High CHT's on -9A

I just finished retrofitting an O-320 to a -9A that used to have a Subaru engine. It has been an adventure. We had to add the scoop to the bottom cowl for the airbox, modify the firewall back as close to stock as we could get it, and so on. The cowl actually fit with a little shimming of the engine.

My problem is this. When I take off, my CHT's climb through 400 as I climb through about 100 ft. I have to climb at about 400fpm to keep them from going much past 425. Even at cruise, with the power pulled back to 2400 (fixed pitch), they stay between 400 and 415. I have trimmed the back of the bottom cowl to allow more air to exit. I have sealed the baffles where air may be able to leak, but they remain high. I don't have the wheel pants and gear leg fairings on yet, so that will help some (higher speed gives more airflow through the cowl, right), but I must be missing something obvious.

I noticed this afternoon that the upper air ramps on the top cowl are missing. I can see how air could escape around the baffles, which would not only steal my cooling air, but would also decrease the pressure differential between the top of the engine and the bottom, decreasing the cooling. But, could missing that part really make that much difference?

My first flight I just had the slot in the middle of the bottom cowl and the opening at the back. The CHT's hit 450 so I just went around and landed right away. WIth the additional sealing of the baffles and the additional trimming of the bottom cowl we have them so they stay in the low 400's, but at 9,000 ft, even on a warm summer day, I would expect to see closer to 350 than 410.

My oil has been hanging right around 180, which I am happy with.

Is it possible that my CHT's are somehow out of calibration? They are not new probes and it is not a new EIS, but I am pretty sure they are the probes that were installed with that EIS in the Zenith 801 I got them from.

Any thoughts, please share.

Jesse said:

I noticed this afternoon that the upper air ramps on the top cowl are missing. I can see how air could escape around the baffles, which would not only steal my cooling air, but would also decrease the pressure differential between the top of the engine and the bottom, decreasing the cooling. But, could missing that part really make that much difference?




Any thoughts, please share.

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I would surely install the ramps and re check the temps. You can tack them if with 5 minute epoxy.

Also, check the ignition timing.

Does anybody have a picture of the ramps for a -9? I have a set that came off a -6 cowl, but the inlets were narrower. Btw, the scoop came off the -6 cowl too.

I noticed this afternoon that the upper air ramps on the top cowl are missing. I can see how air could escape around the baffles, which would not only steal my cooling air, but would also decrease the pressure differential between the top of the engine and the bottom, decreasing the cooling. But, could missing that part really make that much difference?

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Most definitely!

It looks like you have identified several of your problems contributing to hot CHTs.
I would't fly that airplane until all those things are taken care of.

By all means, check your timing and while you are at it, make sure your take off fuel flow is correct for your engine. About <14GPH> I think for your engine.

Photo you requested

As far as I know, the cowling and ramps are identical between the 6, 7 and 9. My 9A cowling has a RV-6 part number.

Cleco'ed in place for bonding.


After some filling in with micro.


The plans clearly state that the inlet ramps are essential for proper engine cooling. The even have a little illustration showing how the turbulent airflow reduces the pressurization of the cooling air through the engine cylinders.

My cowl doesn't have these ramps and temps have never exceeded 390 in the climb since the engine was broken in. Makes me wonder what they'd be if I installed them!

I'd be looking at ignition timing and also use a mirror through the oil door to see if the baffles seal against the cowl correctly.

known issue

I know a fellow who did not install the ramps and had the same issue with high CHT. He installed the ramps and that corrected the problem.

Do the ramps simply reduce turbulence or do they seal against the baffles? The inlets on the -6 cowl are narrower by 1-1.5" compared to my -9 cowl.

It involves some basic airflow theory, but that doesn't mean it is easy to understand or describe.
The cooling system we are using is converting high velocity air into a low velocity pressure. When air enters the inlet we are wanting it to expand,decelerate and fill the large plenum area. The lip at the top back of the inlet is a very sharp corner which air doesn't like to turn around, causes lots of turbulence which can cause a back-up, limiting the amount of air that can come in. At high angles of attack (climb) it is even worse.

The ramps are a critical part of the cooling system... smoothing the sharp corner that the air needs to turn around. Anyone flying without the ramps installed would likely have lower temps with them.

BTW, I believe this is also one of the reasons many home builder designed fully closed plenum systems cool worse that the basic baffle system. Their design for the interface between the cowl inlet and the plenum box has sharp turns in it.

BTW 2 - How is your Dad doing?

OK, I have the ramps installed and am just waiting for the epoxy to cure so I can go fly. It does make some sense that the ramps will reduce turbulence and disturb the flow of air through the cowl. I just hope it makes the difference that it sounds like it might. Some say it will help, some say it will make all the difference in the world. I hope it is the latter.

