'48 Exhaust Shrouds and Ducting

[2Georges]

In the continued efforts of restoring N4085V(1948), I have recently installed the exhausts and shrouds (Hanlon-Wilson model 221), and was about to install new scat hose(2") when I realized that I wasn't at all sure that the installation on my bird was correct in the first place. When everything was disconnected about five years ago to begin the engine work, etc. the left ehaust shroud(just a normal cylinder shape) was ducted to the control box on the firewall, and the right exhaust shroud(fancier shape with conical cap on the front and angular cap on rear) was ducted to the carb air box. I know the IPC calls for a Carburetor Heater Union (PN 0550119), and I have many photos of other 170's with this arrangement, now that I started looking through my "resource library." I don't have any other drawings to substantiate what the correct instation would be. Could someone kindly illuminate me? I'm trying to get things done right!

Thanks,
Andy George (740) 973-7041 in Ohio

[Bruce Fenstermacher]

Well the first thing I can tell you is that if you have the conical inlet on the front of the shroud and the exhaust out the back of the shroud on the right or passenger side you and the scan tube that attaches to it is 3 inch, then you have the shroud for a '53 and later aircraft. While there is no correct later year to use the shroud and muffler setup from the '49 to '52 would be closer.

Are your mufflers identical on each side? Or does the left muffler have a front and back edge that the shroud would rest on. If you have the correct mufflers the right muffler will not have this edge front and back.

Maybe you have the correct mufflers but the wrong shroud. The '49-'52 mufflers and shrouds are interchangeable left and right. '53 and later the muffler and shrouds are not interchangeable right and left.

If you have the '53 and later setup you can us it but you will have two problems to overcome. First the inlet in the right front of your engine baffling should only be 2 inches but the muffler shroud will be 3 inches. You will have to make an adapter from 2 to 3 inches or use 3 inch scat and change out the inlet in your baffle. The next problem of course will be the 3 inch exhaust on the muffler shroud. On the '53 and later there is a 3 inch heat distribution manifold on the fire wall to attach it to. You don't have this. You could adapt the 3 inch down to '2 and then run the right side to the carb heat. The left side would then go directly to your heat manifold.

If you actually have all the '49-'52 stuff and it is all 2 inch scat there is at least 2 ways to run the scat. If you also have a Y pipe with a flapper in it that in attached to the carb heat box then the scat from both mufflers is attached to each outside of this Y pipe and the center of the Y is then attached to the heat manifold on the firewall.

If you don't have the Y then the left scat goes to the heat manifold on the fire wall and the right scat goes to the carb heat manifold.

So what mufflers and shrouds do you actually have?

[2Georges]

Bruce,
Thanks for such a timely reply! My exhausts are not interchangeable. The left side does have a beefy lip on both ends for the shroud to seat on. The right side has no lip. The right side shroud has a series of U-shaped brackets welded front and back to maintain shape and distance around the exhaust heat studs. The cone-shaped inlet on the right side is designed for a two-inch scat hose (I'm not sure now if that was original to the shroud or modified later--it does look "factory"). I don't have a piece that installs on the carb box to accept air from both shrouds. You mentioned that in this case, the left side shroud is ducted to the firewall for cabin heat and the right side shroud is ducted to the carburetor air box for carb heat. Do you know if this is approved? In other words, do you know why the Carb heat union (the "Y" device) would have been removed when the Hanlon-Wilson was installed. Also, I'm simply curious as to the design intent of the right side versus left side shrouds. I assume to increase air flow(heat flow)? I hope this all makes sense.

Andy

[GAHorn]

Andy, the earliest exhaust system was the "pancake" system (see the second illustration below) which was replaced in B-modles by a Hanlon Wilson system (the top illustration) that utilized the exact same mufflers and shrouds on both sides. In the '53 B-model year, (post SN 25373) increased cabin heat was provided via the right muffler, so a larger shroud and associated "standoffs" and larger 3" inlet/outlets were provided. The setup you have on your right side was designed for increased cabin heat for a '53 and later B-model.

