installing something similar on my next build. It reduces temperatures in the climb. There is a lot to be said for fixed ignition timing until established in cruise.

Oddly, I have seen the same problem with carbureted magneto-driven engines when the main jet orifice is too small and the engine was too lean at high power settings. Solution was to ream the orifice (not as painful as it sounds). According to Bart Lalonde (ex AeroSport Power) many carbureted RVs need this done.

p.s. I miss Arizona!
 
Oddly, I have seen the same problem with carbureted magneto-driven engines when the main jet orifice is too small and the engine was too lean at high power settings. Solution was to ream the orifice (not as painful as it sounds). According to Bart Lalonde (ex AeroSport Power) many carbureted RVs need this done.

p.s. I miss Arizona!

Certain carb P/N's flow more than others for just this reason, also known as the 'Mooney' mod. Old news, a google search should bring them up.

SERVICE BULLETIN
#All-62
TO: All Outlets
SUBJECI: Field Modification of AlO-3878 Aircraft Carbureter into AlO-3878-M
'As Applied to Mooney Aircraft Only.
Improved performance can be realized by field modifying the AIO-3878 carbureters
used on the Lycoming 0-360-AlA and 0-360-AID Engines used in·Mooney Aircraft
Models M20B and M20C, Serial Nos. 1701-2296.
This conversion can be accomplIshed by any A & P with standard shop tools and
kit #A666-660.
 
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RV-8 with 0-360- A1A. I live in AZ. Its already getting hot. I flew this morning and right after take off in Climb I saw #1 CHT hit 425.

Vic, more info needed if we are to offer more than shotgun suggestions. Good photos of the installation with the upper cowl off can tell a lot. Use the little paperclip icon.
 
Is there a way to measure to know if the jet on the carb needs to be enlarged? I believe mine isnt getting quite enough fuel and fight high chts and believe its running lean. Do people just automatically enlarge the jet or is there a way to measure fuel flow without a fuel flow gauge.
 
Can someone explain the "eyebrows" mentioned early in this thread?
Maybe a photo to help?

The "eyebrows" are the inlet ramps which are glassed in from the top of the inlet opening to the underside of the top cowling along the top of both inlets. When installed they will be open on both sides allowing air flow. One end should be sealed to prevent a loss of cooling air from the high pressure area to the low pressure area. Sorry but I don't have any photos. Hope that helps.
 
The "eyebrows" are the inlet ramps which are glassed in from the top of the inlet opening to the underside of the top cowling along the top of both inlets. When installed they will be open on both sides allowing air flow. One end should be sealed to prevent a loss of cooling air from the high pressure area to the low pressure area. Sorry but I don't have any photos. Hope that helps.

Is it safe to say, that if you don’t have the eyebrows this could be the source of high CHTs?
 
Is it safe to say, that if you don’t have the eyebrows this could be the source of high CHTs?

No expert here...but on a stock cowl setup I believe it could be a contributing factor. I think the ramps act like a sort of Venturi to smooth out the airflow and keep it going instead of having it curl back into the void above the inlet.

Maybe some more knowledgeable folks will chime in.
 
Last summer, I replaced my original 10-3878 carb with a factory rebuilt 10-4164-1 carb delivering ~4.4 gph more fuel flow at full-rich, full-throttle, sea-level conditions versus the 10-3878 carb. (Fuel flow data provided by Marvel-Schebler engineering.)

Both carbs are recommended by Marvel-Schebler for the O-360-A1A engine.

Also verify that the mixture control cable is fully stroking the mixture lever on the carb. The stock Vans control cable barely gets you there.
 
RV-8 with 0-360- A1A. I live in AZ. Its already getting hot. I flew this morning and right after take off in Climb I saw #1 CHT hit 425. I pulled back on power and leveled the nose and it quickly cooled below 400. Is there something else I can do to control this? It isn't even AZ HOT yet. The OAT was 92 Temp and climb speed was accelerating to 125. Thoughts , Suggestions? TIA

As Dan mentioned...more info is needed in order to provide relevant suggestions...carb model, ignition and timing, baffling...

