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prkaye

Well Known Member
It looks to me like the design of my O-320 FAB is for carb heat to just open the door at the top of the FAB, allowing warm air in the engine compartment to go in. It appears from looking at builders sites that some have actually routed a pipe from a shroud around an exhaust pipe to this opening, thus providing hot air. What are people's thoughts on this? Is it necessary, or is warm air from the engine compartment sufficient for carb heat?
 
Van's sells an adaptor to adapt a tube to the airbox.

cat-med_carbheat-connector.jpg


They also sell a short piece of tube that is cuttout to fit around the exhaust pipe and then connect that to the tube.

cat-med_carbheat-muff.jpg


It is marginal at best.

Lots of people put a carb heat muff on the exhaust and use that instead.

http://www.robbinswings.com/Carb_Heat_Muff.htm

http://www.wicksaircraft.com/catalog/product_detail.php/pid=4733~subid=11813/index.html

http://www.aircraftspruce.com/catalog/eppages/homebuildersheatmuffs.php
 
is it worth it?

Thanks Brian! That looks like what I've seen on some builder sites.
I was hoping for some discussion on whether this is worth-while doing... since it's not included with the FAB box, Van's seems to consider it an "option". Do you guys consider this a "must-have", or is it unclear whether it's worth the extra weight, cost and time?
 
Phil, it is a must have for you in Canada. The plane will not pass inspection without a source of carb heat. Also a gasculator is a must have.
 
Wicks

I have dual mufflers with heat shrouds. Thought I could use one of them for carb heat, but the tight cowling makes the scat tube routing just about impossible.

I took the advice from another VAFer who suggested to use this one:
http://www.wicksaircraft.com/catalog/product_detail.php?pid=4733

SS HEAT MUFF-SNGL OUTLET
1 piece .018 stainless steel construction with one 2" diameter outlet.
Total length: 7-1/4", width2-1/2". Requires two 2" clamps.​

I get a 40F heat rise in very cold weather.
 
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Phil, I built a -9a with VANS venting to the lower cowl. You could not detect a rise in carb temp which was somewhat disconcerting in the humid UK.

Now I have a -4 and I built it like this. As you will see in the graph, clear temp rises when you pull carb heat.

I used the Robbins Wings parts. Excellent!
 
Thanks guys! I'll put in the Carb heat muff. Although I wonder why it makes a difference living in Canada. From what I learned in groundschool, extreme cold is not the risk zone for carb ice.
 
Alfio, I see this one you mentioned has a single outlet:
http://www.wicksaircraft.com/catalog/product_detail.php?pid=4733

Maybe I'm not understanding how these things work, but how does air get INTO the muff? If you're pulling air out of it, air's gotta go in somewhere to replace it...

Vans' Cabin Heat Muffs have a seperate inlet/outlet http://www.vansaircraft.com/cgi-bin/catalog.cgi?ident=1237207761-148-30&browse=heatvent&product=cabin-heat-muff... why would a carb heat muff be set up differently?
 
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prkaye said:
Alfio, I see this one you mentioned has a single outlet:
http://www.wicksaircraft.com/catalog/product_detail.php?pid=4733

Maybe I'm not understanding how these things work, but how does air get INTO the muff? If you're pulling air out of it, air's gotta go in somewhere to replace it...

Vans' Cabin Heat Muffs have a seperate inlet/outlet http://www.vansaircraft.com/cgi-bin/catalog.cgi?ident=1237207761-148-30&browse=heatvent&product=cabin-heat-muff... why would a carb heat muff be set up differently?
Click to expand...

Looking at the pic, it appears the bottom section is left open. That's my guess at least. :)

L.Adamson
 
prkaye said:
Alfio, I see this one you mentioned has a single outlet:
http://www.wicksaircraft.com/catalog/product_detail.php?pid=4733

Maybe I'm not understanding how these things work, but how does air get INTO the muff? If you're pulling air out of it, air's gotta go in somewhere to replace it...

Vans' Cabin Heat Muffs have a seperate inlet/outlet http://www.vansaircraft.com/cgi-bin/catalog.cgi?ident=1237207761-148-30&browse=heatvent&product=cabin-heat-muff... why would a carb heat muff be set up differently?
Click to expand...

Air is sucked in from the open area (a slot about 3/4" x 12"). It is taking in the already warmer cowl air. A 40 F deg carb temp rise is not too shabby. Certified aircraft do require to have a 90 F degree rise.

If you will be using the air to heat the cabin, then you need to bring fresh air to the shroud, that is why the heat muffs have and outlet and and inlet for 2" scat. You can still use the heat muffs for carb heat; just use the scat tubing on only one opening, leave the other one as-is.

I think the heat muff is more expensive, and will not necessarily give you any additional benefit for preheating carb air.
 
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carb heat

Phil,
if you dig into some training books, somewhere there's an explanation that carb ice is most likely when the temp and dewpoint are within about 4 degrees C of each other. In spring and fall especially, when you see fog and mist in low areas, that's a great day for carb ice!
The Van's setup gives me NO drop in rpm, so not much heat. ( others have measured a small heat rise.)
I plan to build a muff around my cross-over exhaust pipe to give more surface area, like the Robins muff.
One cabin heat muff will probably give adequate cabin heat, so you could do the carb heat from a muff on the other side stacks if you want.
I'd consider routing some heat and/or airflow to the windshield, especially if you are doing a tip-up, to keep it clear during initial warm-up and taxi.
 
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