MAC.21 best pipe?

Discussion in 'Outboard Forum' started by Zach Davis, Jun 20, 2019.

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  1. Jun 20, 2019 #1

    Zach Davis

    Zach Davis

    Zach Davis

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    I have a MAC .21 That I have been running for years with a silver bullet pipe. It's on a lawless drive hanging off a Lynx. What do you think is the best pipe for this engine? Just feel like messing around with it.

    Thanks
     
  2. Jun 20, 2019 #2

    Mike Hughes

    Mike Hughes

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    stamped irwin
     
  3. Jun 20, 2019 #3

    Zach Davis

    Zach Davis

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    Thanks Mike
     
  4. Jun 20, 2019 #4

    LarryConrad

    LarryConrad

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    Silver Bullet HP, made my Mac 21 sing on my Leecraft and my Nemesis. Gadgetcraft carbon pipe is supposed to work well too. I ran an Irwin muffled for awhile, before the HP, never tried the stamper.
     
  5. Jun 20, 2019 #5

    topfuel443

    topfuel443

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    We had a MAC outboard that absolutely screamed with an Irwin Stamped pipe back in the day.
     
  6. Jun 20, 2019 #6

    Zach Davis

    Zach Davis

    Zach Davis

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    Thanks for the input James and Larry.

    The Silver bullet has been great. Its really easy to get on pipe. I originally had an older SPP pipe that was crazy fast but it was rare that it would really get on pipe. I run all my boats rich and the SPP needed all the conditions to be just right. I may try it again and see if I can dial it in. I would like to try a Gadget but they are pretty difficult to get. I will see what I can do about an Irwin Stamped.
     
  7. Jun 20, 2019 #7

    LarryConrad

    LarryConrad

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    When I raced my Mac it had to be blubbery rich on shore and nearly 4 cycling when it went in the water. After about a lap and a half or two laps in mill time it would heat up and clean out. If it wasn’t that rich when it hit the water, after the couple laps in mill time when it built some heat it would lean out. I bought a Gadget and a SPP RS pipe to test on my Mac outboards, just haven’t got around to it yet. I’ll have to try a standard silver bullet, and a stamper too. I tried the west coast air cooled setup, and went back to restricted water cooling. I couldn’t get any consistency with the air cooled setup.
     
  8. Jun 20, 2019 #8

    topfuel443

    topfuel443

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    That sounds about like the MAC we ran, when it was on, it was ON. But it had to be darn near perfect.
     
  9. Jun 21, 2019 #9

    Zach Davis

    Zach Davis

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    When you guys were running the stamped Irwin were you reducing the stinger down or running them stock? If so what size copper tube were you using?
     
  10. Jun 21, 2019 #10

    julianconde

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    I have one I’ll Take the measurements and
    Let you know. The best pipe when I ran the Mac 21 outboard was the silver bullet HP pipe. The Irwin stamped pipe was another great option. The pitched up X438 Was also a very very good prop for that motor and very competitive speed wise
     

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  11. Jun 21, 2019 #11

    jaxracer

    jaxracer

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    Issue with the MAC is the spraybar size in the carb. Swap carbs to a CMB or drill it out and a lot of your consistency problems will go away.

    I won the most recent SOWEGA Tunnel Champs winner take all night race with a MAC and SPP NR pipe combo on a Lynx. On and off pipe with ease and zero issues. Change the carb and I think you’ll notice a big difference. I’m running a smaller Geraghty on mine with a Lynx cut x442.

    -Mitch Jr
     
  12. Jun 21, 2019 #12

    Zach Davis

    Zach Davis

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    Thanks Mitch Jr.
    The engine was originally done by Rod Geraghty and I am not sure what he would have done with the carb. It has actually been pretty consistent with the silver bullet pipe I would just like to see it closer to the RPM and speed that it was with the spp pipe when it was on. That said I do have a Geraghty carb on a CMB that I am not running. I will see if it will fit the MAC and give it a try. Thanks
     
  13. Jun 21, 2019 #13

    jaxracer

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    Yeah the CMB, MAC and Nova Rossi all have the same size carb holes so they should all be interchangeable. But sounds good man, keep us posted how she runs. If you have issues getting the SPP consistently on pipe after that, just keep pulling it out an 1/8th of an inch at a time until it lights off when you want it to. I had to run mine a hair longer than I normally would say a Silver Bullet.
     
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  14. Jun 24, 2019 #14

    LarryConrad

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    ADF8075D-7E16-4D37-84B4-077156B5135B.jpeg When Mr. Geraghty prepped my Mac power heads, he made what internal adjustments he deemed necessary for reliability and consistent performance. He machined the inlet bells and changed the spray bars when he flowed the carbs, and restricted the water cooling at the water jacket.
     
  15. Jun 25, 2019 #15

    Zach Davis

    Zach Davis

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    Thanks for the pictures Larry.

    My carbs look the same so I have to believe they received the same treatment. I will mess with a couple of pipes and then try my Geraghty carb to see if I can find some more R's. I am probably do for a new prop as well.
     
  16. Jun 25, 2019 #16

    LarryConrad

    LarryConrad

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    How much fuel have you run through yours? The engine in the photo with the Silver Bullet HP took just over a gallon of fuel through it before it started to really sing, I raced it with a back cut 1440 on my XT 120 Leecraft, on my Nemesis it was fastest through the turns with a x442. The other Mac in the photo is my backup for the one with the pipe.it has been heat cycled but never in the water. The piped engine is still tight and fast after over 6 gallons of fuel and several bearing changes. When my Vision became my primary B tunnel in 2010 I dialed it in with a Cooper piped Novarossi based engine with one of Mark Leyde’s conversion kits in it. That combination worked so well for me, all my other pipes and engines stayed packed away...
     

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