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  1. #1
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    Default will a squarebore to spreadbore adapter plate effect performance?

    The manifold i got for my vortec engine is a dualplane squarebore carb setup. I want to use my quadrajet for the time being and that's a spreadbore. I have a 1 inch thick adapter plate that will allow me to mount the quadrajet. Will this cause any performance issues?
    1986 searay 268 weekender "lunasea" (current)
    1986 grady white 24 offshore with twin 150 yamaha's(current)
    1979 grady white 201 marlin (sold)
    1991 seapro centra 190
    http://picasaweb.google.com/lime4x4/Lunasea#
    https://picasaweb.google.com/lime4x4...hite201Marlin#

  2. #2
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    90% of older chev small block and V6 manifolds are already poo by design.

    Here's a simple way to check if it's going to effect performance:

    Does the manifold/adapter plate combination have a smaller opening than the 4 barrels on the carb?

    So long as you're not introducing a restriction, it should be okay.

    In fact, in a lot of cases, introducing a spacer can actually help performance.

  3. #3
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by lime4x4 View Post
    The manifold i got for my vortec engine is a dualplane squarebore carb setup. I want to use my quadrajet for the time being and that's a spreadbore. I have a 1 inch thick adapter plate that will allow me to mount the quadrajet. Will this cause any performance issues?
    as long as it's not an open spacer you'll be fine, will probably have to adjust idle a/f mixture slightly

  4. #4
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    1986 searay 268 weekender "lunasea" (current)
    1986 grady white 24 offshore with twin 150 yamaha's(current)
    1979 grady white 201 marlin (sold)
    1991 seapro centra 190
    http://picasaweb.google.com/lime4x4/Lunasea#
    https://picasaweb.google.com/lime4x4...hite201Marlin#

  5. #5
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by lime4x4 View Post
    dont use that one, the engine will idle like **** and run like **** anything under 3/4 throttle, get a 4-hole spacer and the proper gaskets.

  6. #6
    Captain MikDee's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by Rat Capri View Post
    dont use that one, the engine will idle like **** and run like **** anything under 3/4 throttle, get a 4-hole spacer and the proper gaskets.
    I don't see anything wrong with that spacer? Why do you think they even make it, if it's gonna be a problem? Plus, usually an increase in plenum height is advantageous, because it gives a longer runner to the manifold, usually increasing torque.
    89'-19' Bayliner Capri Ski boat, with a 125hp Force
    My Previous Boats http://smg.photobucket.com/albums/v2...ee/My%20Boats/
    My motto - Keep It Simple Stupid
    Or It Can Get Real Expensive Later!

  7. #7
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    don't know about that one but i used a 4 holer and then took a die grinder to the intake so the holes funnel smoothly.
    don't remember what brand as it's been some time now.
    different engine and manifold, 390 amc and offy 360 dual port.
    crane cam and lifters plus doug thorley headers
    built to be a mid range engine and runs i think great.
    1968 amx with only 53k miles on the o.d.
    sets in my sons garage now and is his.
    when he was in iraq he asked me how the amx was doing.
    i asked him if he wanted it, quick reply h*** yes!
    then i told him all you have to do is come home alive and it's yours.
    i think i got a bargan.
    opps, sorry but that's how i matched a spread bore double pumper.

  8. #8
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Cool car, Mark Donahue come to mind. that spreadbore adapter will just fine you probably wouldn't feel the difference.

  9. #9
    Moderator Bondo's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    The manifold i got for my vortec engine is a dualplane squarebore carb setup.
    Ayuh,... I agree,... Why would you want to put an Open plenum over a Dual Plane manifold,..??
    Any Grease is Better,..... Than No Grease at All.......

