Exhausts comparison info wanted

Discussion in 'Sterndrives' started by Bill55AZ, Mar 9, 2007.

  1. Bill55AZ
    Joined: Aug 2006
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    Location: Peoria, AZ

    Bill55AZ Junior Member

    I have seen exhausts go thru the outdrive only, or thru the outdrive and then the prop, and thru the hull. Anyone have some good info on the pros and cons of the 3 ways. (I am purposely avoiding the 4th way, directly up from the motor and into the air. I am already deaf enough, thanks to time spent in close proximity to naval guns)
  2. stonebreaker
    Joined: May 2006
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    Location: Shiloh, IL

    stonebreaker Senior Member

    Through the outdrive is pretty restrictive - it can cost you 30-40 hp simply through backpressure on a smallblock V8. Even if you're not into peak performance, the extra hp you're using to push the exhaust through the prop are costing you fuel economy.
  3. redtech
    Joined: Feb 2007
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    Location: suger pine, ca

    redtech Senior Member

    if under 300hp running thru-hub exhaust is fine and going to be the quiettest of your options.
    above 300 ponies thru-transom is a better choice but save your hearing and those that you love from the noise and get captains call or silent choice
    as for loosing ponies thru-hub some but not much the diverging ring on the prop allows for a low pressure cone to develope behind the prop and this helps to pull exhaust from the motor
  4. tuantom
    Joined: Jan 2005
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    tuantom Senior Member

    With all other things remaining the same, I can speak from experience when I say that changing to thru-hull exhaust will not net any noticable performance on a small block (mine's a Ford 351w). It'll be a whole lot louder; but nothing notable in power. My guess is you'd have to make some changes on the intake side as well if you want to see real gains.
  5. Frosty

    Frosty Previous Member

    I had the experience of changing my exhaust.

    It was a failure and had to be removed. Although the noise was nice!!! I lost 300RPM off the top end.

    I also had an anoying problem of the coolant water running from the exhaust that was lowest, I mean if you went from one side to the other and the boat leaned only slighly the coolant water would change from exhaust to exhaust.

    This problem would not normally be noticed in a conventional sytem.
  6. Bill55AZ
    Joined: Aug 2006
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    Bill55AZ Junior Member

    Going to thru hull cost you RPMs? With cars, going to duals can move the limiting factors elsewhere, like cam or intake or carb. All are part of the breathing and they have to be matched to some degree. I suspect you needed richer jets in the carb, but I am guessing from a car engine point of view.
  7. Beech2000
    Joined: Jul 2006
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    Beech2000 Senior Member

    I gain 0 RPM nor loose any when switching captains choice Further, no notable change in power as well. That all might change with fresh 5.7 liter which is in final completion. and scheduled to be completed next week. (Hopefully that is).

  8. Zackman
    Joined: Dec 2006
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    Zackman Junior Member

    I have the 350 MAG with thru transom captains call exhaust. No difference in performance. I’ve heard that there will be no difference until over 400 Hp. It is great though, with the exhaust open I get a lot more attention and people seem to be much friendlier… A lot a people wave using one finger, always assumed it meant “I’m Number One”.
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