Last summer, I replaced my original 10-3878 carb with a factory rebuilt 10-4164-1 carb delivering ~4.4 gph more fuel flow at full-rich, full-throttle, sea-level conditions versus the 10-3878 carb. (Fuel flow data provided by Marvel-Schebler engineering.)

Both carbs are recommended by Marvel-Schebler for the O-360-A1A engine.

Also verify that the mixture control cable is fully stroking the mixture lever on the carb. The stock Vans control cable barely gets you there.

I modified my 10-3878 with the Mooney mod kit #A666-660. It increased fuel flow by 4 USGPM at WOT. The carb then becomes a 10-3878M. The kit included a main nozzle 1 size larger and a restrictor for the economizer cct which provides additional fuel at WOT. Solved my WOT lean run issue.
 
Timing Experiment

Hello all,

after reading this thread, and since I have one electronic ignition, I decided to perform some climb testing with variable advances around 22 degrees and again at 27-28 degrees advance on the electronic ignition which is adjustable in flight. The other ignition is a slick mag set at 22 degrees. I was interested to understand the impact of advance on CHT's for an IO-540-D4A5. I performed several tests on a hot day 90F+ on the ground and around 80F at a density altitude of 6000 ft. This testing was rich of peak EGTs and WOT at 2700 RPM, 100 kts from 5500 ft DA to 7000 ft DA, with data from 6000-7000 ft. On several tests I found no measurable impact of this timing advance on CHT's in this test condition. The data for one such climb test is pasted below. I didn't expect this result as I have read so much about how electronic ignitions increase CHTs and this seems representative of the advance under these conditions. Not sure if my set up is different than others whose CHTs are affected more by advance? Or others would be running even more advance than I? Was expecting some reduction in CHTs during these tests.

The data labels are by column:

Baro Altitude (ft), Indicated Airspeed (kt) , Outside Air Temp (deg C), Density, Altitude (ft), Timing, CHT1 (deg F), CHT2 (deg F), CHT3 (deg F), CHT4 (deg F), CHT5 (deg F), CHT6 (deg F)

Climb test 1
3208 99.1 27.4 5676 19.4 336 334 323 328 339 350
4230 99.8 25.6 6725 19.4 397 384 372 378 387 399
Delta 61 50 49 50 48 49

Climb test 2
3196 99.8 27.3 5651 26.7 336 335 323 328 338 350
4219 99.8 25.5 6699 27.6 395 384 373 378 386 399
Delta 59 49 50 50 48 49
 
Bryan,

To clarify, your EI was set for fixed timing at 19.4, then the test was repeated at 26.7 fixed?

Care to estimate how far rich of peak?

Did you allow the temperatures to stabilize at a maximum?
 
Timing Experiment

Answers

Oil cooler is Airflow Performance 2007X on firewall

Set at 22 fixed for first test and 27 fixed for second test on the electronic ignition.

100 - 150 ROP

If you mean climb until temperatures peaked, then the answer is no. I only compared the rise in temperatures over known climb intervals from a base line. The tests were about 5 minutes apart. Each test started after a decent to lower altitude a few minutes to stabilize engine temps and a slowing to 100 knots to start the test climb at DA 5500 to DA 7000. If the engine is generating more heat the rate of rise should be affected, not just the peak temps?

Thanks,

Bryan
 
If the engine is generating more heat the rate of rise should be affected, not just the peak temps?

I don't know. One would think so, but when running the test at 100 KIAS, the the CHT rise due to timing is just a small percentage of of the rise due to low dynamic pressure.

I've attached a cooling air chart for a 540. In theory you only have about 6.5" H20 dynamic pressure available at the reported altitude, velocity and temperature. Given a standard cowl and baffles, the upper plenum pressure would not be higher than about 75% of available Q, plus some for prop outflow. Let's be generous and say with the outflow you have 6.5" above the engine. The expected CHT would get pretty close to 500F if you could make full power, and be around 435 at 75%.

