The International Cessna^® 170 Association

Anyone with an Avcon or Delair 180 conversion also have a carb temperature gauge?
How much temperature rise do you see when you pull on carb heat?

Hi Dave. Yes, I have an Avcon O-360 on my 170 with a carb heat gauge. The amount of temperature increase will, naturally, depend on the outside air temperature and acquired latent heat within the muffler.

That being said, on an average Alaska summer day (of about 65 - 75 degrees ambient) temperature increase is about 20 to 25 degrees -- but I'm relying on memory alone.

May I ask why you wish to know?

-Scott

I ask because I only get 15 degrees rise, which is pretty useless. Part 23 requires a 120F rise. Of course our airplanes don't have to conform to part 23, but it gives an idea of what we'd like to see. If I'm remembering correctly from 20 years ago, my '73 182 was capable of at least 60F. The Avcon muffler shroud is big like the 182's so I don't see why the heat output is so low. I just installed a new engine monitor with a carb temp gauge so I guess it's always been like this.

Fifteen degrees doesn't seem like much of a rise to me. I don't think I would want to have to depend on it in some serious icing conditions. I believe that I would be looking at the "Heat Sink" conditions in the heat exchanger and striving to develope some improvement.

BL, I agree completely. I need to figure out what's going on with the carb heat. I've already checked the sensor and it's OK and installed correctly, according to JPI. So the question is whether my carb heat system is malfunctioning or defective by design. It's installed per Avcon drawings. The hoses are good and the flapper flaps, so maybe it's a design flaw. That's why I posted the question -- to see what other Avcon owners get for heat. If anyone can add more information I'd be grateful.

I don't remember ever noticing how the heat exchanger on an Avcon conversion is made. That is, how much heat sink "finning" or such is installed on the hottest portion of the exhaust pipe.
In days of old when pilots were bold, and I was a young guy up in Alaska in 1949, I tried making a cabin heat system for my Stinson L-5G by building and installing a heater muff on the left bank exhaust pipe. It was similar to the factory muff on the right bank that provided carburetor heat. It turned out to be semi successful but did not have enough "heat sink" effect to extract enough of the exhaust heat. It was only a smooth exhaust pipe without any attached fins or such to pull more heat out of it. The only way to raise the temperature was to choke off the spill valve and slow down the passage of the air through the muff. Better, but not much more, than nothing.
Boy, that was one helluva cold airplane in an Alaska winter, but I had a lot of fun with it.

I have the Delair conversion ,and have wondered how effective it is because on run up checks there is very little rise in RPM with carby heat on . But icing isn't a real problem where I live. However I do feel the design is a little wanting in heat extraction.

ghostflyer wrote:I have the Delair conversion ,and have wondered how effective it is because on run up checks there is very little rise in RPM with carby heat on . But icing isn't a real problem where I live. However I do feel the design is a little wanting in heat extraction.

You should not get a "rise" at all. You should experience a DROP in RPM (due to restricted air-flow coupled with enriched fuel/air mixture. A "rise" would indicate a blocked/restricted cold-air filter, an ineffective carb-heat system, or both.)

blueldr wrote:I don't remember ever noticing how the heat exchanger on an Avcon conversion is made. That is, how much heat sink "finning" or such is installed on the hottest portion of the exhaust pipe.

No heat exchange enhancements at all. Cold air enters at the top of the shroud (where the 6 is pointing), hot air exits from the lower left port to the carb heat box, and from the lower right port to the cabin heat valve.
http://www.acornwelding.com/pdf/Cessna/ ... 52_172.pdf

It looks to me that the smooth bare skin of the muffler pesents the same problem that I had on my L-5, just not enough "finning" or other surface stuff to pull more heat out of the exhaust gasses.
Anyone with knowledge of the interior of the Hanlon-Wilson stock mufflers on the C-170B, will remember the finned studs attached to pull more heat out of the exhaust.

blueldr wrote:It looks to me that the smooth bare skin of the muffler pesents the same problem that I had on my L-5, just not enough "finning" or other surface stuff to pull more heat out of the exhaust gasses.
Anyone with knowledge of the interior of the Hanlon-Wilson stock mufflers on the C-170B, will remember the finned studs attached to pull more heat out of the exhaust.

Yes and the Hanlon-Wilson the cold air enters in the front and hot air exits out the back giving it a chance to absorb some of the heat. The Avcon looks like even with the fins the inlet and outlet are so close the air would not have a chance to absorb much heat.

# 6 is the inlet, 180 Deg is the outlet to cabin heat and the middle outlet is the carb heat. Is there a difference with cabin heat full on or full off ?

The carb heat is the hole opposite the cold air inlet, the cabin heat is the center hole on the bottom, and the tailpipe goes out the hole on the right.

I've been unable to affect the carb heat with cabin heat. The cabin heat/air valve is stock 170. To minimize flow through the cabin heat outlet I selected cabin heat and de-selected cabin air. Hot air from the muffler was directed toward the firewall by the flapper valve but blocked by the slide valve on the firewall. No change.

I'm not surprised that blocking the cabin heat has no effect. At 2500 rpm the engine is consuming 7500 cubic inches of air per second. That's a cube about 19.5" on a side, so the minimal flow into the cabin probably makes no difference. I wonder if carb heat actually sucks air out of the cabin. I don't know if MP changes when I did these tests. I'm breaking in a new engine so I'm fixating on CHTs.

As I said before, this muffler/shroud setup is a lot like on my '73 182P, and its carb heat could easily peg the analog CAT gauge. It had no fins or studs on the muffler, but it also burned 33% more fuel, so that's a difference.

Google has turned up some interesting things about carb heat, like Van's stock setup is only capable of a minimal carb temp increase.

Thanks for your thoughts. Any other data of actual carb temps in any airplane will be appreciated.

Dave, my carb heat set-up is as I described, so I'm surprised and can see if your sucking from close to where the ram air is entering, the air wouldn't be as warm as it should be. What are the duct diameters for each port?

All ducts are 3". I still don't see how with so much surface area the air doesn't get hotter. More debug is needed. I'll put some thermocouples around the system and see what gets hot where.