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  1. #1
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    Drilled Exhaust - Real World Results and Weigh-In On Benefits

    Guys, I want to get some real-world answers about "drilled exhaust". I've seen this done for years and while I know that noise is one downside result, is there a real benefit? I have a 20" Mercury 2.0L V6 that I'm building for myself and a later project. If I'm going to drill the exhaust, I need to do it now before I start painting.

    My gut says Yes - I say this because my 2.5L Mercury V6 ProMax SS (short shaft) is the best running/performing outboard that I've ever ran. I've owned this ProMax SS for 20 years now and it's acceleration characteristics are unlike any other outboard I've ran. The transition from idle thru hole-shot has no hesitation. There is no blubbering, no hiccups, no delay that many outboards experience when "clearing" the water from the exhaust cavity. Since the SS has a huge, open hole that vents the exhaust above the cavitation plate and does not require all of it out through the gearcase and the thru-hub system, the engine is free to spin up from idle.

    I know the openings wouldn't help WOT operation, but my thoughts are that they can't hurt idle and should aid (as do the reliefs) and they will help to build rpm quicker to clear the exhaust cavity when transitioning from deep in the water to on-plane.

    OK, if the answer is "Go", I'll place 6 1/2" holes in the outer leg (3 on each side), but where do I drill the inner exhaust liner (shown below)? Thanks, Gordon

    Click image for larger version. 

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    Last edited by Gordon02; 06-20-2019 at 10:59 PM.

  2. #2
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    Interesting question, hope to see a nice pro/con debate!
    On my looper I experience a better off-idle accel the more exh I relieve above the gearcase, despite the fact that the prop vent holes has less and less effect the more exh is relieved...
    Last edited by Captain75; 06-21-2019 at 03:57 AM.

  3. #3
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    Quote Originally Posted by Gordon02 View Post
    but where do I drill the inner exhaust liner (shown below)? Thanks, Gordon

    Click image for larger version. 

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    you don't..

    you throw tha liner ona shelf or in tha trash...

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  5. #4
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    Are you doing any other work/mods to motor?


    Michael
    Last edited by mrichartz; 06-21-2019 at 05:38 AM.
    '84 Checkmate Convincor
    Twin v6 closed deck Evinrude loopers Ported,2.7 heads,cooling mods
    milled finger ported to match vertex pistons,
    All casting slag removed
    Sleeves in opposite so no idle reliefs
    Offshore exhaust mids
    148 psi comp,balanced pistons and rods,Stuffed, 106 pullovers
    CDI 8500 digital ignitions ..Boyesen reeds..And more....

  6. #5
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    Post #3 is my suggestion , while powerhead is off you have perfect opportunity to drill 5 1/2” holes at the base of your bass midsection like Mercury sent all the 260 bass mods in the early 90’s , any gains ? It would be minimum IMO but LOTS louder that once done irreversible... Think it out ...

    Jay
    Jay @ JSRE


  7. #6
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    A lot depends on motor and it’s characteristics. I had a cust call me this morning regarding a Yamaha I just built and modified mid to relieve exhaust. He wanted to know if we could relieve more, because he noticed while taking off and the exhaust reliefs cleared the water the motor would instantly spool up, so in this application it apparently is helping. I’ve also noticed on modified carb 2.5’s, the more exhaust I relieve above water line, the better they idle without loading/puddling fuel. In long idle zones you’d have to periodically bump up accelerator to keep it from loading, much better to all but gone by allowing more exhaust out above water line.

  8. #7
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    Remove the liner but keep it. Drill from the top down like the 260s. All you have to do to reverse it is re-install the liner.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

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  10. #8
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    Quote Originally Posted by rock View Post
    Remove the liner but keep it. Drill from the top down like the 260s. All you have to do to reverse it is re-install the liner.

    Rock
    But unless you have done major mods to the 2.0L doubt you will see any difference other than more noise.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

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  12. #9
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    I will agree with above comments, my motor does idle much better with it all opened up. (260 with offshore mid, a drag racer cut of the end, and drilled holes every where) Real increased performance numbers will probably not be quantifiable in my opinion, unless other mods are made to take advantage of added breathing in the mid and top range. If it makes you feel better that the motor idles better, and you like the sound, then I could see how it would be fun, but that is about it. Most of my friends think I am stupid for taking the little step in performance, but I kinda like it
    Whitney Woolard
    Oregon/Washington
    '96 Allison XB2003 w/Mercury 2.5 260ss

  13. #10
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    Guys, you've confirmed It's On! I'll get that 20" stock housing drilled over the weekend.

    Thanks for the inputs and advice. I'm not after the noise, but don't mind it since I've gotten used to my 15" ProMax's 4" hole under the housing (I've had that little baffle plate removed for 20 years too). I give that above-water relief the full credit for how quickly this motor spools-up on my Allison. Never a hiccup, cough, or pause - it responds instantly and always immediate. Gordon

  14. #11
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    Earlier you mentioned 3 holes each side. Like Jay said the 260s had 5 each side and Mercury felt it was a good idea.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

  15. #12
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    Its not a Merc, but this is my solution on my 3 litre OMC's. 3" holesaw, thru the mid and liner.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  17. #13
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    Quote Originally Posted by 58Evinrude View Post
    Its not a Merc, but this is my solution on my 3 litre OMC's. 3" holesaw, thru the mid and liner.
    What year is that motor?
    '84 Checkmate Convincor
    Twin v6 closed deck Evinrude loopers Ported,2.7 heads,cooling mods
    milled finger ported to match vertex pistons,
    All casting slag removed
    Sleeves in opposite so no idle reliefs
    Offshore exhaust mids
    148 psi comp,balanced pistons and rods,Stuffed, 106 pullovers
    CDI 8500 digital ignitions ..Boyesen reeds..And more....

  18. #14
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    Quote Originally Posted by Gordon02 View Post
    Guys, you've confirmed It's On! I'll get that 20" stock housing drilled over the weekend.

    Thanks for the inputs and advice. I'm not after the noise, but don't mind it since I've gotten used to my 15" ProMax's 4" hole under the housing (I've had that little baffle plate removed for 20 years too). I give that above-water relief the full credit for how quickly this motor spools-up on my Allison. Never a hiccup, cough, or pause - it responds instantly and always immediate. Gordon
    So are/did you do any other work/mods to motor?
    '84 Checkmate Convincor
    Twin v6 closed deck Evinrude loopers Ported,2.7 heads,cooling mods
    milled finger ported to match vertex pistons,
    All casting slag removed
    Sleeves in opposite so no idle reliefs
    Offshore exhaust mids
    148 psi comp,balanced pistons and rods,Stuffed, 106 pullovers
    CDI 8500 digital ignitions ..Boyesen reeds..And more....

  19. #15
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    Quote Originally Posted by 58Evinrude View Post
    Its not a Merc, but this is my solution on my 3 litre OMC's. 3" holesaw, thru the mid and liner.

    Drill holes in a Merc mid like that it will crack, there is a reason the Merc engineer's did the 5 half inch holes each side where they did.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

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