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Scat to exhaust clearance?

jcarne

Well Known Member
Patron
Holy cow I don't know why I'm having such a hard time finding this information but what are the requirements on clearance from cabin heat scat to exhaust? I will shield it if needed but I'm wondering if I really need to.

I also attached a couple of pics so you can see what I'm up against.

Thanks in advance!
 

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RV-14 way

Here's how they did it on the RV-14 to avoid melting the SCAT tube.

RV-14 FWF Right Side.png
 
Heat shields are a help, but in this case I strongly recommend relocating the heat muff aft. Heat shields alone will not work.

Side note - taking heat muff air directly off the inlet ramp may not be the best way to go. Most people take it off the rear baffle wall. Doing so will also solve some of your problem.

Carl
 
Here's how they did it on the RV-14 to avoid melting the SCAT tube.

View attachment 293

Ya I have seen that, looks like a cool setup on the 14.

AntiSplat’s heat shields would work great in those locations & they are reasonably priced & well made.

Oh how cool, I didn't know AntiSplat made a heat shield, I like how it curves too. I might just order two of these.

Heat shields are a help, but in this case I strongly recommend relocating the heat muff aft. Heat shields alone will not work.

Side note - taking heat muff air directly off the inlet ramp may not be the best way to go. Most people take it off the rear baffle wall. Doing so will also solve some of your problem.

Carl

Carl that is correct for carbureted engines but on the horizontal injected Van's shows pulling air off of the ramp. This is the location used on the -14 as well. I also don't have the option to pull off of the number 3 pipe as there is a slip joint there. The vertical section is too short for the muff too. I'm limited to number 1 here.

Why do you say a shield won't work? They seem to be pretty darn effective in other spots.
 
SNIP
Carl that is correct for carbureted engines but on the horizontal injected Van's shows pulling air off of the ramp. This is the location used on the -14 as well. I also don't have the option to pull off of the number 3 pipe as there is a slip joint there. The vertical section is too short for the muff too. I'm limited to number 1 here.

Why do you say a shield won't work? They seem to be pretty darn effective in other spots.

I disagree. I have an IO-360-M1B in my RV-8 as well and do not take the air off the right ramp as you suggest. The heat muff is located well aft on my plane. I also suspect (but cannot confirm) the RV-14 has more cowl clearance then you do in your RV-7.

Sure - a heat shield will help but I’d guess your very limited clearance would have the heat shield now touching the SCAT hose. The heat shield is for radiant heat - so it will be nice and toasty warm as it rubs against the SCAT hose.

If you want to keep the heat muff in this general area then:
- Move it as far down the exhaust pipe as possible.
- Rotate it so the SCAT hose (both inlet and outlet) are pointed more down than horizontal.
- This should buy you clearance on the inlet SCAT hose.
- Now you should be able to route the outlet SCAT hose inboard around the #3 exhaust pipe.

Looking at the photos is seems #3 exhaust pipe would work. You say there is a slip joint but I only see a pipe weld. Perhaps I don’t see the issue you have.

If the heat muff is the wrong form factor to mount where you need it, then perhaps looking at other heat muff options would be the next step. A call to Vettermans might be worthwhile.

Carl
 
Silicone SCAT rated -80 to +550F.

Headpipes are quite hot, with the pipe temperature falling to some degree downstream.

A standard sheet metal heat shield is a radiant heat guard. That said, it re-radiates some percentage of the input from the side opposite the source. You can slow the rate of heat transfer by adding an insulator (a conductive heat resistor, if you will) to the back side of the radiant shield. Fiberfrax is a good example. Use aluminum tape.

Having said all that, a quick excursion to the soapbox. We've mostly moved past vacuum pumps and ADFs and six-pack instruments. Given the wide availability of low-amperage heated clothing, and the current trend toward robust electrical systems, isn't it about time to retire SCAT tubes for cabin heat?

Yeah, I hate SCAT tubes ;)
 
I disagree. I have an IO-360-M1B in my RV-8 as well and do not take the air off the right ramp as you suggest. The heat muff is located well aft on my plane. I also suspect (but cannot confirm) the RV-14 has more cowl clearance then you do in your RV-7.

