cylinder head swap
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cylinder head swap

  1. #1
    Senior Member motor head's Avatar
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    Default cylinder head swap

    Have any of you ever changed nothing but heads or heads and cam on a motor and get an increase In rpm with the same impeller? In a jet boat.
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    Default Great Question

    More info is needed.... a change in the heads would change the complete set up from compression to the amount of air the engine will pump. My guess is that peeps will need to know specifics as to what heads and cam you currently have to what you are thinking about changing to.

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    Senior Member motor head's Avatar
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    Yes I know all that comes Into play. I guess what I'm saying is will more cfm (only) which is more hp, turn the same impeller more rpm. And I'm talkin from 320cfm to 360cfm
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    Quote Originally Posted by motor head View Post
    Yes I know all that comes Into play. I guess what I'm saying is will more cfm (only) which is more hp, turn the same impeller more rpm. And I'm talkin from 320cfm to 360cfm
    I thought I was going to be a wise guy and put aluminum heads/ roller rockers and new intake and carb on my stock 7.4l in the Baja. Engine lasted about 15 minutes. Rebuilt it with new cam and it lasted another 7 minutes.

    Then took it to Moritz and they fixed me up.

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    Senior Member motor head's Avatar
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    I will use same cam and everything I'm running now. It's plenty big! My heads just suck! They flow 298 cfm @25" so whatever that is @28" that everything is advertised at. But if I'm going to have to put a smaller impeller In and turn more rpm to really utilize the hp gain then I'll keep it as is for now.
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    gn7
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    Why does everybody assume that more CFM equals more power. Most, but not all, have learned in time that a 1250 dominator will not make more power on a stock 350 over a 750DP. But bring up the topic of heads and CFM automatically means more power.
    If you said more CFM from a port the same size, or the same port buy just cleaned up, I would be more inclined to say yes, there would be a gain. But a larger port to get the extra CFM, more than likely NO. You could possibly lose, specially below redline. Without the addition of a matched cam, the extra port volume is definitely a loser.
    Like Condor posted, not enough info.



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  9. #7
    Senior Member motor head's Avatar
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    Quote Originally Posted by gn7 View Post
    Why does everybody assume that more CFM equals more power. Most, but not all, have learned in time that a 1250 dominator will not make more power on a stock 350 over a 750DP. But bring up the topic of heads and CFM automatically means more power.
    If you said more CFM from a port the same size, or the same port buy just cleaned up, I would be more inclined to say yes, there would be a gain. But a larger port to get the extra CFM, more than likely NO. You could possibly lose, specially below redline. Without the addition of a matched cam, the extra port volume is definitely a loser.
    Like Condor posted, not enough info.
    Ok I get all that. I'm talking about swapping my ported pro crap heads for some trickflow 290 street strips. The port should be close to the same size. This is on a 557ci. Mota

    What is 298cfm @25" be @28"?
    [SIGPIC]

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    Quote Originally Posted by motor head View Post
    Have any of you ever changed nothing but heads or heads and cam on a motor and get an increase In rpm with the same impeller? In a jet boat.
    Short answer is yes.

    Long answer is more involved and varies. See GN7's post above.

    The best gain to be had is in improving the single most limiting factor. Once everything is optimized for your application, you're left with increasing displacement or supercharging.

    Are your heads your most limiting factor? Really not enough info to tell at this point.

  11. #9
    Senior Member motor head's Avatar
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    557ci, 1050 Dom, 10.5:1comp, pro comp heads. I understand motors just maybe not as well as y'all....I just don't totally understand jet pumps yet lol. If you go by an impeller chart if you pickup more hp @ more rpm then you should turn more with the same impeller but iv learned the hard way that torque is the biggest factor.
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    gn7
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    Quote Originally Posted by motor head View Post
    Ok I get all that. I'm talking about swapping my ported pro crap heads for some trickflow 290 street strips. The port should be close to the same size. This is on a 557ci. Mota

    What is 298cfm @25" be @28"?
    Is this BBF?



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  13. #11
    Senior Member motor head's Avatar
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    When I hurt my 472ci motor all I changed was stroke and cam picked up 500rpm. Same compression even.
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    Senior Member motor head's Avatar
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    Quote Originally Posted by gn7 View Post
    Quote Originally Posted by motor head View Post
    Ok I get all that. I'm talking about swapping my ported pro crap heads for some trickflow 290 street strips. The port should be close to the same size. This is on a 557ci. Mota

    What is 298cfm @25" be @28"?
    Is this BBF?

    Yes sir
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  15. #13
    gn7
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    Quote Originally Posted by motor head View Post
    What is 298cfm @25" be @28"?
    This is the multipler chart.


    So a port that port that flows [email protected] 25" would flow 315



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  16. #14
    Senior Member motor head's Avatar
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    Quote Originally Posted by gn7 View Post
    Quote Originally Posted by motor head View Post
    What is 298cfm @25" be @28"?
    This is the multipler chart.


    So a port that port that flows [email protected] 25" would flow 315
    Lol!! See THEY SUCK!!!!! and they are on a 557!
    [SIGPIC]

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