I checked the mag timing and they are both within 1/2 degree or so of 25.

I still don't have the wheel pants on, so if the cooling is ok with the ramps in, then it should get even better with the pants on, so I'll report what I find.

For those flying with 150HP FP -9A's, do you have a power chart for fuel flow and MAP/RPM?

Thanks to all who have responded.

Jesse said:

OK, I have the ramps installed and am just waiting for the epoxy to cure so I can go fly. It does make some sense that the ramps will reduce turbulence and disturb the flow of air through the cowl. I just hope it makes the difference that it sounds like it might. Some say it will help, some say it will make all the difference in the world. I hope it is the latter.

I checked the mag timing and they are both within 1/2 degree or so of 25.

I still don't have the wheel pants on, so if the cooling is ok with the ramps in, then it should get even better with the pants on, so I'll report what I find.

For those flying with 150HP FP -9A's, do you have a power chart for fuel flow and MAP/RPM?
Thanks to all who have responded.

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Keep in mind that the cowl, baffling, seals, etc. are a system.
Installing the ramps will require modifications to the baffling and seals so that they follow the contour of the ramps (anyone have a link to good photos showing these details). It will not work, to just have the baffles go beside the ramps

I did have to modify the baffles for the ramps. The side baffles, that is. The baffles that go over the top of the engine are not affected by the ramps. I would like to see the pictures of other installations to see how the baffles fit with the inlets in the middle.

I flew and think that the inlets gave me about 20 degrees so far. I need to tweak the baffles a little bit so they seal better, but the ramps made a big difference. Before, I couldn't climb below 105kias (I think my pitot reads low, too, but that's for another day). In the initial climb it used to break 400 at about 50-100 ft. Today it didn't until about 500 ft, and the hottest cylinder went to 422 and stayed there. Once I leveled off, they went right down barely under 400. I would like to see another 20 degrees lower at least, but I am much happier now. I was able to climb today at 90kias and they didn't climb any higher. Not surprisingly, although interesting, the front cylinders were hotter in a steeper climb and the rear cylinders were hotter in the shallower climb.

I haven't finished calibrating my fuel flow, so I can't depend on any fuel flow numbers. I am wondering if my carb is running too lean for takeoff and initial climb, and maybe that is contributing to the temps.

Another issue could be that this engine only has 10 hours SMOH. I hear that they tend to run a little hot during break-in. Is this true? They are new ECI steel barrel cylinders, massive electrode spark plugs, new slick mags.

I have done a lot of work on RV-10's, but am new to the -9 world. I don't have any plans except the big drawings, and I don't have all of them. Can anybody give me a link (or post) a picture of the exit area of the bottom cowl?

Thanks again for all of the suggestions and please keep them coming.

Jesse said:

Another issue could be that this engine only has 10 hours SMOH. I hear that they tend to run a little hot during break-in. Is this true? They are new ECI steel barrel cylinders, massive electrode spark plugs, new slick mags.

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This is a big factor.
Engine break-in produces a lot more cyl heat. Try and fly early in the morning when temps are cool, so that you can easlily keep CHT's under control. Once you have more break-in hrs, the CHT's should come down more.

How is your Dad's recovery going?

There have been discussions a while back about the necessity to close off the air path under the ramps. Typically closing off one end will suffice. I closed mine during construction so don?t know how important it is. My CHTs will get slightly above 400 if I climb out about 1400 fpm under 120 mph. My front two cylinders also are hottest during climb-out, I think it is the turbulence at the entry point caused by a high angle of attach. I also block off about 2 inches of the #1 cylinder with aluminum tape which seems to even out the CHT temps. I have a 160hp O-320 installed new and my oil temps are always low, about 180 max if I block off the oil cooler in the summer, this seems typical for 9?s with 320 engines. I know many will argue that oil temps below 180 are not good but its better than dealing with high temps.

Regarding your comment about the exit area at the bottom of the cowl; the only exit area I have is the cowl tunnel where the exhaust pipes exit. I can email you some pictures if you PM me your email address.

Thanks for the additional comments. Regarding my dad, he is feeling a lot better. Still no feeling in his hands and not much in his legs, but he is walking, driving and aching to fly more.

I expect the next 10 hours should help with the break-in and that will help some. I also will see if sealing off one end of the ramp helps. The -10 has the Ramos sealed against the outboard side if the cowl by design, which probably makes things better.

Should I be worried about seeing 410-420 in the climb during break-in?

647jc said:

Regarding your comment about the exit area at the bottom of the cowl...

