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top cowl air passage for proper cooling

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drmax

Well Known Member
Hello. I'm including this photo of my current inner cowl layout. I was told,
that I should block the tunnels (where the rag goes through) on either side, to get all the air on the top of engine. I wonder then how the lower side of eng. gets it's air for cooling? My hottest cyl. is #3 at nearly 380 cruise. My upper rubber baffle appears to be in good shape. I won't do anything until I hear from you all. 0-320 160 HP
 
Well, I was going to attach a pic, but won't because I'm not allowed, which stinks. Where the ramps are on air entrance top side of call...beneath the ramps are hollow, allowing air do dump outboard of engine rubber baffles. Should these be plugged up? Thx DM


[ed. Use these instructions to insert images if you'd like: http://www.vansairforce.net/articles/ImagesInForums/images.htm br,dr]
 
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If you are talking about the fiberglass pieces that curve upward from the top of the cowl inlet, and blend into the inside top of the cowl, then yes they should be blocked off at either end or wherever is easiest.
 
Mike S said:
If you are talking about the fiberglass pieces that curve upward from the top of the cowl inlet, and blend into the inside top of the cowl, then yes they should be blocked off at either end or wherever is easiest.
Click to expand...

Hello Mike...so then the lower eng gets cooling from forced air from the top, down between the heads? Thx for the info. I gotta run. Late for work. DM
 
I believe the over(!)-simplified idea is that what cools the engine is the movement of cool air coming in to the upper side of the engine compartment and flowing down through the cooling fins, carrying heat away out the cowling exit area that is shared by the exhaust pipes. In order for downdraft flow to happen there needs to be a pressure differential between the top and bottom of the cowl...high pressure where the cool air comes in on top of the engine and lower pressure beneath the cylinders. If the fiberglass ramps on the inside top of the cowl aren't sealed it allows a very big "leak" of air between the upper and lower cowl and it will reduce the needed pressure differential.

A lot of us that have battled cooling problems have been on a seemingly never ending quest to seal the upper plenum as much as humanly possible. The usual leak culprits are ill fitting rubber/silicone baffles, the ramps we're talking about, the big hole that feeds the oil cooler and loose fitting curved aluminum around the cylinder fins. The search for leaks includes turning the hangar lights off and having a friend shine a bright flashlight all around while maneuvering your head in every possible way to spot light leaks that imply air leaks.

# 3 is frequenly the hottest and if you search these forums you will see a lot of discussions. The 2 most common solutions include either putting a washer as a spacer between the cylinder and the baffle or pound a curve in the baffle there. Both solutions are in order to allow air to travel from above to below, around the cooling fins. All this said, 380 degrees is not at all hot ond many would call it very close to ideal.

All for education and recreation:D
 
anybody got a pic for us imagination challenged types?

ok, now i am piqued and confused, anyone have a pic of the closed ramps on the upper cowl?

thanks

stump
 
jjconstant said:
I believe the over(!)-simplified idea is that what cools the engine is the movement of cool air coming in to the upper side of the engine compartment and flowing down through the cooling fins, carrying heat away out the cowling exit area that is shared by the exhaust pipes. In order for downdraft flow to happen there needs to be a pressure differential between the top and bottom of the cowl...high pressure where the cool air comes in on top of the engine and lower pressure beneath the cylinders. If the fiberglass ramps on the inside top of the cowl aren't sealed it allows a very big "leak" of air between the upper and lower cowl and it will reduce the needed pressure differential.

A lot of us that have battled cooling problems have been on a seemingly never ending quest to seal the upper plenum as much as humanly possible. The usual leak culprits are ill fitting rubber/silicone baffles, the ramps we're talking about, the big hole that feeds the oil cooler and loose fitting curved aluminum around the cylinder fins. The search for leaks includes turning the hangar lights off and having a friend shine a bright flashlight all around while maneuvering your head in every possible way to spot light leaks that imply air leaks.