Sure - a heat shield will help but I’d guess your very limited clearance would have the heat shield now touching the SCAT hose. The heat shield is for radiant heat - so it will be nice and toasty warm as it rubs against the SCAT hose.

If you want to keep the heat muff in this general area then:
- Move it as far down the exhaust pipe as possible.
- Rotate it so the SCAT hose (both inlet and outlet) are pointed more down than horizontal.
- This should buy you clearance on the inlet SCAT hose.
- Now you should be able to route the outlet SCAT hose inboard around the #3 exhaust pipe.

Looking at the photos is seems #3 exhaust pipe would work. You say there is a slip joint but I only see a pipe weld. Perhaps I don’t see the issue you have.

If the heat muff is the wrong form factor to mount where you need it, then perhaps looking at other heat muff options would be the next step. A call to Vettermans might be worthwhile.

Carl

It's hard to see in the picture but there is a slip joint right in the sweet spot on number 3 or else I would have used this spot as you suggest. I can get a different clocking on the heat muff but I would like to try and make this somewhat convoluted routing work as it will slow the air down a bit, I have heard many a people say that slowing air down somehow with this 2" pipe was the key to getting enough heat. How is the heat in your -8? It gets pretty dang cold where I live.

The pictures also I think play a bit of trickery. The inlet scat has at least 3/4" clearance to the number 1 exhaust and about 1/2"-3/4" to the cowl. This scat is also pretty taught so I'm not worried about it flapping around. The outlet scat also has around 3/4" clearance to the number 3 exhaust pipe. The heat shields only take up about 1/4" of that gap. Thoughts? and thanks for the help!
 
Silicone SCAT rated -80 to +550F.

Headpipes are quite hot, with the pipe temperature falling to some degree downstream.

A standard sheet metal heat shield is a radiant heat guard. That said, it re-radiates some percentage of the input from the side opposite the source. You can slow the rate of heat transfer by adding an insulator (a conductive heat resistor, if you will) to the back side of the radiant shield. Fiberfrax is a good example. Use aluminum tape.

Having said all that, a quick excursion to the soapbox. We've mostly moved past vacuum pumps and ADFs and six-pack instruments. Given the wide availability of low-amperage heated clothing, and the current trend toward robust electrical systems, isn't it about time to retire SCAT tubes for cabin heat?

Yeah, I hate SCAT tubes ;)

Haha funny you mention that Dan, I saw in a couple of older threads your disdain for scat tubing. Unfortunately I think I am bound to using it as I will see some pretty low negatives out where I live in the winter unless I just stop flying. lol

I was planning on using your method with fiberfrax to protect the cowl, I hadn't thought about putting some on the heat shield, great idea!
 
. SNIP..I have heard many a people say that slowing air down somehow with this 2" pipe was the key to getting enough heat. How is the heat in your -8? It gets pretty dang cold where I live.
SNIP[/QUOTE

What I do:
- I put a restrictor on the SCAT hose flange. This is a simple piece of aluminum cut the same diameter of the SCAT flange with a 3/4” hole drilled in the center. The restrictor is held in place with a little aluminum tape, and the SCAT hose slides over it all. This reduces the air volume so you get less cabin heat flow, but what you get is hotter.
- I add a touch of SS scrub pad to the inside of the heat muff. Not packed in, just loose. The objective is to add a lot of heat transfer surface area. This will also reduce the total air flow slightly, but not totally.

I did the restrictor on all three RVs. I did the SS scrub pad on the 8A and the 8 but not the RV-10 (it had so much heat as is there was no need). I’m happy with the results.

As on other option to gain clearance, shift to 1.5” SCAT hose using a couple of reducers. This will still be way more air flow than needed.