The early "Y" system was originally used to provide adequate carb-heat due to the less-efficient pancake muffs. B-model aircraft dispensed with that system because the more efficient H-W muffs provided sufficient heat for carburetors.

While there is nothing "wrong" with using the later system for your airplane, technically they are not the intended parts for your cabin heat system. You will have to get "imaginative" to get them all plumbed to your system. I'd recommend you consider trading your right-side setup for another left-side 2" setup...(changing from a pancake system such as your airplane originally had to a H-W system would normally use only the early, SN 20267-25372) H-W system in the illustration below, which is two left mufflers)... or I'd recommend you use the right side you presently have for cabin heat and left side for carb heat. (That way you'd have potientially more cabin heat, and the smaller left side is fully adequate for carb heat all by itself.)

B-model exhaust system.JPG

pancake exhaust system.JPG

[2Georges]

George,

Thanks for all the information. That all makes sense to me. Fortunately, the inlet and oulet for the right side shroud is already made for the 2-inch scat hose. So I will probably take your suggestion to route the right side to the cabin heat and the left side to the carb box for heat. Also, I meant to tell you that I took your suggestion regarding fabricating new fuel lines for my new fuel pump installation. I gave up trying to locate the original material (Everdur) and had new lines made up of teflon coated with stainless steal braid from Specialty Hose Corporation. The don't look "original," but they do look attractive and I was able to fit everything quite nicely. The details are finally starting to come together. My dad and I have been working on this restoration for almost twelve years, and we finally feel we have rounded third base!

Andy

[Bruce Fenstermacher]

Andy

If your inlet and outlets are 2 inches on the right shrouds as you describe then they have already been modified. They should be 3 inches.

In your case you are really ahead of the game though since that is what you need for your model aircraft. George is correct in his thinking that if the inlet and outlet was 3 inches, as they should be, then you would want to figure out how to use that side for cabin heat.

But since they are not 3 inches the right side is essentially no different than the left and it really doesn't make any difference as far as heat goes, which side supplies heat to what. In this case the easiest, cleanest and one of the correct methods used by Cessna would be to hook the heat from the right side to the carb heat box and the left side right to the cabin heat manifold on the left side of the fuselage.

Like many things on my plane (a '50-'52) I have a mix of parts. I've personally thought of changing out to the later 3 inch scat in order to increase cabin heat. And so I've made it a point to survey every 170 I've seen to specifically see how the scat was hooked up and what was stock and what wasn't. I've seen a lot of systems.

One of the auxiliary heating systems available on A models was to port the heat up through the center cabin tunnel rather than the heat manifold on the left. This may have been available on '48s as well. Anyway my friend has this system and I've seen about 3 others as well. It was not a very popular option. In any case this is about the only time I've seen the Y pipe used. I think the thought was that more heat would be supplied to the cabin to be used with this system. I've actually tried the Y pipe system on my plane without the tunnel vent ducts and found it does not provide more heat or at least I didn't think so.

One other point about the Y pipe. The carb heat box has an inlet on the right side. Ordinarily attached to this is a 90 degree elbow pointing the inlet to the rear of the plane. The Y pipe replaces this elbow. So you would either have a elbow or the Y pipe.

So as George and I have already stated the most correct way to do this would be to get the parts that make better sense for your airframe. But what you already have for heat shrouds isn't really incorrect other than if they had the 3 inch inlet and outlet like they should, they would not be the best fit. The fact the inlet and outlet are modified wouldn't bother me in the least unless you think it was shoddy workmanship and unairworthy. In your case you can either have the elbow or the Y pipe off the carb heat.

[GAHorn]

Andy

If your inlet and outlets are 2 inches on the right shrouds as you describe then they have already been modified. They should be 3 inches....

An unfortunate choice of words, perhaps. "They were originally 3 inches..." might be clearer. They should be 2" to mate up to your airplane's accessories.

It probably matters little which muffler you attach to your cabin heater since your heater-manifold is inadequate for truly cold climes anyway. I was thinking that the larger "oven" created by the larger right muffler-shroud would be more efficient for cabin heat regardless of the 2" reduction in inlet/outlets. It certainly is more than necessary for carb heat.

