Compression Ratio Question on BBC
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Compression Ratio Question on BBC

  1. #1
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    Default Compression Ratio Question on BBC

    How would I determine the compression ratio on a 454 BBC with flat top pistons and 97cc closed chamber heads? And what would it drop too with 110cc open chamber heads?

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    "It's HONDO, honey" Kyle's Avatar
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    To say the least it's above 8 and yes it will drop do you have the casting numbers on the heads I can look it up for you that way?
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    Currently running a set of oval port 97cc closed chamber heads # 3856206 and am maxing out at 4800 RPM's. Thinking about going to a set of # 336781 or 353049 heads. Is there much to be gained by jumping to the open chamber heads? Or better to just have some head work done on my current set of heads? Looking to gain some more top end RPM's currently running 71 at 4800 RPM's and that’s all she's got. Just a side note, the boat jumps out of the water and hits full speed in a heart beat, but stops at 4800 RPM's like its hitting a wall.

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    mo balls than $cents$ IMPATIENT 1's Avatar
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    open chambers have better flame travel than your heads but if i had to guess, doin the swap would kill the performance you have now with the closed chambered heads, just imo

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    jetboataholic HawaiianJet's Avatar
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    What size impeller do you have in the jet?
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    (A) impeller

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    The open chamber heads will breathe better than the closed chamber heads, but, the about 1 point compression you will loose (my off the cuff guess) will counter that.

    That said, if you do a bit of porting (bowl clean-up and blending) along with a set of large valves (and relieving the chamber arround the intake valve), you get even BETTER flow (more, WITH VELOCITY, not just "more, from huge port rectangle port heads), and CAN come out on top.

    I run Stage 1 ported '049's on my flat-topped 454 and pull an Aggressor B (comparable to your Berk A) to 5,400RPM, with awesome throttle response.

    I MIGHT have 8.3:1 compression, if I get lucky.
    I was pulling the same impeller 5,000 flat before the work I had done, no other changes.

    Wish I had 10:1. LOL
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    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by bahner454 View Post
    Currently running a set of oval port 97cc closed chamber heads # 3856206 and am maxing out at 4800 RPM's. Thinking about going to a set of # 336781 or 353049 heads. Is there much to be gained by jumping to the open chamber heads? Or better to just have some head work done on my current set of heads? Looking to gain some more top end RPM's currently running 71 at 4800 RPM's and that’s all she's got. Just a side note, the boat jumps out of the water and hits full speed in a heart beat, but stops at 4800 RPM's like its hitting a wall.

    The forward speed of the boat has absolutely no effect on RPMs. You could tie your boat to a dock and stab the throttle to the floor and it will still swing to 4800 RPMs. Berkeley has done the test to prove that. All the engine is doing is turning a water pump and all the pump is doing is pumping water. It has no idea that it's attached to a moving object...it sees no load difference between forward, stopped or reverse. Once you stab the throttle to the floor it will spin the pump up to max RPM. That max RPM is dependant on the impeller's horsepower requirement curve and the engine's horsepower curve. Once the pump is moving a certain volume of water through it, the impeller calls for more horsepower than the engine can supply at the RPM it's topping out at. Therefore, once that happens the engine can't spin the pump any faster.

    As far as increasing speed, you may also wanna try tuning your loader, shoe/ride plate, droop/wedge, etc etc. Hull efficiency/setup has a lot to do with MPH. You only increase horsepower if you're lookin' for more RPM.

    Another option as well is to cut your current A impeller down to a B...that may gain you some RPM as well. But also, RPM doesn't always equal MPH.
    Last edited by Jetaholic; 09-22-2008 at 04:01 PM.



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    Quote Originally Posted by Jetaholic View Post
    The forward speed of the boat has absolutely no effect on RPMs. You could tie your boat to a dock and stab the throttle to the floor and it will still swing to 4800 RPMs. Berkeley has done the test to prove that. All the engine is doing is turning a water pump and all the pump is doing is pumping water. It has no idea that it's attached to a moving object...it sees no load difference between forward, stopped or reverse. Once you stab the throttle to the floor it will spin the pump up to max RPM. That max RPM is dependant on the impeller's horsepower requirement curve and the engine's horsepower curve. Once the pump is moving a certain volume of water through it, the impeller calls for more horsepower than the engine can supply at the RPM it's topping out at. Therefore, once that happens the engine can't spin the pump any faster.