  10. #10
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    it's not going to stay that way.. In a month or so i want to replace the quadrajet with an Edelbrock 1409 in a month or so
    1986 searay 268 weekender "lunasea" (current)
    1986 grady white 24 offshore with twin 150 yamaha's(current)
    1979 grady white 201 marlin (sold)
    1991 seapro centra 190
    http://picasaweb.google.com/lime4x4/Lunasea#
    https://picasaweb.google.com/lime4x4...hite201Marlin#

  11. #11
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by MikDee View Post
    I don't see anything wrong with that spacer? Why do you think they even make it, if it's gonna be a problem?
    it's not a problem if yhe engine stays above 4500 rpms all the time

    Quote Originally Posted by MikDee View Post
    Plus, usually an increase in plenum height is advantageous, because it gives a longer runner to the manifold, usually increasing torque.
    that's only true if velocity increases or at least stays the same, that's not gonna happen with that open hole spacer......should only use a spacer for the manifold type you're using

  12. #12
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    so i should get this type of spacer then??
    http://cgi.ebay.com/ebaymotors/EDELB...Q5fAccessories
    1986 searay 268 weekender "lunasea" (current)
    1986 grady white 24 offshore with twin 150 yamaha's(current)
    1979 grady white 201 marlin (sold)
    1991 seapro centra 190
    http://picasaweb.google.com/lime4x4/Lunasea#
    https://picasaweb.google.com/lime4x4...hite201Marlin#

  13. #13
    Captain MikDee's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Personally, I don't think it matters one way or the other
    89'-19' Bayliner Capri Ski boat, with a 125hp Force
    My Previous Boats http://smg.photobucket.com/albums/v2...ee/My%20Boats/
    My motto - Keep It Simple Stupid
    Or It Can Get Real Expensive Later!

  14. #14
    Lieutenant Commander mkast's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    From Dennis Moore:

    CARBURETOR TO MANIFOLD ADAPTERS
    When General Motors' Rochester Division decided to quit making the
    Quadrajet carburetor, Mercruiser switched to the Weber carburetor.
    Unfortunately the Quadrajet four barrel is a spread bore design
    carburetor and the Weber is a square bore. All Chevrolet marine engines
    with standard performance intake manifolds have the Quadrajet spread
    bore carburetor mounting flange. An adapter is used by Mercruiser to
    mount the square bore Weber carburetor on the spread bore manifold. This is not an ideal situation, to say the least, for maximum performance. The Rochester Quadrajet is one of the finest carburetors ever built It has
    been discontinued because the entire automotive industry has converted to
    fuel injection. Purchasing a used Quadrajet marine carburetor from a
    Boat dealer and installing it in place of the Weber,and doing away with the
    adapter,would be an excellent idea. The Weber four-barrel is also an
    excellent carburetor. Installing a square bore intake manifold while
    retaining the original Weber carburetor would be another good
    option in order to do away with the adapter. Either way,getting rid of the
    adapter should be one of your first steps for increased engine
    performance.

  15. #15
    Moderator Bondo's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by lime4x4 View Post
    so i should get this type of spacer then??
    http://cgi.ebay.com/ebaymotors/EDELB...Q5fAccessories
    Ayuh,.... Actually,.. This would be the Best adapter to use,...


    You're gonna change it in a month or so,...
    Break out the plastic,+ do it Right the 1st time,.....
    Any Grease is Better,..... Than No Grease at All.......

  16. #16
    Commander bifflefan's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    for all the nay sayers,
    When you put on a spacer of any kind you are increesing the plemtem space and that is always a good thing. thats why in racing they limit you to a 1 inch spacer. as for the open spacer to the duel plane intake it does change the flow abit but you wont see any problem with it. these have been around for many many years and have done fine. now if you have a racing motor and are trying get the very last single HP out if it then things are different and i can write about that all day long, and well into next week. as i havent read anything about it being more than a stock engine (meaning it doesnt make more than 50 more hp than the factory model) then you will have no problems at all with that adapter. if fact it will run better in the top end, without a noticable gain or loss at off idle.
    Hope this helps. i guess 25+ years of racing didnt hurt after all.

  17. #17
    Lieutenant Commander mkast's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by bifflefan View Post
    i guess 25+ years of racing didnt hurt after all.
    Just curious, was this in boats?