The simplest flexible-hose (Sceet) routing is probably the best solution, and I'll venture is Bruce's thinking?

[HA]

I DOES matter which muffler he uses for heat, remember he said that the LH muffler has lips at the ends for the heat muff to ride on, that one has less airflow (by design), 2" scat hoses, and is meant for carb heat. The RH muffler has no lips and the heat muff has tabs to space it away from the muffler, that combined with the conical front air inlet and extra fancy outlet is to get more airflow over the muffler for cabin heat. Sounds like he has the setup similar to my (relatively) newer 172 except I have 3" scat hoses on the RH heater muff and of course by then Cessna had moved the (bigger) cabin heat box on the firewall to the RH side so it's a straight hookup from the RH muff. He can still hook the RH up for his heater, just going to be a long hose run.

Andy - does your carb airbox have the carb heat inlet on the LH or RH side? that might present a challenge too. All the bits and pieces from the various years have a way of intermingling in the parts bins, you have to really study the parts books to see what you have and how it is used to your best advantage. You have a good setup from a newer airplane (you knew that), you just have to adapt the hoses to make it work.

[Bruce Fenstermacher]

I DOES matter which muffler he uses for heat, remember he said that the LH muffler has lips at the ends for the heat muff to ride on, that one has less airflow (by design), 2" scat hoses, and is meant for carb heat. The RH muffler has no lips and the heat muff has tabs to space it away from the muffler, that combined with the conical front air inlet and extra fancy outlet is to get more airflow over the muffler for cabin heat. Sounds like he has the setup similar to my (relatively) newer 172 except I have 3" scat hoses on the RH heater muff.

HA, Andy said that his later heat muff shroud had 2 inch intake and exhaust. Just like the left side. The diameter of the shrouds no matter what style is the same. The only difference in inside area would be that little area just inside the front funnel intake and that created at the V shaped exit. Very little difference. So in his case there would be no difference.

The big difference, and the reason Cessna redesigned the shroud I'm sure, is that with the stock 3 inch inlet and exit allowed a larger volume of air to pass through the system and since it was passing straight through it would be less restrictive than changing directions like the old style.

But no matter what year or style system and shrouding you have Cessna never used anything bigger than 2 inch scat from the original style shroud either from the right or left side to the carb heat box on a 170. The newer larger 3 inch scat and shrouding was always for cabin heat only. This may have changed in later 172s, I don't know.

[2Georges]

Gents,

I really appreciate all the input. To answer HA's question, The the flange is on the right side of the carb. In fact I'm pretty sure it is an original part because there are holes drilled in the flange that would be for the machine screw to mount the carb heat "union" called for in the parts book. From all the information presented, I am leaning toward hooking the right shroud to the cabin heat and the left to the carburetor heat. My thinking is that the right side arrangement that I currently have was designed for cain heat, even though originally with a three inch hose. A couple questions though.........My friend Clarence that has been helping me pointed out that generally speaking, each 180 degree turn in the ducting has the potential to greatly reduce the effeciency of the air flow. Is this a valid consideration? If I proceed as planned each scat hose will have to make a 180 degree turn in order to attach to its corresponding flange. Also, just purely out of curiosity, with three inch hoses on the right side shroud, did the forward scat hose hook to a three inch vesion of the little air scoop in the front baffling, or just maybe a three inch hole/flange in the front baffling?
When I get things hooked up I will try to post a picture of the install. I figure worst case, if I change my mind later, I can just switch the hoses from their respective destinations which would only require trimming the scat hoses shorter.

Cheers,
Andy

[jrenwick]

I have a little second-hand experience with modifying a carb heat system, because a mod is required when you install a C90 in a J3 Cub. The mod on my Cub is done by field approval (which was worked out by another Cub owner in the Twin Cities). For the approval, the FAA required the mechanic to show that the rise in temperature was at least 90 degrees Fahrenheit. The owner instrumented the airplane, made some test flights, and received a letter from the FSDO approving the mod.