    As far as increasing speed, you may also wanna try tuning your loader, shoe/ride plate, droop/wedge, etc etc. Hull efficiency/setup has a lot to do with MPH. You only increase horsepower if you're lookin' for more RPM.

    Another option as well is to cut your current A impeller down to a B...that may gain you some RPM as well. But also, RPM doesn't always equal MPH.
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    Quote Originally Posted by Jetaholic View Post
    The forward speed of the boat has absolutely no effect on RPMs. You could tie your boat to a dock and stab the throttle to the floor and it will still swing to 4800 RPMs. Berkeley has done the test to prove that. All the engine is doing is turning a water pump and all the pump is doing is pumping water. It has no idea that it's attached to a moving object...it sees no load difference between forward, stopped or reverse. Once you stab the throttle to the floor it will spin the pump up to max RPM. That max RPM is dependant on the impeller's horsepower requirement curve and the engine's horsepower curve. Once the pump is moving a certain volume of water through it, the impeller calls for more horsepower than the engine can supply at the RPM it's topping out at. Therefore, once that happens the engine can't spin the pump any faster.

    As far as increasing speed, you may also wanna try tuning your loader, shoe/ride plate, droop/wedge, etc etc. Hull efficiency/setup has a lot to do with MPH. You only increase horsepower if you're lookin' for more RPM.

    Another option as well is to cut your current A impeller down to a B...that may gain you some RPM as well. But also, RPM doesn't always equal MPH.

    This post cracks me up...
    read the first sentence of the first two paragraphs
    they contradict each other

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    Quote Originally Posted by Outlaw View Post
    This post cracks me up...
    read the first sentence of the first two paragraphs
    they contradict each other

    pass the CORN steelcomp
    Quote Originally Posted by Jetaholic
    The forward speed of the boat has absolutely no effect on RPMs.

    As far as increasing speed, you may also wanna try tuning your loader, shoe/ride plate, droop/wedge, etc etc.
    And just how exactly do they contradict each other?
    Last edited by Jetaholic; 09-22-2008 at 07:50 PM.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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    I disagree jetaholic. on a stationary pump with the same power input, the steady inlet pressure and smooth flow will be a nice starting point. The Changing,"Dynamic" conditions will affect how the pump works. The suction pressue is a biggie among other things. think about the ram effect at speed.

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    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by DEL51 View Post
    I disagree jetaholic. on a stationary pump with the same power input, the steady inlet pressure and smooth flow will be a nice starting point. The Changing,"Dynamic" conditions will affect how the pump works. The suction pressue is a biggie among other things. think about the ram effect at speed.
    I see what you're saying. However, for some reason some people have this idea in their head that it somehow works like a car, where the more speed the boat picks up the more the RPMs are supposed to increase like they do on a car as the wheels unload the drivetrain. Not so on a jet boat. A jet pump loads the engine similar to how a high stall torque converter loads an engine while you're standing on the brakes. The only difference is, on a car the car would never move as long as you have the brakes locked up. The boat moves because of the escaping water through the nozzle...however the RPMs do not increase as the boat picks up forward speed.

    If anything, the RPMs will drop slightly as the boat increases speed, due to the movement of the boat increasing dynamic pressure at the intake, which loads the pump harder.
    Last edited by Jetaholic; 09-22-2008 at 08:40 PM.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

  16. #14
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    Quote Originally Posted by SmokinLowriderSS View Post
    The open chamber heads will breathe better than the closed chamber heads, but, the about 1 point compression you will loose (my off the cuff guess) will counter that.

    That said, if you do a bit of porting (bowl clean-up and blending) along with a set of large valves (and relieving the chamber arround the intake valve), you get even BETTER flow (more, WITH VELOCITY, not just "more, from huge port rectangle port heads), and CAN come out on top.

    I run Stage 1 ported '049's on my flat-topped 454 and pull an Aggressor B (comparable to your Berk A) to 5,400RPM, with awesome throttle response.

    I MIGHT have 8.3:1 compression, if I get lucky.
    I was pulling the same impeller 5,000 flat before the work I had done, no other changes.

    Wish I had 10:1. LOL

    Hey Smokin, what kind of specs did you port to on the 049's?

    I'm sure others are interested as well, what bowl percentage, throat, and choke point did you port out to? And what size vavles were they in the heads?

    Inquiring minds would like to know

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