  18. #18
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    this post could go on forever with opposing arguments. i would think you would need dyno data to backup any of these arguments....kc

  19. #19
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    See, this is why, when I used my truck vortec motor, I did this:
    Attached Images Attached Images

  20. #20
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Bondo i don't do PLASTIC
    1986 searay 268 weekender "lunasea" (current)
    1986 grady white 24 offshore with twin 150 yamaha's(current)
    1979 grady white 201 marlin (sold)
    1991 seapro centra 190
    http://picasaweb.google.com/lime4x4/Lunasea#
    https://picasaweb.google.com/lime4x4...hite201Marlin#

  21. #21
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by Bubba1235 View Post
    Sorry but no. You have to remember this is not a race car, it has no transmisison and you can't bring the revs up and drop the clutch. A Marine engine has to develope a ton of torque from a dead stop. When you drop the hammer and go WOT the velocity of the fuel air mixture drops DRASTICALLY. The engine has almost no vacumm at that point. Unless the intake manifold, any spacers, and the carb are all matched and setup properly you will have nothing but trouble coming out of the hole.
    isnt that the same thing a drag car does, say with a power glide two speed auto trans?

  22. #22
    Commander bifflefan's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Quote Originally Posted by Bubba1235 View Post
    Sorry but no. You have to remember this is not a race car, it has no transmisison and you can't bring the revs up and drop the clutch. A Marine engine has to develope a ton of torque from a dead stop. When you drop the hammer and go WOT the velocity of the fuel air mixture drops DRASTICALLY. The engine has almost no vacumm at that point. Unless the intake manifold, any spacers, and the carb are all matched and setup properly you will have nothing but trouble coming out of the hole.
    Not to try to argue with you or anyone for that fact, but a dyno. has no trans eather and thats where we all the testing. as a matter of fact it has a "water brake". and all the data is read by computer from idle, WOT at 1000 rpm to WOT at 7500 rpm.
    as another poster said this could go on for ever with no difinite results.

    No i raced cars.

    Just put the spacer on. its not going to give you a noticable drop or gain. then when you get the money for the right one, you have your own results for a comparison and can post them.

    I can say one thing for sure, that a Holly of any kind (marine or other) will give a marker performance gain of the that Edelbrock P.O.S.
    that is a Carter revamp from the 60's and there is a reason that they are out of business.

  23. #23
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    If I understood the op's question. He was asking if he could do it temporarily and not have problems - not whether it would make an incremental difference in performance. It won't be a problem.

    I also think that we make a lot of blanket statements about what is needed in a boat engine, etc, etc, etc... without fully considering the situation (ie need tons of torque down low, etc...). My current small block engine has a single plane intake, 200 cc runner heads and is propped for top end. I can pop a heavy slalom skier or boarder out of the water with under half throttle no problem... even with an adapter plate in place between the pos edelborck carb that's been on it most recently and the square bore manifold I've had on it... If there's anything that I've learned from the conversations with the guys in the machine shop I use - Some conventional wisdom, may not always be spot on.

  24. #24
    Captain MikDee's Avatar
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Too true, beating a "dead horse" here!
    Just do it!
    89'-19' Bayliner Capri Ski boat, with a 125hp Force
    My Previous Boats http://smg.photobucket.com/albums/v2...ee/My%20Boats/
    My motto - Keep It Simple Stupid
    Or It Can Get Real Expensive Later!

  25. #25
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    Default Re: will a squarebore to spreadbore adapter plate effect performance?

    Well i dropped the 350 vortec engine.Using the adapter. Idles nice and smooth has good transition of power .Boat use to take 1 min 20 seconds to plane tonight it came onto plane in 31 seconds. I'm not cing and ill effects from using the adapter plate
    1986 searay 268 weekender "lunasea" (current)
    1986 grady white 24 offshore with twin 150 yamaha's(current)
    1979 grady white 201 marlin (sold)
    1991 seapro centra 190
    http://picasaweb.google.com/lime4x4/Lunasea#
    https://picasaweb.google.com/lime4x4...hite201Marlin#

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