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  1. #1
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    Outboard Exhaust Tuning - Who Did It Best?

    I've been wanting to start this thread for a while now. A recent post in a Mercury V6 History thread on the origins of the 2 x 4 inline Mercury motivated me.

    I've owned 40+ outboards and am currently restoring two Chrysler 72 CI 3 cylinder outboards. I'm also building my own 2.0L V6 Mercury project and the ProMax SS on my Allison that I've owned for 20 years is just flawless. Having said all this, my absolute favorite outboard of all time is the 3 cylinder 70 horse Johnson - I've owned two - one in 1980 and one now, both are just pleasant motors.

    The 49 CI 3 cylinder Johnson/Evinrude series used a Tuned Exhaust like you see below. I've always preferred 3's and 6's simply because the engineers normally build the crankshafts to fire 120 degrees apart thus giving the exhaust time/room to evacuate in even pulses. The 3 cylinder OMC's took it even further - they have cast channels in the exhaust that created "equal-length" tubes. These and the old "bubble backs" used in the hotter OMC V-4's like the 135/140's are designed to make the length from port to the exhaust plate equal. I have not seen these used in the Mercury in-lines nor the V6's. I never understood OMC's use of the heavy cross-flow pistons in the V-4's while at the same time designing elaborate exhaust bubble back manifolds. While not a V-4 fan, my two V-4's screamed - all 99 cubic inches of them.

    Not including racing engines, what other production makes/models have creative performance designs that make them somewhat remarkable? There are enough engineers and builders here to know which outboards had something special - I'd love to hear about them! Thanks, Gordon

    Click image for larger version. 

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    Last edited by Gordon02; 08-20-2019 at 11:12 PM.

  2. #2
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    the 3 cyl omc works great for production work under 6k but when you go over it seems the log style makes more power or is it just a function of getting the exhaust out?

  3. #3
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    I can't one eye open, you'll enjoy the F5 and 72° spacing with its manifold. The independent tuned stacker pipes CHRYSLER. The triples as above or V6. Regular old L4 chest too. Port to port timing is key, the crank pin spacing and distance between, in all. Tomorrow

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    F5 72* pin spacing , inline 5 cyl. Manifold linking tuning pairs port to port with sufficient length for proper arrival time prior to closing. Low duration exhaust and transfers to match. Ex height from 103.7 to 102.5* from TDC depending on year bore size. Low rpm tractor which did 150 ISO prop and 170BIA crank.
    Attached Thumbnails Attached Thumbnails US5134851-3.png   US5134851-4.png   12345.1.jpg   F5 150 -- 150XR-2.jpg   F5 150 - 150XR-2.jpg  


  5. #5
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    Quote Originally Posted by Gordon02 View Post

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    Honestly, there is no "best". The best you can do is weed out the worst first. Tell me how many "engineering blunders" can you point out on this little jewel .. ???

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    Chaz, the 70 OMC is "my" favorite. My family and friends learned to ski and grew-up being pulled by the 1980 Johnson 70 that I bought brand new. When I think about the thousands of pulls that little 49 CI outboard made effortlessly while teaching my kids to ski and kneeboard, I'll always be impressed. I'm not qualified to point out problems with the manifolds.

    I'm hoping to hear about innovations and cool engineering designs that contributed to performance from the 100 years of 2-stroke outboard history. As said, my Mercury's split exhaust chest doesn't do anything to even the length of travel.

    Here is one question for you or others - A friend's Dad back in the late 70's had an old 16 foot Bomber bassboat fitted with a 3 cylinder Chrysler 85 horse outboard. That boat out-performed everything equally powered that pulled up to it. It would almost clear the water on holeshots and still run in the mid 40's. The Bomber was a heavy boat with no pad and somehow that ugly, ancient designed cross-flow 3 cylinder would pull so hard coming out that you could see most of the hull completely out of the water. That motor had something that none of my V-4's (90 and 115) had. That memory is why I've got 2 restorations going on for my own collection. Gordon

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    With the triple 85, even though open chest and not a manifold the pairing signals are doing their job. The 120° crank spacing and the distance port to port of pairs with the ex port duration and transfers timed to complement for good low rpm flow as bonus. This combination allows for port to port signal arrival to be in optimum range much lower in the rpm range raising dynamic pressure and cylinder charge quality and maintain the range to high rpm. The 72ci displacement also helps as does CHRYSLERS superior x flow design.

