stakeside

purpose of heat riser on manifold

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Posted (edited)

My DA6 truck engine has a heat riser on the manifold that is frozen up. lots of rust prevents me from safely removing without breaking the cast iron. I noticed most of the DA6 engines do not have this manifold riser. What is the purpose of this since not used on other engines?

I have two choices: 1 remove and replace with a blank plate, or 2 leave in current position with slot facing vertical. I do not know of any replacements.This is not my engine.

 

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Edited by stakeside
e (see edit history)

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Posted (edited)

Essentially used at startup for proper warmup, especially in cold conditions (I thought that pic looked familiar lol).

 

3rd option 

If I can get it loose you can have it. Up to you...

 

 

41.jpg

Edited by 30DodgePanel (see edit history)

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It prevents carburetor icing on cool days (less than 15 oC?) with a bit of humidity. Start your own engine and measure the temperature of the neck between the carb. and the inlet manifold. This is caused by the venturi. The air-fuel mixture speeds up in the venturi then slows below it. The higher speed creates the vacuum that pulls fuel from the float chamber. Beyond the venturi, the mixture expands (and thus slows) and that absorbs heat, this cooling the neck beyond the venturi. Use your inexpensive IR thermometer on it. Mine is always cool or cold to the touch for a while after starting.

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On 5/10/2019 at 10:41 AM, stakeside said:

My DA6 truck engine has a heat riser on the manifold that is frozen up. lots of rust prevents me from safely removing without breaking the cast iron. I noticed most of the DA6 engines do not have this manifold riser. What is the purpose of this since not used on other engines?

I have two choices: 1 remove and replace with a blank plate, or 2 leave in current position with slot facing vertical. I do not know of any replacements.This is not my engine.

 

 

The manifold riser on my truck has small cracks in the flange. It will be hard to remove intact.

I have found out if the slot on the end is vertical the flapper is in open position. Since I do not drive in the extreme cold it should be ok to leave riser as is. I have the radiator shutters that can be closed on cold days if needed until the engine warms up.

Are there replacement risers available?

 

 

 

523816191_DA1.JPG.e8045d95c30fd7f49252361d51ce2642.JPG

 

 

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3 hours ago, stakeside said:

 

Are there replacements available ?

 

 

 

I've never been able to locate any. 

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5 hours ago, stakeside said:

 

 

My DA6 sedan's manifold heat control is missing some parts and is "open".  I was too afraid to break the fragile cast iron to deal with it. I left it as is and have had no problems with it whether cold weather or warm weather.

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I am sure more tech people on this forum than I will chime in here...

 

Hot air does not hold as much fuel than cold air, the simple principle of venturi effect of carburetors...that being said, if hot air is being mixed with fuel prior to entering the compression chamber and not getting the full effect of the venturi, then from an efficiency standpoint I have to assume you guys are enriching the mixture to compensate for the hot air, less dense air....

 

Which then leads me to believe that the pistons are running hotter than designed, which then could lead to premature head/piston failure or even ring failure...

 

Am I off base here?

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6 hours ago, 30DodgePanel said:

 

 

 

I've never been able to locate any. 

 

Antique Auto Parts Cellar/Then and Now Automotive makes some heat riser kits for later cars. It would not surprise me if the bi-metallic spring they make would work in your application. The other parts needed are brass bushings which can be made, etc.

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Posted (edited)
9 hours ago, Surf City '38 said:

Am I off base here?

Yes Sir. You are correct, hot air contains less oxygen than cold air, hence the intercooler. But the hot exhaust gas is directed around the mounting base of the carb. to warm the carb. not the fuel air mixture. Heating the neck below the venturi prevents it cooling enough to freeze moisture in the air touching it as it flows through. I suppose once the inlet manifold is warmed a bit by the hot exhaust gas the whole carb. will begin to warm up too. These engines don't have a thick insulator under the carb., just a paper gasket.

Edited by Spinneyhill (see edit history)
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56 minutes ago, Spinneyhill said:

Yes Sir. You are correct, hot air contains less oxygen than cold air, hence the intercooler. But the hot exhaust gas is directed around the mounting base of the carb. to warm the carb. not the fuel air mixture. Heating the neck below the venturi prevents it cooling enough to freeze moisture in the air touching it as it flows through. I suppose once the inlet manifold is warmed a bit by the hot exhaust gas the whole carb. will begin to warm up too. These engines don't have a thick insulator under the carb., just a paper gasket.

 

So probably not enough to be worried about since its not a direct heat transfer, more of an indirect.....

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The heat riser is there to provide a HOT SPOT under the carburetor so as to vaporize any liquid fuel that has fallen out of the air stream.  It doesn't do a very good job of preventing carburetor icing and the hot air stove on more modern vehicles is much more effective.  See Intake Manifold Heat.  Keeping the carburetor as cool as possible minimizes the possibility of percolation.  See Vapor Lock.

 

The purpose of the heat riser is explained starting at 5:20.

 

 

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Thank you @fraso, I didn't know that and am not clever enough to think of that.

 

Dyke's (1929) Instruction No. 13 discusses this. The manifold hot spot is one way to heat the fuel to improve vaporisation. The reason given is that "low-gravity" or low grade gasoline is hard to vaporise and needs to be heated. "High-gravity" gasoline needs less heating or vaporising. Low-grade gasoline mixture is not a true vapour. "Instead of forming a gaseous mixture, it condenses inside the combustion chamber and manifold. Therefore a plentiful supply of heat is required." In addition, condensed fuel runs through the cylinder into the crankcase and dilutes the oil.

 

A couple of pages further on there is a section headed "HOW TO DETERMINE THE SIZE OF CARBURETOR TO USE". It says that if the carb. is too small for the engine, it will become very cold while in operation and water may condense in the carburetor:

CarburettorIcing_Dykes.thumb.jpg.93bc52b679ad1b3e00215476c6eb549a.jpg

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Speaking of heating fuel check out Dan Strohl's article on Hemmings Blog from yesterday about Adiabatic engines. Certainly different then a heat riser but very interesting . The whole engine is super heated including the fuel. Instead of 30% efficiency, which is normal,  they get double. Combustion chambers run at 2000 degrees F. 

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Vaporizing gasoline absorbs a lot of heat at at part throttle (when manifold vacuum is high) so the heat riser system also replaces that heat thereby preventing the manifold from collecting condensation or frost.

 

See Model T Forum: Intake Manifold Iceing

 

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