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Air Dams Again?

morganjp

Well Known Member
After reading through many past threads on this issue I am still at a loss as to what to do about my high CHT's on # 1 and especially #3 on climbout. this is an 0-360 carburated engine on the RV-6A with the standard cowl.

While in cruise lean of peak all is happy with the CHT's on all 4 cylinders. 350 to 370 range.

EGT's are always fine and close to being the same.

On climbout # 1 will go yellow and #3 will go red if I don't back out of the throttle. I have air dams on the front of both #1 and #2. They are both curved and vary from 2 1/2 to 2 in in height.

I put the washer in the back between the #3 cylinder and the baffle but that did not help. You can see the baffle markings in the inside of the top cowl so I think the fit there is pretty good.

I swapped the #3 and #4 CHT probes to make sure that the readings are correct and they are. #3 is the hottest and #4 is the coolest. #4 reads hottest when that probe is in the #3 cylinder.

I am thinking if cutting down the air dam in front of #1 but how much do I take off and will that make # 3 run even hotter?

Any advise is appreciated.

John Morgan
 
i removed the dam

John,

I had similar issues. I removed the dam from in front of 1/3 side, and used a piece of aluminum tape (available in local hardware store aviation aisle) as a temporary dam. i started at about half the height of the original dam. i got much better temperature evenness between 1 & 3 and better temp overall. You can try different heights of tape to see which works best, then use that measurement to create/modify your dams.

I have the same setup as you. I also found a gaping hole behind the #3 cylinder against the block where the aluminum baffle material came nowhere close to the block, it started about even with the centerline of the cylinder and went down, where the aluminum material has a bend. Something to look at as well. if i find a pic of that i'll post it.

Stump
 
Making my air dams removable...

Working on baffling now and my air dams are removable - I riveted a strip of angle onto the bottom of each air dam using the existing holes; drilled the holes larger in the reinforcement angles on the baffles to accommodate the set rivets; match drilled three holes on each side from the baffle reinforcement angles (horizontal edge) into the bottom of each angle piece on the air dams; and then riveted #6 platenuts in these three holes under the baffle angles to secure the air dams. I hope that makes sense... anyway, they are removable without taking off the entire #1 or #2 baffle! Hopefully this will make it easier to trim them if needed for cooling.
 
removing or cutting the majority away will help.

LOP cruise should yield CHT's around 300-315 in winter (less if Alaska :p ) and Summer around 330-340dF

If not in that range keep working on your cowls as it indicates something is less than optimal.

DanH on here has some great threads and photos.
 
Climbs are always going to be challenging CHT wise, depending on how long/steep you climb and what the OAT is. It's also pretty easy to bring CHT's down significantly (at least on my #3 and #4, my two hottest respectively) by going LOP. Thus, those two measures (CHT's in climb and CHT's LOP) don't help me decide when I've got cooling where it needs to be.

This summer, the first post-break in summer, will be when I know whether or not I have cooling issues. Is it fair to say that one should be able to fly straight and level at full power at pretty much any summer OAT, and expect to be able to keep CHT's below 380F when ROP? (This is what I'm currently aiming for to tell me when I've got a setup that is cooling reasonably well.) Is this a reasonable litmus test?
 
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If you were asking me is that a reasonable litmus test, a full power straight and level ROP run should see something around 365-370'ish but this is a guide only.

LOP in cruise about 30-40 less. High power LOP down low you could even see lower like 310-315dF.

Here is middle of summer, high OAT and 325dF at 82% power LOP.
80LOP_zps6e22f7f3.jpg
 
I think it depends on what weather you fly in. Here in Minnesota, I fly in 100 degree weather in July, and -10 in January.

I cut the air dam down about 1/2" last July and didn't go much farther because I wanted to see what the effect would be in January. As it turned out, I could get the CHT to about 350 in that January weather and I was more concerned with getting the CHTs to all be about the same. I'll end up using a little aluminum tape to bring them up further.

However, let me suggest this about your particular problem. It looks like your hottest CHTs are on the same side of the engine. So I don't think the air dam is necessarily the problem if both the front and back temps are about the same, though high.

I think there may be some significant leakage around the baffles or other areas that could be sealed.
 
Bob;

yeah #1 & #3 are much higher than 2 & 4. I will trim the air dam a little and test it. The only leakage that I am aware of is the intentional opening with the washer between the baffle and the #3 cylinder.

John Morgan
 
Check This Thread

http://www.vansairforce.com/community/showthread.php?t=89844

It's AMAZING what a little work on the "wraps" around the cylinders will do to tighten up the openings. It made the biggest difference on my O-320 of anything I did. It's imperative to keep the air in the fins as long as you can and where the wraps don't lay tightly on the fins you are losing a tremendous amount of cooling. (Thanks DanH)
 
Changes

Bob,
I'll try to dig out some pictures. But basically it amounted to work on the "wraps" that go around the fins that are held together with the small rods or safety wire as some do it. If you don't take a lot of care in fitting/bending these tightly around the fins, they will leak and the edges and the end that is bent to accept the rod will not lay tight against the fins. This gap then allows air to pass over the fins and out rather than staying down within the fins all the way to the end. Once put together, it's very hard to make yourself take the baffles off to re-fit. Red RTV is your friend here. I re-bent the best I could then carefully sealed the edges on all four cylinders. Also the inter cylinder baffles that come with the engine needed quite a bit of sealing on my engine.

I did end up using two washers behind No. 3 after looking at some mods some others had done involving building a little box to add some room behind it.

After talking to DanH at length on the subject, I believe he has the right idea. SEAL SEAL SEAL. I also believe that the O-320's run hotter, at least in a 9A, than the O-360's do. Don't know why that is but after reading all these threads on high CHT's, I believe it.

The jury is still out on mine a little bit as the last mods were done when the 90 degree OAT's were pretty much over last fall. I just got it out of the paint shop this week and it's finally warming up down here in LA (lower Alabama) enough to put it to a test.


EDIT: Here's a picture shamelessly stolen from DanH that shows what I'm talking about very well. I don't think he'll care!!;)

CylinderWraps.jpg
 
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I would imagine that the only significant finning difference between parallel valve 320 and 360 engines would be in the "I don't care" zone of the steel barrel to accomodate the 1/2" difference in stroke. Bet the heads are the same.
 
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