Exhaust Heat Riser Question

[454Man]

I think its called the heat riser

GM called it the "heat stove".  It is only useful if the Thermac system is still installed and operational.




The TAC system, Fig. 82, is designed to improve carburetor operation and engine warm-up conditions. It achieves this by keeping the air entering the carburetor at a temperature of at least 100°F., or more.
Carburetor air temperature is controlled by a pair of doors, Figs. 83 thru 85, located in the air cleaner snorkel, which channel either preheated or under hood air to the carburetor.
Preheated air is obtained by passing under hood air through ducts surrounding the exhaust manifold, causing it to pick up heat from the manifold surface. The heated air is then drawn up through a pipe to the air cleaner snorkel.
Underhood air is picked up at the air cleaner snorkel in the conventional manner.
The two air mixing doors work together so that as one opens, the other closes and vice versa. When underhood temperature is below approximately 86°F., the cold air door closes, causing the hot air door to open. Hot air from the exhaust manifold stove is then drawn into the carburetor. As the underhood temperature increases, the cold air door begins to open until the temperature reaches approximately 115°to 130°F, at which time the cold air door is fully open and the hot air door is fully closed.
The doors are controlled by a vacuum motor mounted on the air cleaner snorkel. This motor, in turn, is controlled by a sensor inside the air cleaner which regulates the amount of vacuum present in the vacuum motor according to air cleaner temperature. Whenever manifold vacuum drops below 5---8 inches, depending on the unit, the diaphragm spring in the motor will open the cold air door wide in order to provide maximum air flow.
The vacuum motor and control door assembly in the left snorkel on outside air induction units does not have a sensor and is controlled only by manifold vacuum. This snorkel remains closed until full throttle is obtained. With manifold vacuum at 6-8 inches, the door will open, allowing maximum air flow.

This makes sense thanks for the detail :-)

 

[greasemonkey54]

thanks for the detailed explanation

[rich weyand]

I was surprised when I put the system back on my truck (it had the stupid two-chrome-pie-plate open-element air cleaner setup on it when I bought it) that the thermostatic air cleaner is proportional, not one or the other.  It actually mixes the air from both sources in the middle range of temperatures.  Above that, the cold is open and the hot shut, and below it, the hot is open and the cold shut, but in the middle range of temperatures it varies the two to maintain its setpoint.  Surprised the heck out of me.

[Irish_Alley]

i just might have to rig something up for mine. its fun in the summer but once it gets cold its a pain but one thing is, it shouldnt cause me to idle for 5 minutes before i can take off

[Irish_Alley]

http://forum.73-87chevytrucks.com/smforum/index.php/topic,24561.0.html
from bd

[greasemonkey54]

SO it has gotten into the 40s a few nights and so far now issues with cold starts or cold driving that I did not have before. We shall see when it gets really cold tho.

here is what she looks like now...

[philo_beddoe]

Where can i purchase that heat stove to air cleaner tubing?

[LTZ C20]

Where can i purchase that heat stove to air cleaner tubing?

Well you can search wrecking yards for the orginal piece. It's a bent piece of steel tube. Some resto parts places might have them, or ebay or craigslist.

The other option is the universal, foil type, flexible tubing. You bend it to the shape you need. That should be available at any autoparts stores, they are usually super cheap.

[rich weyand]

Where can i purchase that heat stove to air cleaner tubing?

Well you can search wrecking yards for the orginal piece. It's a bent piece of steel tube. Some resto parts places might have them, or ebay or craigslist.

The other option is the universal, foil type, flexible tubing. You bend it to the shape you need. That should be available at any autoparts stores, they are usually super cheap.

You can't bend those tight enough to miss the valve cover.  BTDT.

[SkinnyG]

The factory riser tube was metal on my '77.  I modified it to clear the tall valve covers with an old header tube.  Works dandy in a Canadian winter.

[rich weyand]

I think the one I PMed you is the correct one, philobeddoe.  What year/model/and engine are you talking about?

[philo_beddoe]

I'm in the process of installing a new heat riser, upper and lower heat stove and the tube that connects to the bottom of the air cleaner snorkel.  Dont remove that stuff.  Nothin like factory stock.

[MAILMAN]

I have a 1978 GMC C15 Heavy Half.  It has a 350 and the heat stove has cracked right at the tab for the mounting screw.  Are there any sources for purchasing a replacement?  Thanks in advance.