Blown out aluminum OT pipe
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Blown out aluminum OT pipe

  1. #1
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    Default Blown out aluminum OT pipe

    Hey,

    Anyone else seen something like this? I have cost aluminum OT's, water cooled and the right side pipe looks like it got super-heated, bubbled and split at the top bend. The left side had no issues. This was after a number of (10 maybe) high-load launches and 1/2 mile runs.

    The plugs all look good, i pulled the water logs and each exhaust port seems sooty black so it does not appear to be running lean on that side. Water was running through the log the whole time, as far as I can tell, and the motor was not overheating.

    FYI, only the bottom channel of these OT's are water cooled, this split happened at the top.
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  3. #2
    Senior Member Roaddogg 4040's Avatar
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    That looks like it might just be a poor casting.

    Steve

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    Senior Member jockorace's Avatar
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    Your last sentence should answer your question. That many long runs put continual heat blowing upward against that DRY bend location. Those pipes were never designed for that extreme use. JMO.....Jocko

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  6. #4
    Some guy obnoxious001's Avatar
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    The blown engine, depending on A/F ratio, is likely to be putting out exhaust at around 1200-1300 degrees. Guess what temperature aluminum melts at?

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    Old Timer LeE ss13's Avatar
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    Quote Originally Posted by obnoxious001 View Post
    The blown engine, depending on A/F ratio, is likely to be putting out exhaust at around 1200-1300 degrees. Guess what temperature aluminum melts at?
    x2
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    To add to what others have said, if only the bottom of the bend is cooled a problem like this was destined to happen.

    The hot gasses are hitting the top of the bend directly and only grazing the bottom of the bend. Furthermore, when the pipe was bent, the top wall thickness was decreased while the bottom would have been thickened a bit. My guess is that during a shut down, the boat decelerated fast enough to allow the water in the channel to backed up and provide a lot of cooling immediately. This would cause the already brittle aluminum to contract and the outside of the bend is always the weak spot.

    Aluminum is very fatigue sensitive and the thin spot on a bend is that much more sensitive to the stresses during normal use. Add to that the thermal stresses and it's not a question of if, but when this will happen.

  9. #7
    Some guy obnoxious001's Avatar
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    Quote Originally Posted by boatbroke View Post
    Furthermore, when the pipe was bent, the top wall thickness was decreased while the bottom would have been thickened a bit.
    Pretty sure all of those aluminum OT pipes were cast, not bent.

  10. #8
    Lookout I'm Crankin it Baby Rattle Can Lou's Avatar
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    Quote Originally Posted by obnoxious001 View Post
    Pretty sure all of those aluminum OT pipes were cast, not bent.
    Barry is right...and they probably won't do well on a healthy blower motor.

  11. #9
    Some guy obnoxious001's Avatar
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    Quote Originally Posted by obnoxious001 View Post
    The blown engine, depending on A/F ratio, is likely to be putting out exhaust at around 1200-1300 degrees. Guess what temperature aluminum melts at?
    Quote Originally Posted by Rattle Can Lou View Post
    Barry is right...and they probably won't do well on a healthy blower motor.
    I probably should have further qualified my post to include something about the tune up. Depending on mixture and timing, exhaust temperatures might be even higher than that. Consider what happens if you have a rich fuel mixture and/or retarded timing, and the fuel is still burning as it exits the engine?

    I once saw some guys talking about adding more fuel on a blown engine on the dyno, battling high pyrometer readings.

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