Point is, given these test conditions I suspect the CHT rise due to timing may be more or less lost in the robust noise. CHT is racing upward regardless of timing.

Set up level flight at 19 BTDC, stabilize a 10 minutes, record the CHT, the change the timing, stabilize and record again.

EDIT: Whoops, the elbow needed drawn one more curve to the left. That makes the pressure requirement slightly less...but remember, we were being generous ;)
 

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CHT Testing

Dan,

the chart you shared seems about right for what I am seeing leaned for best power. My climbs today started at around 84% power and ended around 73% power. OAT of 77F at DA 5500 (3500 ind) to 60F at DA 9000 (7000 ind), ground temp was 93F. I didn't reach 435 even with 30 degrees timing, I suppose I could check pressure differential and correlate the cooling, but it seems inline or better than suggested.

This afternoon I climbed until the CHTs stopped rising, or were rising extremely slowly, i.e. less than 1 degree in 250 feet. I performed climbs at 100, 110 and 120 kts IAS from 3500 indicated to 7000 indicated alt. All of the climbs I leaned to 17.4 GPM which was my target best power fuel flow. I performed each climb twice at 22 degrees advance and 30 degrees advance. Here is a summary of the data... this yielded some difference in CHT due to timing advance but it was rather minimal compared to what I expected originally. 5 degrees F rise at 110 kts and 120 kts and 10 degrees F rise at 100 kts. For the final climb of the day I used a full rich mixture setting 21.8 GPM at 120 kts and this dropped the CHTs by 45 degrees F from the same climb at 17.4 GPM. It seems the affect of fuel is far more pronounced on CHT than a timing change from 22 to 30 degrees. The best compromise is probably to just use the extra 1/4 gallon of total fuel over the 5 minute climb to cool the engine and/or stick with the 25 degrees fixed timing to avoid an extra advance adjust switch and complexity. In my previous testing I noticed little impact on climb performance at rich mixtures. Here is a summary. Perhaps I am nearing the limit of what is expected for cooling from this set-up, cooling seems acceptable with extra fuel flow and finding a compromise fuel flow at reduced power climbs seems even more feasible.

Climb from 3500 ft to 7000 ft

______Peak CHT Avg___Peak CHT Avg____Fuel_____CHT
IAS___22 Timing_______30 Timing________GPM_____Delta F
100___414.5_________424_____________17.4______9.5
110___412.5________417.2____________17.4_______4.7
120___393.5________397.8____________17.4_______4.3
120________________352.7____________21.8_______45.1


Here is a chart showing the actual data

https://hosting.photobucket.com/images/i/BryanFlood2001/CHT_cooling.jpg
 
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try louvers on bottom aft cowl, they extract in climb, behind boundary layer separation in cruise.
DAR Gary
 
Mel - Reread your post. Even though the eyebrows are sealed by rubber gasket material, I suppose under certain conditions they could 'unseal' and provide a bypass for the air.

I was able to improve my CHT's during climb by 20 degrees by sealing my engine better. My leaks were not in the baffling material up top or inlet ramps, but rather all the "little holes" in the back and bottom of the engine. Was able to find those leaks using Dan H's flashlight trick. De-cowled the engine, closed hangar door and turned off lights to make things dark in the hangar. Then shined a flashlight from bottom up under the engine. Any place I could see light besides through the fins, was an air leak. Was AMAZED as how many leaks there were on back side and underside of engine (all of which is air escaping from other points that are not going across the cooling fins). After filling all those holes (6 in all) with RTV, temps went down 20 degrees in climb. Here are a few pics of those holes...

