Exhaust Flapper Valve

edhamlin
edhamlin Senior Contributor
edited November -1 in HUDSON
(Yeah, I know this question has been asked before).... need to settle a disagreement between me (the old timer) and my mechanic nephew:



Which way does the thermal spring push the flapper?



In my '41 Six, the flapper shaft has a thermal spring on the passenger side, and (at one time) a couple of ordinary coil springs on the driver's side. The springs worked against each other. The pin attached to the ordinary springs has rusted away. The flapper is still free to move, but the thermal spring holds it in position all the time.



I say the thermal spring forces the flapper closed when it heats up. My nephew says that the thermal spring relaxes when it gets hot and allows the ordinary springs to close the valve. Which is true?



I need to know this because I want to safety wire the flapper closed (in the position it would be when the engine is hot) all the time. I can't tell which position is correct. The engine seems to sound the same regardless, and I can't feel any resistance on the flapper either way. My engine is running hot, and I suspect that the flapper is the culprit.

Comments

  • junkcarfann
    junkcarfann Expert Adviser
    The thermal spring works to turn the flapper. The smaller ordinary coil springs are to keep the assembly taut so it does not rattle...at least that is what I assumed when I last saw mine. Which way it turns, I do not know...Mine did not work, so I took it out, and plugged the small hole with a bolt, and the covered the large hole with a piece of flat stock for "performance enhancement".
  • Uncle Josh
    Uncle Josh Senior Contributor
    The valve is NC, Normally Closed, which routes exhaust gasses under the carb to warm it up when cold. When it warms up, the carb heat is no longer needed so the bi-metallic spring lets it open.
  • edhamlin
    edhamlin Senior Contributor
    Uncle Josh wrote:
    The valve is NC, Normally Closed, which routes exhaust gasses under the carb to warm it up when cold. When it warms up, the carb heat is no longer needed so the bi-metallic spring lets it open.



    OK, I know should know this, but: does the thermal spring get looser or tighter when it gets hot? That's the main point, and it would tell me which way to tie it off.



    I wanted to remove the cover and see which way the flapper works, but the nuts are frozen on the studs and I'm afraid to try and take them off, for fear of breaking a stud inside the manifold. Also I don't have a replacement gasket for the cover.
  • Park_W
    Park_W Senior Contributor
    The thermal spring grows in length when it gets hot, so look at it and figure which way the flapper shaft would move if the spring gets longer.
  • Geoff
    Geoff Senior Contributor
    edhamlin wrote: »
    OK, I know should know this, but: does the thermal spring get looser or tighter when it gets hot? That's the main point, and it would tell me which way to tie it off.



    The thermal spring loses tension as it gets hotter, allowing the flow of hot gas to open the valve, hence the exhaust will go straight through, instead of around the by-pass.
  • edhamlin
    edhamlin Senior Contributor
    Thank you all!



    I'm not normally this dumb, I just have a mental block about this particular thing...
  • Uncle Josh
    Uncle Josh Senior Contributor
    You aren't dumb. The inate carnality of inanimate objects make things look like they should operate contrary to all reason.
  • Geoff
    Geoff Senior Contributor
    Plus, if something is so simple nobody can misunderstand, most people will.
  • edhamlin
    edhamlin Senior Contributor
    Well, I took everyone's advice and then took a "scientific" approach. I removed the thermal spring, put one end in a vice and heated it with a torch to see what it would do. Viola! It became TIGHTER. This told me which way the flapper would be moved when the exhaust heats up. The answer, at least for my '41 Six, was:



    When looking at the thermal spring from the passenger side, it holds the flapper in a CW direction when cold. When heated up, it moves the flapper fully CCW.



    I then safety wired the flapper in the full CCW (hot) position. I suspect that this will make cold starts a bit more difficult, but should eliminate the risk of burnt valves should the flapper get stuck.



    Thanks again for everyone's advice. This is a great forum - lots of help, no abuse, very supportive.
This discussion has been closed.

Categories

In this Discussion