# 3 is frequenly the hottest and if you search these forums you will see a lot of discussions. The 2 most common solutions include either putting a washer as a spacer between the cylinder and the baffle or pound a curve in the baffle there. Both solutions are in order to allow air to travel from above to below, around the cooling fins. All this said, 380 degrees is not at all hot ond many would call it very close to ideal.

All for education and recreation:D
Click to expand...
Thank you!!! For the big air hole where the oil cooler is at...i just made an alluminum blocker plate on the fwd side of my cooler to raise my eng oil temp. This may further cool my engine, if I'm reading this correctly.
 
where the rag is run through....should be blocked off. I'm using some left over polystyrene 1.5" pink foam and rtv'ing a piece in either side.
I plan to fit these closer to the inbd side, as so that the wind won't have so much a chance to swirl around in there with debris and chew the foam out. I suppose fiberglass would be a better alternative...but I don't have any ready available.

http://i48.tinypic.com/33kx1sm.jpg
 
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33kx1sm.jpg


Yes, that is it.

I used spray can expanding foam, and a layer of glass over the exposed end just to protect it.
 
that is even better

I like the sray can foam idea. This stuff if very durable. Did you just block off the outbd end a pour a little in there, or did you block of both ends and fill it up? That stuff is hard to predict how much it will expand. Thx for the idea. ;)
 
Blocker plates for the oil cooler may provide more air to cool the cylinders at the expense of the oil cooling. My engine was climb limited initially because of too high oil temps. I swapped oil coolers to a bigger one and now CHTs are the limiting factor, but at a much more aggressive climb angle, so it's MUCH better.

Closing up the openings in the ramps is a really great thing to do, but I'm not sure you have a "problem" requiring anything more if your hot cylinder in climb is only going to 380....
 
curved part glassed in the upper cowl

Stump said:
ok, now i am piqued and confused, anyone have a pic of the closed ramps on the upper cowl?

thanks

stump
Click to expand...

Stump,

I am not near the computer to send a picture but I will try to explain. When you at your plane from the front the air enters the inlet. On the upper cowl there is a curved piece that is glassed in. It smooths the flow of air. This once glassed in per the instructions would have the sides open. What guys are talking about is to glass in at least one side of the to keep air from going under and through.

I hope this helps. Take you top cowl off and you will see what I mean. The baffling should make a good seal along these inlet ramps but the seal up by the spinner can be troublesome.

Cheers
 
Question to those that know the material about the spray foam: is it appropriate for use in the engine compartment or is it something you would use to establish a shape and then dissolve with solvent after the fiberglass has cured?
 
drmax said:
I like the sray can foam idea. This stuff if very durable. Did you just block off the outbd end a pour a little in there, or did you block of both ends and fill it up? That stuff is hard to predict how much it will expand. Thx for the idea. ;)
Click to expand...

I just sprayed it in the center side opening, let it grow way oversize, then took a hacksaw blade and trimmed off the excess. Laid up a layer of glass over it to protect the exposed end of the foam. I also shaped it to make the baffle seal seat better.
 
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Results?

My -10's CHT's generally run 360-380 in cruise and I think my ramps are open on both ends.

What have you guys seen your CHT's do after the mod?

Thanks,
 
jjconstant said:
Blocker plates for the oil cooler may provide more air to cool the cylinders at the expense of the oil cooling. My engine was climb limited initially because of too high oil temps. I swapped oil coolers to a bigger one and now CHTs are the limiting factor, but at a much more aggressive climb angle, so it's MUCH better.

Closing up the openings in the ramps is a really great thing to do, but I'm not sure you have a "problem" requiring anything more if your hot cylinder in climb is only going to 380....
Click to expand...

I'm not blocking off air to the cooler, to create a lower cht temp. My oil temp is low at 160 degress...I need it hotter. I'll block this off to create a highter oil temp, at the same time getting better cht cooling.
 
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drmax said:
I'm not blocking off air to the cooler, to create a lower cht temp. My oil temp is low at 160 degress...I need it hotter. I'll block this off to create a highter oil temp, at the same time getting better cht cooling.
Click to expand...