Here's a link to the RTV with a long nozzle and pressure to make application MUCH easier. https://www.oreillyauto.com/detail/c/ultra-black/chemicals---fluids/glues--adhesives---sealants/gasket-makers---sealers/166cc2694828/permatex-ultra-black-9-5-ounce-maximum-oil-resistance-rtv-silicone-gasket-maker/perd/85080?q=gasket+makers+%26+sealers&pos=17
 

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Dan,

the chart you shared seems about right for what I am seeing leaned for best power. My climbs today started at around 84% power and ended around 73% power. OAT of 77F at DA 5500 (3500 ind) to 60F at DA 9000 (7000 ind), ground temp was 93F. I didn't reach 435 even with 30 degrees timing, I suppose I could check pressure differential and correlate the cooling, but it seems inline or better than suggested.

The old Lycoming cooling charts are emperical, and if I understand correctly, data taken with "standard" baffling described in a Lycoming book which is no longer available. I suspect diligent homebuilders can install better baffling, as you likely have.

At a fixed fuel flow, the BSFC is progressively going richer with altitude. If we assume 260 hp as baseline, 84% at 17.4 is 0.48, while 77% is 0.55. This auto-enriching effect would counteract some of the EI's timing contribution to CHT.

For the final climb of the day I used a full rich mixture setting 21.8 GPM at 120 kts and this dropped the CHTs by 45 degrees F from the same climb at 17.4 GPM. It seems the affect of fuel is far more pronounced on CHT than a timing change from 22 to 30 degrees.

Agree, and the relative magnitude is indeed interesting.

Remember, adding fuel is a moderate power reduction, and it effectively retards timing.

A lot of owners find increased fuel flow acceptable in return for lower climb CHT. Others lean in the climb to maintain all available power, and control CHT in other ways.
 
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Me- hot jugs on an RV-7.
Did a lot of baffling stuff described here. And some other stuff.
Helped a little, but not enough.

After a year of tinkering, but the bullet and installed TWO Aerosplat cowl flaps. Did mess around.
Does the job. Happy I did it.
No more tinkering with baffles.
 
Legacy RV-6 Carb'ed O-320 D2J w/ Slick mags timed to 22BTDC, Sterba wood prop with lots of pitch, carb built to flow additional fuel at WOT
As a part of the process to determine why CHTs get so high during climbout, I installed a temporary airspeed indicator to act as a pressure gauge with one probe to the baffle plenum (upper deck), and one probe to the middle of the firewall, to see if there is a pressure differential.
at 160 mph indicated, power set at 2100 RPM, CHTs all 350 to 360, the airspeed showed 90mph with the pitot port connected to the upper deck probe, and the static port connected to the firewall probe.
I then disconnected the firewall probe, so the static was seeing ambient pressure, and the airspeed indication went up to 180 - demonstrating there is a pressure differential between the upper deck and the lower cowl area, and ALSO a differential between the lower cowl area and outside of the cowl.

I repeated the test at WOT climb at 140 mph, and the readings were all 20% lower, but maintained the proportions. After climbing 2000 feet, CHTs were at 425 and rising.

The only way I can keep the CHTs below 380 during the climb is to accelerate to 140 or more, and reduce the power to 2100 RPM.

Any sage wisdom out there to help identify next steps? I've been thru the baffles eliminating leaks.
I'm thinking now it is time to open up the exit space to improve outflow.
 
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As a part of the process to determine why CHTs get so high during climbout, I installed a temporary airspeed indicator to act as a pressure gauge with one probe to the baffle plenum (upper deck), and one probe to the middle of the firewall, to see if there is a pressure differential.
at 160 mph indicated, power set at 2100 RPM, CHTs all 350 to 360, the airspeed showed 90mph with the pitot port connected to the upper deck probe, and the static port connected to the firewall probe.
I then disconnected the firewall probe, so the static was seeing ambient pressure, and the airspeed indication went up to 180 - demonstrating there is a pressure differential between the upper deck and the lower cowl area, and ALSO a differential between the lower cowl area and outside of the cowl.

You did not mention altitude, so the following assumes 5000 ft and standard day conditions, just to make the illustration simple on the attached cooling chart. Note the chart is empirical, using some "standard" baffle setup described in a book no longer available from Lycoming.