Carl
 
Another idea

I am building a 7a tipper. I am at the same point in most of my build as you. You are FAST! I know the plans have 7 use the front inlet for heat. I also have the snorkel hortz. . The baffle sub plan show the inlet heat duct coming off the rear baffle on #3 which is what I used. I know you already installed this baffle. This is a better location and would work. I also want more heat so I installed the heat muff , got a large 2" SS muff, on the tailpipe on the pilot side just before exit in the cowl tunnel. I used vettherman alternate method of hanging the exhaust with adle clamps to lower engine mount with alum. 1/8 x 3/4 6 strip.
Now I noticed you used the updated nose landing gear. This is where the scat tubing will be crossing from #3 to the inlet of the muff and outlet to the heat duct to cabin. I have the old nose gear and this area is wide open. Your may be crowded here but it would be cool. The reason I put the heat muff on pilot side is because there is about 1 to 2" more room on that side from the ball joint and you can keep the muff off the joint of exhaust. With method of attachment you have to really keep up with lubing the exhaust joints as the exhaust is solid and no viberations. Sorry don't know how to post picture yet.
Jim
 
Jereme,

A few under-construction photos of how I routed my SCAT for a horizontal induction lower cowl on an RV-7A. Some of the heat shields aren't tightened/turned yet. I got the 90-degree bend for the heat muff from Van's accessories catalog.
 

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Folks,
Please note that there are heat muff with opposite side inlet/outlet. This is what I had installed in my RV7 which looks like the first picture above for the inlet and the outlet was routed on the outside of the #3 exhaust. I still have one somewhere in my hanger.
 
Carl that is correct for carbureted engines but on the horizontal injected Van's shows pulling air off of the ramp. This is the location used on the -14 as well. I also don't have the option to pull off of the number 3 pipe as there is a slip joint there. The vertical section is too short for the muff too. I'm limited to number 1 here.

FWIW I used the aft of #3 location for the air source and was able to route scat to a muff on #4 exhaust, which has a straight enough section for the stock muff. RV8 IO-360-M1B horiz induction.
 
Thanks everyone for the great response to this question, it helped me big! Pics below.

I have decided to give the method in the pics below a try with some heat shields and Dan's fiberfrax trick (I even have some leftover from the firewall). It seems that I have pretty good clearance to the number 3 exhaust pipe and about 1/2"-3/4" to number 1. If this method doesn't work I really think it will be because of the proximity to number 1. In which case I will order a muff with a different clocking that will make it a more direct shot to the inlet. Thanks again for all your help.

Carl, I'm also going to try some of your air trickery to get that heat transfer up, thanks for the suggestions.
 

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I don't know if it's possible at this point, but you want reverse flow on the heat exchangers.

Input should be farthest away from the exhaust valve (where it's slightly cooler), output closer to the exhaust valve.

You'll pick up a little more heat this way.
 
I don't know if it's possible at this point, but you want reverse flow on the heat exchangers.

Input should be farthest away from the exhaust valve (where it's slightly cooler), output closer to the exhaust valve.

You'll pick up a little more heat this way.

Ed - you are correct! I think that makes two of us now who have done it this way. Heat exchangers 101 - go counter-flow. I've mentioned it many times in this forum over the years, but most still set them up for concurrent flow.

Also, I'm wondering if 1.25" scat, or perhaps even 1", would be large enough. I changed one of my heaters to 1.5" scat, with a intake on the left front ramp. I had to tape off about 80% of it to get maximum comfort. There is still a surprising amount of airflow with that setup.

I've experimented with quite a few things w/r to heaters over the years (MN winters are a good motivation). There is an optimum air flow - too little or too much = less cabin warmth.

Also, there is a huge difference between putting a heater on a crossover (1 cylinder) vs the tail pipe (2 cylinders). Even though the crossover mount is at a lot higher temperature position, there is much better heat from a tailpipe mount fed by 2 cylinders. These heaters take a significant amount of heat from the exhaust gases, so how much exhaust mass flow is very important.
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Just wanted to do a follow up to this old thread.

I have about 150 hours on the setup as pictured above. I was initially worried about 2 things on installation:

1. clearance of the scat to an exhaust pipe
2. if it would move and rub the cowl

After flying with this setup I can confirm that neither of those concerns are an issue. This setup is working beautifully with no rubbing and absolutely no heat discoloration. I'm betting you could get scat quite close to an exhaust and not have an issue. Mine is around 1-2" away from it and it is a non event. I installed some anti-chaf material on the cowl where I thought the possibility of rubbing existed; the scat has not moved one bit and doesn't even contact the cowl. Long story short this appears to be a viable option for those with a similar setup. Next time I will request a special weld on the heat muff but this method works just as well.

Now, just need to get some aileron boots installed so my legs aren't so cold. :)
 
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