I'm saying this just to raise a flag that hooking up some baffles and scat tubing might not be good enough, from the FAA's point of view, if the carb heat system isn't installed per the original type certificate.

[HA]

I'm just pointing out the airflow issues - I have the same exhaust/heat setup on my 172 as Andy now has, except I have 3" scat hoses to my RH heat muff and the cabin heat valve located on the RH side of the firewall. So, his LH muffler has lips at the ends and the air comes in at the side and out at the side of the shroud, 2" hoses.

His RH muffler has no lips at the ends, and the air comes in at the front and exits at the rear - if it had the lips on the muffler ends then the air wouldn't pass over the shroud, so really poor airflow through the heat muff and not much heat. With no lips for the shroud to ride on, it's optimized for more airflow over the muffler to serve as a cabin heater, no matter if you have 2" or bigger hoses.

Andy, my carb heat inlet to the airbox is also on the RH side, there is a 90 degree elbow attached to that which my scat hose from the LH shroud hooks to at the rear of the box. An adapter like that would make it easier to hook things up the way you need it.

And I've seen setups where they hooked both heat outputs to a y-valve for more heat, and shunted part of it off for carb heat when required. gets complicated.

[Bruce Fenstermacher]

His RH muffler has no lips at the ends, and the air comes in at the front and exits at the rear - if it had the lips on the muffler ends then the air wouldn't pass over the shroud, so really poor airflow through the heat muff and not much heat. With no lips for the shroud to ride on, it's optimized for more airflow over the muffler to serve as a cabin heater, no matter if you have 2" or bigger hoses.

HA, I don't really follow what your saying about the lips. If one tried to use a muffler with lips on the the side that had the front intake and rear exit then no air would pass through. It would be blocked by the lips. I agree that a straight through design might provide more heat than one that required the air to change directions 180 degrees like the old system.

And I've seen setups where they hooked both heat outputs to a y-valve for more heat, and shunted part of it off for carb heat when required. gets complicated.

This is what I described earlier. But using the Y running into the 2 inch cabin heat manifold does not provide more heat. I've tried it. Despite having twice the pressure from 2- 2 inch scat tubes rather than one, the 2 inch cabin heat manifold and single scat tube to it effectively blocks any extra heat from the cabin.

Andy I've established that the carb heat could come from either side and in your case with your parts with no difference in performance. But running the right side to the left side of the firewall doesn't make sense to me because of the longer scat run. I wouldn't be surprised that any bend in the scat would slow the air and lower the air pressure. I'm also sure that a longer run of scat leaves more opportunity for heat loss.

Running the scat from the left shroud to the cabin heat manifold and running the carb heat from the right shroud would produce the shortest scat runs for each. Besides that if you had the correct right shroud and muffler that is the way it was run by Cessna and so I think that is the most correct why to go

When Cessna went to the 3 inch scat on the right side there was no scoop in the front baffle just a hole. There are many 170s with 2 inch scat that do not have scoops in the front baffle, just a hole, as well.

[GAHorn]

I don't know which airplane Bruce is referring when he says that is the way Cessna did it... because all the airplanes with the H-W system we're discussing uses the LEFT muff for carb heat and the RIGHT muff for cabin heat ....and that's the way I recommend you to also connect them. I do not think you will lose any effectiveness due to 180-degree turns in the carb heat because the factory method also had that exact arrangement. (Whether you wish to attempt a field approval for this different installation or not may hinge upon how much you enjoy trouble and paperwork. In any case, I believe most inspectors will be much happier in copying a factory design (left to carb, right to cabin) than attempting to re-invent a wheel by hooking the muffs differently than intended by the factory.)

[jrenwick]

The point I was making -- probably too obliquely -- was that carb heat is a safety-of-flight item; if you make a change to it, you need to be sure, by actual measurements, that it's producing enough heat to melt the ice. I wouldn't want to find out the hard way that it wasn't doing the job, and I don't think you can just ASSUME it's doing it unless you're faithfully copying an actual factory installation.