A deltaP of 90 MPH across the engine fins equates to 3.42" H2O, which would be quite low for a high power setting. As you can see below, the expected result would be a high CHT. We can assume a lower altitude (more mass per unit volume), but the result would not change a lot.

There are issues with your measurement method. Consider...how do you get a 180 MPH indication in the upper plenum at 160 MPH indicated airspeed? For reference, upper plenum pressure in a good system is perhaps 80~85% of available dynamic pressure. Best guess? When you disconnected the firewall port (lower cowl pressure), the available "ambient" static (perhaps cockpit pressure?) was well below freestream.
.
 

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Dan, thanks for the response. (I was hoping you would)
Yes, the ambient was measured in cockpit.
One thing that I have observed on every flight that supports the idea the lower cowl area is also pressurized is that there is a 4 inch gap in the upper right cowling-to-firewall attachment where the hinge pins go in, that rises up about 1/8 inch during flight...
One thing I didn't mention is this airplane has a 4 pipe exhaust, so a lot of the outflow area is occupied by pipes.
So, I'm thinking my next step is to open up more outflow area, maybe by using louvers or cowl flaps.
 
Yes, the ambient was measured in cockpit.

Usually best to make both the upper and lower plenum measurements with one leg of the meter (or here, an ASI) plumbed to aircraft static. And it is important to use the right kind of probe, if you expect to know if you're looking at static pressure, dynamic pressure, or total pressure. I generally try to measure static, because inside a cowl it is too hard to know which direction the air is really moving.

One thing that I have observed on every flight that supports the idea the lower cowl area is also pressurized is that there is a 4 inch gap in the upper right cowling-to-firewall attachment where the hinge pins go in, that rises up about 1/8 inch during flight...

Ahhh, but we want the lower cowl to be pressurized, and still cool the engine. To do it, the real goal is to increase upper cowl pressure, or to be more precise, convert a higher percentage of the available dynamic pressure into upper plenum static pressure.

Think of it this way. Assume for a particular airspeed and air density, we have 10 units available as dynamic pressure. The good system might convert 8 of them to upper plenum static pressure, drop 5 across the engine fins, and have three remaining to boost outlet velocity, i.e. reduce cooling drag. The less desirable system converts 7, drops 5 and has two for exit velocity. The so-so system converts 5.5, and the owner must cut a huge exit to get lower cowl pressure down to where it will flow 5 through the fins. All three will cool the engine if 5 is enough. However, the first one will be faster, and cool better in climb.

Real common? That's a system which converts 7, but only measures a drop of, oh, let's say 4 across the fins, because the baffling leaks in all kinds of places which are not through the small fin passages. The leaks tend toward equalizing the upper and lower pressures. Exit temperature tells the tale; leaks don't pick up heat.
 
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High CHTs and Canopy Cracks

Having an 8 and many friends who do as well, I believe that high CHT's and canopy cracks are just one of the few drawbacks to owning these beautiful flying machines. After having nearly 8 years and just shy of 1,000 HRS on my O-360 A1A RV-8 I still struggle with high CHTs on climb out on anything but winter days. Though I have improved them over the years all through a combination of:

1. Re-doing my baffles
2. Sealing up leaks
3. Installing intake ramps (this did two things, improved flow dynamics and sealed off the big side leak you get without them). Not sure why they were left off in the build.
4. Washer trick/pounding out the baffle a little (could do more) behind #3.

I have had dual P-Mags for most of my time and those definitely bump up the CHT's on climb out. I have played around with them, jumper out with modified curve etc and I have finally just gone back to the more conservative jumper in for CHT purposes.

On a California winter day I will see around 385 on climb out. Anything past 80 deg I will have to pull power and increase speed to keep under 400deg. One trick I use when it's really hard to keep them lower during climb is securing a mag (once safely airborne with sufficient alt of course) in the climb which pretty quickly drops the CHT's albeit with less power output. This also allows me to lean a little too to smooth the motor, all while keeping CHTs under control which has been an interesting technique I created.