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We fiddled with this and messed around with that and checked this thing and double checked the other time. Baffling, timing, mixture control, wiring, cowling ... full power climb would go sailing past 450 like there was no tomorrow. Did two thing that fixed it.

First: Opened the exit are 5/8 of an inch. Don't know how much of a difference it made because never know how hot it would get if yo left the power at 100%, but it would likely have exceeded 500 It did reduce it at least 20-30 degrees. In a cruise climb from sea level to 8000' the CHT stayed at or below 430.

Second: Drilled the carb jet first to #41 then to #40. CHT dropped another 20-30 degrees. Sea level climb to 8000' at Vy stays at or below 430. Cruise climb stays around 400 and doesn't usually exceed 410.
Still considering going to #39, but the plugs and cylinders look good and the temps are reasonable. The extra drill step may eliminate the taper in the jet and complicate matters, so for now we are planning to stay at #40.

Jesse said:

Should I be worried about seeing 410-420 in the climb during break-in?

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Depends on who you ask. Lycoming say it is ok and we are ok with those temps.

Jesse said:

Thanks for the additional comments. Regarding my dad, he is feeling a lot better. Still no feeling in his hands and not much in his legs, but he is walking, driving and aching to fly more.

I expect the next 10 hours should help with the break-in and that will help some. I also will see if sealing off one end of the ramp helps. The -10 has the Ramos sealed against the outboard side if the cowl by design, which probably makes things better.

Should I be worried about seeing 410-420 in the climb during break-in?

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Happy to hear your Dad is able to get around.

Sealing off the ramps does nothing unless the baffles are trimmed incorrectly. If the baffles are trimmed so that they follow the inlet ramp (Inboard and outboard), and the baffle seal, seals against the ramp it is not needed.

If the highest CHT you see is is 420, during break-in, you are probably doing ok and you should wait doing anything else until after fully breaking in.
Never give up looking for leak paths in the baffle system though. It is rare that I inspect an RV and not find at least somewhere in the system that can be improved.

I am sending you O-320-A and -E fuel consumption chart and performance chart through email. They are scanned from Lycoming Operator's Manual.

During break-in CHTs are always higher. They will come down soon. I am doing phase I for a friend's 9A (160 hp and constant speed prop), highest CHT during initial climb to 1,000' was always above 410F at 100 kts. Continue climb to 3,000' will show above 430F. After 20 hrs, they are lower by at least 20F.

Even my 9A after almost 1,300 hrs still see above 420F if I continue to climb. During cruise at 65% power highest CHT is about 370F, still high compare with a lot other RVs. I know my cooling system is not as effective as many others'. I tried my best to seal all leaks I could find, enlarge exits and drill nozzle (read from Lycoming publication advise against it). However, all my numbers are within manufacturer's limits. So far my engine has been running trouble free. I will continue to use Lycoming numbers as my guide.

rvbuilder2002 said:

Sealing off the ramps does nothing unless the baffles are trimmed incorrectly. If the baffles are trimmed so that they follow the inlet ramp (Inboard and outboard), and the baffle seal, seals against the ramp it is not needed.

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+1 for the above. I flew without them sealed for 6 months or so. Everything looked like I had good seals on the ramps in all the necessary spots. New engine. Climb CHTs were around 410 very early on. Came down but still tickled 400F so I went ahead and sealed them up just to be sure. Did not see any instant difference. Climb CHTs have gradually come down to where I take notice if they break 380. I attribute it all to break-in.

All great comments. I'll keep tweaking a little on sealing the baffles, then will just give it some time to finish breaking in. I was very pleased to be able to climb at 90kias and not exceed 420, and that it came down to just under 400 when leveled off.

Btw, I'm seeing 138kts true at 7500ft wot with no pants. Does that seem reasonable? Also, the canopy is not fitting well to the windshield (yet), so we get a fair bit of wind into the cabin, which has to be causing some drag.

And a little fun shot of the panel.

Another possibility is flashings blocking air passages through the cooling fins. Some people have lowered temps by 40F after filing these down. I don't have any further details on the problem though.

Glad to see you are working on things to get the heat issue resolved.

While you are down, take a few moments to dull the dash cover with a rubber
edge or anything to keep that razor blade edge from hurting you or anybody in an accident.
Not complaining just looking out for you.

Keep CHT <400

Mike Busch is adamant that you should take immediate action to keep CHT's below 400 because the yield strength of the cylinder heads dramatic drop over 400. I only saw around 410 during the first 30 min of first flight at 75% power. At 30 min TT CHT's dropped 20 deg!

During the hot summer months I put in cowl louvers and it drops CHT's by 20 deg. With louvers I've never busted 400 on climb out during summer. Once daytime OAT runs less that 70, I put the sold plugs back in.
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