S

Slats

Active Member
I have searched the threads and read a lot about engine cooling. A lot of the threads are pretty detailed but none sort of describe the basics.

I would appreciate anyone clarifying or correcting what I have come to understand:

that ram air from the engine cooling inlets is used to create a high pressure area on the top of the engine.

baffles are used to contain the high pressure air and force it down through the fins to a low pressure area underneath and to the rear of the engine compartment where it exits.

the area of the exit effects overall cooling considerably.

Attached (I hope ) are three photos which show my upper rear and forward baffles.

http://s1255.photobucket.com/albums...3/jcslats/?action=view&current=BaffleRear.jpg

http://s1255.photobucket.com/albums...3/jcslats/?action=view&current=BaffleFwd2.jpg

http://s1255.photobucket.com/albums...3/jcslats/?action=view&current=BaffleFwd1.jpg

I have some red high temp RTV on order from Spruce and unless someone corrects me I intend to use it to seal off the gaps between the baffles and the engine casing in an effort to force more air down through the fins.

Thanks to all who have helped in the past and who will help here, again now.

Jim
 
Red RTV

I used clear myself.:D You want as much air going thru the fins as possible for the max amount of cooling. recently I found areas on my -3 that some rtv had come off of, engine temps were up 20 deg.:eek: after sealing them up ( they were small) Temps went back down.:) The more that you can seal up, the better the cooling will be.

Just my $.02
 
Looks like the gaps to fill are way too big for RTV. It's really like making the whole upper engine watertight like an aquarium for the best air flow through the cylinder fins and out the lower cowling.
It equates to sealing any and all induction leaks for peak efficiency.
No compromises work here.
 
rvaitor87 said:
...You want as much air going thru the fins as possible for the max amount of cooling. recently I found areas on my -3 that some rtv had come off of, engine temps were up 20 deg.:eek: after sealing them up ( they were small) Temps went back down.:)
Click to expand...
OK ... but, if you covered half of the 24 sq in hole behind cyl #4 then wouldn't the temps come down at least 50 deg? and they don't seem too ... can anyone explain that :confused:
 
MauiLvrs said:
OK ... but, if you covered half of the 24 sq in hole behind cyl #4 then wouldn't the temps come down at least 50 deg? and they don't seem too ... can anyone explain that :confused:
Click to expand...

The oil cooler opening? It's not really a hole, but rather it is filled with fins and has a resistance to flow similar to the passages between cylinder fins. See the Stewart Warner oil cooler charts.

Jim, your description is correct. And yes, you need to close those baffle gaps....they are holes for sure.
 
DanH said:
The oil cooler opening? It's not really a hole, but rather it is filled with fins and has a resistance to flow similar to the passages between cylinder fins. See the Stewart Warner oil cooler charts.
Click to expand...
I'll look at the charts.... but I'll bet it is equal to at least a 4 sq in hole if not more.

Not that I'm saying gaps shouldn't be sealed ...
But filling a few tiny gaps mathematically isn't likely to make a huge difference when covering the oil cooler or heater hole doesn't.
Just saying it's math and physics... how does it add up?
 
Hmmmm!

Hi

When I first looked at your photos I felt something didn't look right and wondered if this was a vans supplied kit or from somewhere else. The gaps at the prop flange are very large compared with mine.

The front bracing bracket is definitely different to mine. Yes it's a good to seal with high temp rtv but you need to close those gaps a bit first. There are some photos on my blog but am not sure if the will show what you need to see as I have converted to a sealed plenum.
 
Cool it...

Jim,
The standard ratio of inlet/outlet area is 1.0/1.2. Measure the inlet hole sq in and the exit area. From much experimentation I have found most people forget how much area the exhaust pipes, especially Vetterman 4-pipe on the RV4. You can trim the lower cowl lip to let more air out if you need. It really makes a big difference. Also, take all the previous gap inputs to heart, every tiny bit helps. You may have to use gap seal vs silicone on those big gaps.

None of it however is valid if you have more air coming in than out.

V/R
Smokey
 
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Cooling air and black holes

smokyray said:
Jim,
The standard ratio of inlet/outlet area is 1.0/1.2. Measure the inlet hole sq in and the exit area. From much experimentation I have found most people forget how much area the exhaust pipes, especially Vetterman 4-pipe on the RV4. You can trim the lower cowl lip to let more air out if you need. It really makes a big difference. Also, take all the previous gap inputs to heart, every tiny bit helps. You may have to use gap seal vs silicone on those big gaps.

None of it however is valid if you have more air coming in than out.

V/R
Smokey
Click to expand...

Just a small clarification, I think: Inlet/outlet area ratio will affect mass air flow rate, exit velocity, upper and lower plenum pressures, etc., and therefore cooling efficiency and drag. But regardless, unless you have a black hole under the cowl, the amount of cooling air coming out will always equal the amount coming in.
 
MauiLvrs said:
I'll look at the charts.... but I'll bet it is equal to at least a 4 sq in hole if not more.

Not that I'm saying gaps shouldn't be sealed ...
But filling a few tiny gaps mathematically isn't likely to make a huge difference when covering the oil cooler or heater hole doesn't.
Just saying it's math and physics... how does it add up?
Click to expand...

its all about area and pressure ratios and upper plenum volume..

stock inlets are about 50-60 sq in, where as a parallel valve engine has a cyl fin flow through area (the area which contributes to cooling) of only 25 sq in (angled valve is about 34 sq in). Even with a large OC inlet area the combined flow through area is less than the stock inlet area which combined with a sufficiently large upper plenum volume acts as a pressure reservoir resulting in very little change in upper plenum pressure as a function of OC inlet size - therefore cyl fin mass flow is only marginally affected - therefore little change in CHT as a function of OC inlet size. This works because as Dan H points out, no matter how big the OC inlet is the OC itself creates a pressure drop (resistance to flow) which is on par with the delta P across the cyl fins and probably a little more. If you cut a 25 sq in hole in the back of your baffle without an OC installed I'm confident CHTs would suffer.

