Burnt Valves?

Discussion in 'Chevy' started by TXFlyGuy, Dec 2, 2019.

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  1. Dec 2, 2019 #1

    TXFlyGuy

    TXFlyGuy

    TXFlyGuy

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    Posed this to our engine tuner, asking if he had any concerns about our short-stack exhaust, and potential valve issues.
    His answer was no. And he has more experience with engine building/tuning/exhaust than anyone that I know.
    What do the experts here say about this? My reading over the years tells me this is indeed an Old Wives Tale.
     
  2. Dec 2, 2019 #2

    TFF

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    Shock cooling valves can be an issue.
     
  3. Dec 2, 2019 #3

    TXFlyGuy

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    With a liquid cooled engine, the cylinder heads remain at a constant temp. With all of the SBC that have been hot-rodded with shorty exhausts, for decades, I do not know of anyone with a valve problem.

    Yes, there have been problems due to improper mixture control, and other issues.

    My engine tuner claims no problem, with the ECU controlled, liquid cooled V8.

    If you have some good data on this, we would like to learn more.
     
  4. Dec 3, 2019 #4

    TFF

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    It’s all engine management in the pilots hands. Your a professional pilot so you should have the discipline to not yank the power back to descend. You are not shock cooling the cylinders, you are shock cooling the valve head. Close the throttle where it’s cold, like altitude, and the cold air reverses up the pipe, it’s a good chance you will warp a valve. It’s not air or water it’s reversal of air. Not far for air to travel with a short pipe. I would have a short pipe too.
     
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  5. Dec 3, 2019 #5

    TXFlyGuy

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    That being the case, it will be easy to manage, at least in theory.

    However, on all of the transport jets that I flew, slowly pulling the power to idle at TOD is standard practice.
     
  6. Dec 3, 2019 #6

    Toobuilder

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    Jets are a continuous combustion device and do not have the exhaust reversion issues of a recip. Some airplanes like the "short stack" B-25 had minimum idle speeds to prevent reversion and exhaust valve damage. In your case, some minimum idle speed will probably avoid the same issue. Of course, your airframe team should have worked this all out for you so you don't have to look for advice on the internet.
     
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  7. Dec 3, 2019 #7

    TXFlyGuy

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    I am taking everything a step further rather than relying on base info from the manufacturer, in the pursuit of a safe operation.

    We are in uncharted territory with the complete "clean sheet of paper" design of the new exhaust. But no worries, as Hutter Performance has been down this road before. Did I mention that the engine tuners at Hutter are pilots?

    Until someone can show some factual evidence of damage to the engine resulting from power reduction/cooling, I'm in the camp that says this is a long term myth.

    Here is some good reading on shock cooling, myth or reality?

    https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/
     
    Last edited: Dec 3, 2019
  8. Dec 3, 2019 #8

    BJC

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    At least 12 Thunder Mustangs were built. There might be some relevant info among that crowd.

    Lots of Cassutts, and other airplanes too, such as my neighbor’s Hatz, use stub exhaust stacks and do not baby the throttle settings.

    I, too, would like to get some factual data on “shock cooling”.


    BJC
     
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  9. Dec 3, 2019 #9

    Hot Wings

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    Good!? I found it superficial and used examples that seem to have been picked to support the authors views.

    Shock cooling damage is a real thing.......for some engines.......in some installations.....in some situations.
    Probably the most infamous aviation example is the Rotax/Mini 500 combination - a liquid cooled engine in a helicopter.
    Using that example in an article as an illustration of the dangers of shock cooling would be an example of the same kind of data picking as that article from the other point of view.

    Per the article even Lycoming knows that there is chronic damage from repeated rapid cooling. They have designed their engines so that there is no real danger of acute damage and accept the chronic damage.
     
  10. Dec 3, 2019 #10

    TXFlyGuy

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    Bob Hoover did not seem to be concerned. Or maybe his planes were of such a nature that they were immune to the myth of shock cooling?

    We are still waiting for conclusive and factual real world evidence of shock cooling damage, to engine valves in liquid cooled engines. Since that is within the purview of this thread.
     
  11. Dec 3, 2019 #11

    Toobuilder

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    Exhaust reversion due to short stacks is not the same phenomenon as "shock cooling" in this context.
     
  12. Dec 3, 2019 #12

    TXFlyGuy

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    Point well taken.

    What about the exhausts on the P-51, Stewart Mustang, Thunder Mustang, and FEW Mustang. They all have short stacks. Are they suffering from valve damage? Why, or why not?

    As AvWeb pointed out, the ultimate cooling shock occurs each and every time the engine is shut down. Where cold ambient air can migrate up through the exhaust.
     
    Last edited: Dec 3, 2019
  13. Dec 3, 2019 #13

    Toobuilder

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    As I said in the first post, a higher RPM idle than would otherwise be needed is usually the fix. The longer the pipe, the lower the chance of reversion.
     
  14. Dec 3, 2019 #14

    TXFlyGuy

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    Yes. Easy fix. Just requires a bit of planning.

    But what a bout the ultimate shock cooling, after engine shutdown, with the associated cold air migration?
     
  15. Dec 3, 2019 #15

    bmcj

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    This makes it sound as though you have already made up you mind before you even asked.


    It seems that on a more substantial exhaust system, the migration path and latent heat of the head would help protect it by warming the air before it gets to the valve. I can’t speak to short stacked systems though. Also, it will be like a closed end tube, so there will be no “through-flow” drawing air through the system. The piston is as far as the air will be allowed to travel, and that is only if the valve is open.
     
  16. Dec 3, 2019 #16

    TXFlyGuy

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    We are seeking factual data, supported and documented, as to damage caused by by using short stack exhaust headers on liquid cooled, ECU controlled engines.
    That is why the question was asked, and I personally don’t know.

    Many theories exist. But they are theories, not real world evidence.

    If this is a reality, there should be volumes published documenting this phenomenon.
     
  17. Dec 3, 2019 #17

    TFF

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    Your airplane is different than anybody else’s. You are going to have to test it out for yourself. You know what the concerns are. You are going to have to fly yours through it’s test program and see where it sits. You don’t know if it’s not going to apply, have a once and your done breakage, or something that’s going to take 500 hrs to show.

    I know two people with Safari helicopters. One has a factory demonstrator and one built it himself. The guy that owns the demo helicopter is tired of it. He has had it two years and he has had to take it back to the factory over ten times for them to fix stuff that’s not supposed to break. Like four gearbox rebuilds. He has not been able to fly it more than ten hours at a time before taking it back. Only the factory has wrenched on it. At least they are taking care of it. The guy who built his has had not one problem. They are all different.
     
  18. Dec 3, 2019 #18

    bmcj

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    Perhaps an EGT sensor near the valve can be used to monitor the cooling rate after shutdown?
     
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  19. Dec 4, 2019 #19

    BBerson

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    I would be concerned about rust. I suppose plugs are used?
     
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  20. Dec 4, 2019 #20

    TXFlyGuy

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    Already installed.
     

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