I notice little, if any, difference in CHT cooling when I block my oil cooler. I believe for two reasons; there is only so much pressure you can generate in the plenum before air is lost to the outside world and an increase in oil temp affects the cylinders too.
With stock baffle and cowl, your biggest gain in CHT cooling will come from sealing up the baffling and internal air leaks as much as you can and/or effectively increasing the plenum to exit pressure differential. You, somebody, not me, could write a book on different ways this can be accomplished; ie, cowl inlets, outlet, plenum design, etc.....

Find the leaks. They like to hide in the inter cylinder baffling and around the front seal.
 
pic of finished blockage

I used spray-in foam and shaved off flush on either side. I did do a flashpoint test with a blow torch with the foam on top of some aluminum. The stuff only melts. A direct flame with a lighter, however is a different story. I will attempt to glass when time permits.
amd3x3.jpg
 
I'd like to make a comment or two about "blocking" the air passage under the inlet ramps.

The first is that I totally agree it is possible and even probable to have a big cooling air leak in that spot unless you do something to prevent it.

I think one root cause of leakage in some builds is that the parts supplied by the factory do not provide a seal surface for the nose baffle material to rest upon. Customization is required. You can see that in the image posted by DRMAX above. The baffle wear pattern goes right to the ramps and then has to jump an open gap to get onto the ramp (if it ever does at all). I have seen this problem on other builds in my locality too. The open gap is a major leak path. The gap also (in some I saw) messed up the nose baffle area so badly that the whole front leaked.

So, when I was buiding my cowl, I added a couple of ramps (shown in attached image). I got very good baffle seal impressions on them from the start of flight operations and continue today.
Even with that. I wondered about leakage and decided to add some glass to do the 100% block. I did that about 30 hrs ago. I saw zero change in CHTs. I believe it is simply because I had no leaks in the beginning.

I can't seem to post a pic from this borrowed computer today.
Here is a link to the pics on my build page:

Cowl Inlet Ramps
 
update

I got my CHT down to 300 on #3, and maybe 280 on the other 3, with temp at 44 F. I will give most of this reduced cooling factor due to me putting an oil cover plate of the entire fwd cooler side, plugging that hole and I recon. some of it has to do with foaming up the ramps. I'm happy now. :)
 
Explain please ?

I could be wrong but if the front baffles seal correctly against the outside front sides of the cowl, the open ramps wont cause a "leak" since they are completely contained within the high pressure area of the upper cowl. If you are trying to seal against the ramps that might be a problem since the high pressure air needs to blow against the baffle to make it seal. I might be wrong but if the baffles are sealing correctly against the cowl (meaning all turned inward so pressure pushes seal against cowl ) the ramps and side seals can't cause leaks. As I remember it there is no part of the baffles that seal against the ramps.

However, what also seems to be very important is a constant smooth hi volume flow of air through the cylinder fins and the open ramps can cause a lot of disruption in the airflow. Disruption equals poor flow! I think that's why sealing them helps some and not others.

Lots of discussion on cooling drag loss in nosedraggers due to all the extra weldments causing reduced airflow in the low pressure side which is also a problem to getting good flow from the top. If the pressure differential is not large enough ..... Poor flow. You need to be sure in nosedraggers that you dont have scat tube or other stuff blocking the flow of exit air The things that helped me most with chts (7a) were

1) good sealing around spinner and front cowl (took several efforts)
2) washer in the back on #3 baffle (running too hot)
3) closing off 1/2 of oil cooler outlet ( running too cool anyway)
4) adding front cyl baffles on #2 (was running too cool)
5) cleaned up exit air area in lower cowl to increase exit flow thus increasing total flow.
6) did seal sides of upper inlet ramps to smooth airflow

Hope any of this helps and tell if I've missed something. It's all a discussion :)
 
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Bill,
I understand your point about not including the ramps in any sealing activity. You are right about no leaks in that case. Since I have experience only with mine, I can just say it was better for me to include the ramps in the seal. Different engines and cowls could allow the option you mention.
 
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I felt around those seal, prior to foaming, with cowl on. It was more than possible for air to blow by those seals. Majorirty has went with closing off those ramps for better cooling. I went with the majority.
 
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