The $100.00 dollar Q? octaine vs compression ratio
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The $100.00 dollar Q? octaine vs compression ratio

  1. #1
    Senior Member H20MOFO's Avatar
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    Default The $100.00 dollar Q? octaine vs compression ratio

    So... I have this 427 bbc 11.5 to 1 alum head motor........... man this is going to be harder than I thought. I guess what I'm asking is what is dynamic compresion and how would you calculate for that . In other words what is the real/best way to know your octaine needs. Oh I'm at altitude also...I guess I need some dyno software or something.
    Another Hot Boat refugee

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    Senior Member hotbo's Avatar
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    you can go to kieth blacks website they have a very easy to use compression calculator and figure your static and dynamic comp.out very quickly need to know the ivc.intake valve closing on your cam and youll be good to go,i use it all the time,hope this helps.travis

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    Senior Member H20MOFO's Avatar
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    Thanks hotbo. I'll check that website out. BTW I didn't know the term static compression was used on a N/A motor, I thought it was more for the blower/turbo guys.
    Another Hot Boat refugee

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    cfm
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    More altitude can effectively use more compression.\
    What elevation for what compression ? I dunno - I'm near sea level.
    However I do know that I'd be asking builders in your area and or those in your exact altitude.
    Less air = less effective pressure.

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    Senior Member H20MOFO's Avatar
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    So.... what's the magic #(dynamic), say for 91 octaine, 9.8-1ish? I can't get 93 here at all. Maybe later in the season I'll get brave and try some e-85.
    Another Hot Boat refugee

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    Senior Member Brendellajet's Avatar
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    lots of people safely running 91 at over 11:1 with aluminum heads.

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    Senior Member Cat Sass's Avatar
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    In a jet boat with a cold water supply(like in your avatar) I bet you could get away with 11:1 on 91. The altitude will help and you will need to be jetted real good. learn to read plugs next

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    I agree with that ...... i bet you guys wish you could get 93.... what do you think about mixing fuels????

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    Senior Member Bigblockbill's Avatar
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    I ran an 11:1 motor at 4500 ft elevation no problems on 92 for years. By my basic back of the envelope calcs at 4500 ft you loose 1 point of compression... making a 11:1 motor here equal to a 10:1 motor at sea level.

    Altitude Sucks

    As far as mixing goes, I have mixed Av Gas and pump gas on a 12:1 motor. It actually is a good thing to do to help the AV Gas get back the molecules its lacking The only thing you got to watch out for is Av Gas needs a larger jet, so as you mix you will have re-jet. Now the hard part... how do you know you have the same ratio from tank to tank all the time?
    Last edited by Bigblockbill; 04-29-2008 at 10:50 AM.
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    jetboataholic HawaiianJet's Avatar
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    When you come down from altitude, would that 11:1 motor turn into a 12:1 motor? Or is that just crazy talk?
    HJ

    ***FOR SALE*** 1978 19' Full Stringer Hawaiian Molokai Jet Boat w/ Berkeley 12JC-AB and Chevy 440 Tall Deck ***FOR SALE***

    Quote Originally Posted by mako shark mark View Post
    Oh yeah, Forgot to mention that the hull is a special mixture of speedcote, herculiner and rain-x, all blended together- good for another 8 mph top end.

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    mo balls than $cents$ IMPATIENT 1's Avatar
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    Quote Originally Posted by HawaiianJet View Post
    When you come down from altitude, would that 11:1 motor turn into a 12:1 motor? Or is that just crazy talk?
    a 11 to 1 compression motor would be figured at sea level, not at the bottom of the sea

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    Senior Member hotbo's Avatar
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    Quote Originally Posted by IMPATIENT 1 View Post
    a 11 to 1 compression motor would be figured at sea level, not at the bottom of the sea
    just cant get a break can ya H.J.


    i would venture to say 11.1 would be okay,but it also comes into play with your cam profile

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    jetboataholic HawaiianJet's Avatar
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    Red face

    Wow!!!! I re-read Bigblockbill's post, and now I feel like a complete idiot for asking that! I think I need to shut-up for a little while!
    HJ

    ***FOR SALE*** 1978 19' Full Stringer Hawaiian Molokai Jet Boat w/ Berkeley 12JC-AB and Chevy 440 Tall Deck ***FOR SALE***

    Quote Originally Posted by mako shark mark View Post
    Oh yeah, Forgot to mention that the hull is a special mixture of speedcote, herculiner and rain-x, all blended together- good for another 8 mph top end.

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    Senior Member H20MOFO's Avatar
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    As far as reading plugs, I give up on that. Supposedly I have a set of dry headers on the way I'm going to set up with both an a/f and a pyrometer. When they arive, I'm sure I'll be asking more Q?'s. I assume the pyrometer is going to be the most help with the octaine thing??
    Another Hot Boat refugee

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