Ex manifold heat riser valve...

[philo_beddoe]

I need the final scoop if those heat riser valves are really needed, this thing.



I just installed new ex manifolds including the heat stoves and pre heater hose to air clnr assy.  If its a matter of waiting an extra 2 minutes for my engine to kick down to normal idle on cold mornings, then the heck with it.   Anyone agree?? 

[Captain Swampy]

Weld it open or shoot can it.

[bd]

I need the final scoop if those heat riser valves are really needed

How cold does it get in your neck of the woods?

[philo_beddoe]

Im in new england, could get -15, could be in the 40's. We have a saying, "if you dont like the weather, wait a while, it'll get worse!"

[philo_beddoe]

Heres the set up, tube not installed yet.

[bd]

Remember this...

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?

...and the answers...

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 (TAC) to improve cold drivability.  Because of the severe winter cold in your area, I recommend a functioning heat riser.... 

When the heat riser functions correctly, it provides subtle improvement in cold drivability and fuel mileage.  Two significant weaknesses in the design are, 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 TAC is functioning....

Paraphrasing another post in that same thread... as air flows through the carburetor's venturi above idle the airflow speeds up, compresses and heats slightly.  The main fuel nozzle sits just below the point of greatest constriction in the venturi where developed airflow pressure becomes the lowest.  Fuel is forced into the low pressure area by aspiration and mixes with the airstream.  As the airstream exits the venturi the mixture expands, slows and cools dramatically.  Cooling is exacerbated by the introduction of the fuel, which has a lower boiling point than water.  When the liquid fuel vaporizes, partially changing state to a gas, it supercools the intake charge.  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 TAC helps to alleviate this phenomenon by heating the intake air.  The heat riser raises the initial temperature of the intake manifold to help prevent puddling of cold fuel on the walls and floor of the manifold.  Once the engine comes up to temperature and the heat riser gradually opens, hot oil splash against the base of the intake manifold from the valley area makes the heat riser unnecessary.  TAC may continue to function based on the temperature of the air inside the air cleaner housing.

The point of all this rhetoric is that the choke, TAC, and heat riser function as a system - each component overlapping and compensating for specific aspects of driveability during cold operation.  If you eliminate part of the system, you invite cold driveability issues.  Can you run without a heat riser?  Of course.  Will it affect driveability?  The colder the climate, the greater the effect.  At -15°, I expect it to be beneficial.  At 40°, not as much.

Take a look through this current thread:  Electric vs Manual Choke.

[philo_beddoe]

Yeah bd, thanks for the reminder. At least this winter i'll have stove and pre heater tube. Last winter nothing and it was coldest winter since i was born. We'll see how it goes w/o the heat riser.  I cant remember one post from the next, thanks for the young sharp minds out there.

[bd]

Post the results in the Spring.  I'm sure it will run, albeit with slightly increased warm up times on the colder days and maybe a very slight dip in fuel economy depending on miles driven following each start up.

[philo_beddoe]

Yeah bd, thanks for the reminder. At least this winter i'll have stove and pre heater tube. Last winter nothing and it was coldest winter since i was born. We'll see how it goes w/o the heat riser.  I cant remember one post from the next, thanks for the young sharp minds out there.

[philo_beddoe]

Today was the official first day driving with the new pre heater tube and w/o a heat riser valve. 20 degrees last night, started up and after about 60 seconds, a little tap of the pedal and she kicked down nicely.

In summary: Heat riser valves are not needed. It would be different if they could be changed out w/o disassembling ex manifold. Then when they froze up or rusted or whatever, you could quickly swap it out. But, it us what it is. I live in a cold climate in winter and so far, I'm good.

I'll repost results at -15 degrees. Lord I hope it don't get that cold!!

[VileZambonie]

Most guys wired them open or eliminated them altogether when they failed. They help reduce emissions more than you will notice them improving drivability. In extreme cold, don't wide open throttle, let it warm up before you take off and you will be fine. For your stove pipe, do you have the thermostatic valve operational on the air cleaner? Is the TVSV connected and functioning?

[philo_beddoe]

Yes the t-stat valve, tvsv is functioning properly, i checked it right after imstalling new air clnr assy. I was actually surprised it was working properly.