Heat riser valve

[philo_beddoe]

I saw a few posts, but not exactly...  What is the function of the heat riser valve?  I am installing new manifolds and stove on pass side with tube to air clnr assy.  Is the heat riser valve an important part, when they rust out and fail, most do not reinstall when replacing exhaust.  What is your take?

[LTZ C20]

If your truck is required to take a visual emissions test, then it MIGHT be required. It is best to check with your area of registration if you are required to take a visual test. If no visual test is required or only a "tailpipe/sniffer" test is required or you have no emissions requirements at all, then you may be able to remove or leave it off. Again, first check area emissions requirements.

[Peter S]

The function of the heat riser valve is to force the exhaust to pass thru a passage in the intake manifold when the engine is cold. This will bring the intake/carb to operating temperature faster.

[philo_beddoe]

Ok, thanks.  15 years or older, no emissions!! Amen!

[Irish_Alley]

hold up.
philo dont you remember this post
http://forum.73-87chevytrucks.com/smforum/index.php?topic=26679.0
peter also take time to read this post it has the heat riser or stove in it. i know of the part youre talking about but im not sure of the name. someone else like bd or rich will know it

[philo_beddoe]

Yes i do.  Before posting i did a search and found it.  However, it appears the post is mainly talking about the tube that goes from the upper stove to the air cleaner assy.  I know he does mention 'heat riser", but i was looking to discuss the actual heat riser valve that is inside and sits between exh pipe and manifold. Today i dropped $1200 in parts and new exh manifolds are on the list, new studs and i want to put the heat riser valve back in.  Last winter we barely got above 10 degrees.  I did not have my square body back then.  I had a f-350 powerstroke diesel.   Whoops, that reminds me, i gotta cancel my subscription to the ford powerstroke forum.   

[Irish_Alley]

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.

the valve operated the blender door in the snorkel. when cold it will pull the air from the exhaust manifold, once warm it will open to pull air from the end of the snorkel

[philo_beddoe]

http://images2.carpartsdiscount.com/auto/archive/pictures/127793/515/1/P/39C3290/chevy_camaro_1969_exhaust_manifold_heat_riser_valve_oem_3887023.jpg

Yup, this little guy.  Should i reinstall?

[philo_beddoe]

This one has no commercial background writing.

http://www.impalas.com/images/T/EMR002_big.jpg

[bd]

The function of the heat riser valve is to force the exhaust to pass thru a passage in the intake manifold when the engine is cold. This will bring the intake/carb to operating temperature faster.

The heat riser passage crosses under the floor of the intake plenum between the right and left cylinder heads and helps maintain atomization of the air/fuel charge to decrease coalescing and puddling of cold fuel within the intake runners.  It works in conjunction with the THERMAC air cleaner to improve cold drivability.  Because of the severe winter cold in your area, I recommend a functioning heat riser if you can locate one, or free yours up using aggressive penetrating solvent that carries high temperature lubricant and some TLC (a hammer, vise and channel lock pliers). 

When the heat riser functioned correctly, it provided subtle improvement in cold drivability and fuel mileage.  Two significant weaknesses in the design were, 1) seizing of the butterfly in the closed position, which increased exhaust backpressure, degrading performance and fuel mileage and increasing exhaust heat, and 2) separation of the butterfly plate from the valve body and blowing downstream through the exhaust.

Not having a functioning heat riser is not the end of cold driving, as long as the THERMAC air cleaner is functioning.  When both fail or are missing, driving becomes a heel/toe proposition.  There is only so much you can accomplish with a choke.  lol

Irish, the heat riser is a weighted exhaust butterfly valve counter-balanced by a bi-metal thermostatic spring, generally installed at the outlet of the right exhaust manifold.  The round wire spring simply decreased valve rattle.  At some point during the 80's it was superseded to a vacuum actuated Early Fuel Evaporation (EFE) valve, the primary difference being refined control of the butterfly.  You are confusing an exhaust heat riser valve with the thermostatic air cleaner (THERMAC).

[Irish_Alley]

Ahh makes sense as to why I couldn't remember the correct name for the one he was talking about cause it was the heat riser and what I thought was the heat riser was the thermac

[rich weyand]

Yup.  Heat riser is the crossover doodad that runs exhaust through a channel in the intake manifold to heat up the carb. Heat stove is the doodad that runs heated air from around the manifold or headers up to the thermac air door.  The latter warms the intake air, the former warms the carb until it comes up to temp.  The heat riser is unnecessary once the engine is up to temp, while the heat stove/thermac is necessary continuously while driving in cold weather conditions for correct mixture (basically since the carb is setup for normal temps) and to prevent carb icing during operation.  (Yes, you can find posts from a couple years back where I was confused as well.)

If you have the thermac but not the heat riser, the carb will come up to temp, but more slowly.  If you have the heat riser and not the thermac, the carb come up to temp but will cool off again when the heat riser valve opens, and you can have problems as well as crappy winter gas mileage.  If neither works, it will run like a POS when the weather is cold.

I have the heat stove on mine, but no heat riser.  Takes a couple minutes, but it warms up fine.  Then again, mine is garaged, and half the garage walls are under ground, so even when it is ten below, the truck is nowhere near that temperature when I start it.

[bd]

Rounding out the preceding discussion:  As air flows through the carburetor's venturi above idle the airflow speeds up, compresses and heats slightly.  The main fuel nozzle resides just below the point of greatest constriction where developed airflow pressure becomes the lowest.  Fuel is drawn into the low pressure area by aspiration and mixes with the airstream at this point.  As the airflow exits the venturi the airflow expands, slows and cools dramatically.  Cooling is exaggerated by the introduction of the fuel, which has a lower boiling point than water.  If the ambient air is laden with moisture, the sudden cooling of the airstream will condense the air/fuel/moisture stream to below the freezing point of water and ice will form and collect around the throttle plate.  The THERMAC helps to alleviate this phenomenon by heating the intake air. 

The heat riser raises the temperature of the intake manifold to prevent puddling of cold fuel.  Once the engine comes up to temperature, hot oil splash against the base of the intake manifold from the valley area makes the heat riser unnecessary.

[philo_beddoe]

Like "todays thought" post,,,, imagine living in a neighbor hood full of you chevy heads.  I'm pickin the house between rich and irish, with bd across the street!!!  Great info, thanks.