Two things left to try for me:

1. Increase fuel flow (as mentioned on this thread)
2. Anti-splat cowl flaps (has anyone tired these?)
 
I have somewhat the same issue. My CHT and my oil temp on a warm day climb out where high. I also run dual PMags with the jumper in. I installed 2 anti splat cowl flaps and it really helped out. Did not solve the problem but its well worth installing them.
 
Having an 8 and many friends who do as well, I believe that high CHT's and canopy cracks are just one of the few drawbacks to owning these beautiful flying machines.

Most such problems are builder induced. And anyway, we can change anything we wish.

On a California winter day I will see around 385 on climb out. Anything past 80 deg I will have to pull power and increase speed to keep under 400deg.

Photo below is top of climb, just after leveling to accelerate. Climb was leaned for best power mixture (reference EGT method) at regular intervals all the way up, cooling 30 more HP with 25% less exit area than a stock cowl. August 2021, south Alabama.
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I see cht reaching 450 on climbout on a not super hot day, ~80F at WOT, on Cyl 1 & 3. I am going to check for leaks and the airseals and try reducing power during climb sooner. I already have one antisplataero cowl flap on the right side bottom of the cowling but that seemed to be ineffective at reducing the max temp.
 
I think I finally got mine under control.
1) Installed ramps in upper cowl.
2) Installed "bump" on lower firewall
3) Discarded trapeze exhaust hanger system devised by builder, and went with the tubing and hose hangers per plans.
Now, CHTs on climb run 375 or less, 325 to 350 in cruise - except #1 runs around 300??
 
Do you have the deflector installed on the face of #1?
Yes, I do, and I increased the height of it - with no obvious effect,
Thanks for asking.
CHTs are pretty even in climb. The disparity becomes evident above 170mph indicated. power setting does not affect it unless I pull the power way back, to the point the other three CHTs start dropping.
 
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Question

I follow these threads quite closely as I too suffer from high cht’s on climb out. This started to happen after upgrading to 160hp and adding 1 pmag. The thing I really find odd is that my first flight always sees a faster climb in cht’s. if I stop for lunch a jump back in, I can climb for considerably longer before my cht’s start climbing above 400 degrees. Any ideas why this would occur? Is this possibly a clue as to where my solution may lay? If nothing else, it has always had me perplexed.
Thanks
 
Hi chts

I follow these threads quite closely as I too suffer from high cht’s on climb out. This started to happen after upgrading to 160hp and adding 1 pmag. The thing I really find odd is that my first flight always sees a faster climb in cht’s. if I stop for lunch a jump back in, I can climb for considerably longer before my cht’s start climbing above 400 degrees. Any ideas why this would occur? Is this possibly a clue as to where my solution may lay? If nothing else, it has always had me perplexed.
Thanks

I tried everything to lower my climb CHT. 0320 160 hp. Including jetting out my carb and cowl flaps. What finally worked was contacting Marvel Schibler. I upgraded to a new carb with a higher flow rate. No longer and issue. If you have a fuel flow transducer, check the flow gph on climb and report back. Should be high 12-13 gph on climb full throttle. If you have this flow rate and still have high CHT’s, check your intake tubes for leaks and do the mods on the baffling that you can find info on VAF. The cowl flaps do seem to also work to lower temps.

Another test that works. Remove your air filter and go for a test flight. BE SURE to first lean for best power before take off as you will be running very rich!!(In fact, you wont be able to taxi unless you lean) The increased air flow will suck in more fuel from the Venturi effect. You will see a dramatic cooling effect on increased fuel flow. Take note of the gph on climb. This will be your answer.
 
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Fuel flow

Thanks Al for the reply, unfortunately I do not have a fuel flow indicator. Makes it a little tougher to figure things out.
Would you mind sharing which carb you got?
 
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