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    Quote Originally Posted by Chaz View Post
    Honestly, there is no "best". The best you can do is weed out the worst first. Tell me how many "engineering blunders" can you point out on this little jewel .. ???
    Dont forget omc made 3 cyl race engines even the 31m custom and it had log style like a mod 50 and nothing like a production engine less the very first 55hp which is similar

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    Quote Originally Posted by powerabout View Post
    Dont forget omc made 3 cyl race engines even the 31m custom and it had log style like a mod 50 and nothing like a production engine less the very first 55hp which is similar
    How about the triple Stinger exhaust, any pics?

    The transition from ex port to manifold passage is where gains are made. The manifold itself isn't chosen for only purpose of spent gases having a way out, it's to improve the pairing and scavenging signals timing for the small 49 ci consumer motor. The manifold likely enable more ex duration to helping the little motor on the high end.
    Last edited by FMP; 08-22-2019 at 09:26 AM.

  10. #10
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    Ahh yes, I visited the school where I attended grades 1 thru 5 . I remember the flag pole that had to be 100' tall. The gym that was maybe half of that. The outdoor basketball hoops that took all I had to make a double underhand shot.
    In reality, it's now deep in the hood, a tagged up ****hole. The flag pole is 20' at best and I would have to duck my head to clear the nets, if there were any still hanging.
    I wish now I would have left my euphoric childhood memories as they were ..

    Here is one of West Bend's finest "George Jetson" three cylinders .. It reminds me of the exhaust log above. Two units tied together haphazardly, with a third left hung out all by itself.

    I get it that you guys like the old Westie / Chizzler stuff and want to keep them going. But as far as the best ... they tanked twice for a reason



  11. #11
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    Chazzzz it really doesn't matter which engine design we try to discuss, Merc v6, inline twister 2x , simple triples , twins , stackers or the guys who had their names on patent papers working OMC , Merc or wherever you always manage to try and degrade the accomplished results. Why is that?
    Gord wanted to open a conversation on exhaust, who did it best. Let's cut to the end, Bombardier and the new Etec and G2 with or without power valves, how much do think you compare to all the minds that built these engines and the before mentioned. You're much smarter than all.

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  13. #12
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    Frankie , If your wanting to capture the days of your youth. Then say your restoring an antique.

    But all this : as we roared against the mighty river our Chrysler 85 took us to breathtaking velocities achievable by no other inferior product. The perfect harmony of intake and exhaust pulses ...

    Then ya hang a picture of an exterior log. Top guy looks to start out pretty good, nice oval-shaped port. Then does what pilots call a wing-over and makes a sharp 90* over itself.
    Whats with the second hole .. did they forget how to make an oval ?
    Maybe they thought air likes sharp corners or likes to run into walls and change directions . BTW .. the exit is down, no lets run it up into a wall .. it'll find it's way out eventually.
    The bottom, hey .. they can repeat on oval. I see they tried to even the length between the top and bottom "runner" . But all that squirrel cage does is add a bunch of kinked up turns darting in and out between an odd-looking bolt arrangement.
    But now that we made it past all that, lets leave the cumulative cross-section of the exit no bigger than when it started. What should we do with it .. I know, make another sharp 90* and put it back into the bottom of the block or mid where it's going to have to 90* again to face down ...

    Now tell me where I'm wrong on my brief overview .. without even needing to look at the picture again ..

    The difference between you and me .. you'll settle for what they sell you. And when you "port it" you'll knock some of the casting flash off.
    Me, I'll tie it in a mill and saw the back half off , put the ports where they need to be and then weld the whole thing back together, machine it up and run it. If it's a slug, I got no problem saying so . But if it's not, better put your raincoat on .. the roost is heavy

  14. #13
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    And remember .. Any idiot can make something over complicated.

    It takes a genius to make it simple ...

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    You just keep on thinking like that because four-strokes you understand.

    Two units tied together with one left hangin on its own. You must be kidding.
    Last edited by FMP; 08-22-2019 at 05:28 PM.

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    31m exhaust
    Attached Thumbnails Attached Thumbnails 31 M exhaust cover off.jpg  

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