I measured upper plenum pressure through a full speed sweep with OC inlet blocked completely and wide open (from 7 sq in area to 0) and only noted a change of about .3" H20 at Vmax with even less change in the delta P across the cyl fin - which means the mass flow was nearly the same no matter how the OC inlet was configured manifesting in no discernible change to CHTs. Please note: I didn't say there was zero pressure change, just that it was small: <.3" H20.

Sealing all those gaps is important because their total flow through area can add up quickly.
 
RV8R999 said:
Sealing all those gaps is important because their total flow through area can add up quickly.
Click to expand...

...and even more important, there is poor transfer of engine heat to air flowing through those gaps. You want the air passing through a gap between hot parts, the hotter the better. And it should be turbulent flow.
 
Cooling Part Deux

Thanks to everyone for their input.

I gapped sealed and siliconed for three days, I think I moved the C of G. I took her up for a quick spin yesterday with no noticeable improvement. All three temperatures (oil, CHT and EGT) are high and so I have assumed that they are related and relatively accurate. I have bought a thermometer and will run her up to compare those readings with the guages, just in case. If there is a better way of calibrating the guages, I'd love to know what it is.

jcslats


jcslats


I assume that these gaps need to be done also, can silicone be applied or do I need to fit aluminum?

jcslats


jcslats


In these two photos you can see the wraps in red and also natural aluminum plates between the cylinders. It looks to me like the middle ones are blocking air flow. I think they should be smaller and on top. Does anyone have anything similar on their plane?

Thanks again,

Jim
 
baffles

Picture #5, the natural aluminum parts apear to be standard Lycoming intercylinder baffles. No real need to alter those.
The two red portions( cylinder #1) should be tied to the matching parts on cylinder #3 with a tie rod or safety wire. .Same on left cylinders Yours do not even appear to be drilled. Drill a small hole in the center of the small flanges and tie them together. This is probably causing most of your temperature problems. At high speeds the baffle moves away from the cylinder and the air is going around the fins instead of between them.
 
Gaps

Hi

Any gap which lets air out other than through the fins on the cylinders/head neads sealing......... its wasted energy and is effectively just presurising what should be the low pressure side of your cooling system. I am not suprised you have poor engine cooling with gaps that large! Air like water will just take the easiest route out.

As I mentioned before I am suprised by the size of the gaps especially at the prop end. It looks to me that the simplest way to reduce the gap there would be to move the fixing point on the engine back allowing the baffle to com back toward the crankcase, then to seal with RTV.

I am sorry I cant show you photos at the moment because my engine is out.

If after sealing those gaps you still have high temps there are a couple of more tweeks you can make but first step is to seal it up so the air that comes into the cooling system is doing its job not acting against you.
 
I don't see any flap seals on the ends of the forward bulkhead. The seals on the top of the forward bulkhead don't appear to be tall enough.

BaffleFwd1.jpg


BaffleFwd2.jpg
 
WAM120RV said:
As I mentioned before I am suprised by the size of the gaps especially at the prop end.
Click to expand...
We had similar gaps on ours and sealed them temporarily with tape while working on a real fix. Flew it with the gap open to see how much difference it made. Surprisingly, it wasn't much. :(

jrs14855 said:
The two red portions( cylinder #1) should be tied to the matching parts on cylinder #3 with a tie rod or safety wire. .Same on left cylinders. ... This is probably causing most of your temperature problems. At high speeds the baffle moves away from the cylinder and the air is going around the fins instead of between them.
Click to expand...
Fixing that I would expect would make a NOTICEABLE difference. This article : Design for optimum cooling efficiency , explains why

We did two things that made a HUGE difference. :D

First, we opened the outlet exit by installing a 5/8 inch spacer adding about 6 sq in of exit area. This makes the inlet/exit ratio (including the exhaust and stuff) about 1.0-1.2 depending on how you measure. We don't know how much of an improvement this made because we could never continue a full power clime from takeoff to 8000 feet before the mod without the CHT going sailing past 450. After it stayed between 430 and 450 from SL to 8000 feet. So we got at least a 20-30 deg improvement.

Second, we drilled the carb jet from #42 to #40. CHT now stays between 380 and 430 from SL to 8000 feet depending on OAT and climb speed. So we got at least another 20-30 deg improvement.

We spent a lot of time filling little leaks and tweaking things. But, these two improvements gave us 40-60+ degree drop in CHTs. :eek:

In cruise at 50 ROP we generally have CHTs of 330-350 at 65%-75% power depending on Alt and OAT (and all of the other stuff that makes it difficult to make two consistent measurements).:D
 
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DanH said:
I don't see any flap seals on the ends of the forward bulkhead. The seals on the top of the forward bulkhead don't appear to be tall enough.
Click to expand...
Generally, should be able to see where the seal is rubbing on the cowl after a while...
These, maybe not...
 
Engine Monitor

Thanks again for all the great input.

The prop flange gaps and rear casing gaps are now sealed.

The upper baffles are in fact contacting the cowling. I think it could be better but they do contact.

I have wired 6 of the 8 wraps and will give her a test fly to see how things go.

Next up, oil temp.

Jim
 
Learning Experience

A little update for all those who are helping.

I have made many of the improvements as suggested and I am convinced that they have helped considerably. Unfortunately, we will never know exactly how much.

It turns out that part of my alarm was caused by the cht/egt guage being pooched and wildly inaccurate. It has been swapped out and the new guage is registering acceptable temps.

Thanks again,

Jim
 
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