Calculating compression ratio?
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Calculating compression ratio?

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    Senior Member 74glencoe's Avatar
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    Default Calculating compression ratio?

    Ok, trying to figure out compression ratio on a proposed build, maybe need an education on the subject. Have a couple assumptions I need help with.

    Inch to cm conversion is inch x 2.54?

    Volume of a cylinder is: stroke x 3.14159 x 1/2 bore squared, right? Of course must add in head gasket cc, head cc, and subtract any dome?

    "Top dead center cc" is head cc + head gasket cc + cc of any amount the piston is short of the deck?

    compression ratio is how many times the top dead center volume fits into the total cylinder volume? (Total cylinder volume/top dead center volume?)

    so, tell me if Using stuff from geometry works for this or not, thanks guys!
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    Some guy obnoxious001's Avatar
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    Quote Originally Posted by 74glencoe View Post
    Ok, trying to figure out compression ratio on a proposed build, maybe need an education on the subject. Have a couple assumptions I need help with.

    Inch to cm conversion is inch x 2.54?

    Volume of a cylinder is: stroke x 3.14159 x 1/2 bore squared, right? Of course must add in head gasket cc, head cc, and subtract any dome?

    "Top dead center cc" is head cc + head gasket cc + cc of any amount the piston is short of the deck?

    compression ratio is how many times the top dead center volume fits into the total cylinder volume? (Total cylinder volume/top dead center volume?)

    so, tell me if Using stuff from geometry works for this or not, thanks guys!
    Tons of free calculators online.... http://www.summitracing.com/expertad...on-Calculator/

    Using the actual math is cool, but the calculators should be fast and efficient. I have one loaded on my computer, have not actually used the one from Summit that I linked.

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    northern member Canuc's Avatar
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    bore sq'd x.7854 x stroke gives you cu. in. x 16.387 gives you cc's . you need to know quench clearance + chamber vol. - dome vol. to know what you have at tdc . hope that helps .
    Last edited by Canuc; 01-15-2014 at 11:11 PM.
    Quote Originally Posted by gn7 If your ran a cammer your odds of finishing WERE ZERO.

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    gn7
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    Quote Originally Posted by obnoxious001 View Post
    Tons of free calculators online.... http://www.summitracing.com/expertad...on-Calculator/

    Using the actual math is cool, but the calculators should be fast and efficient. I have one loaded on my computer, have not actually used the one from Summit that I linked.
    KB pistons has one of the better ones on the net. It will also calculate dynamic CR as well is you have the rod length and intake valve closing timing. Brooks Racing Components had a good one as well, but it has taken input for a couple years now. No idea why.

    If it doesn't ask for the head gasket diameter separate from the cylinder bore diameter, its not that accurate.

    Some go so far as to ask the piston clearance above the top ring, and the distance from the crown to the top ring to account for crevice volume.

    If you are guessing at the combustion chamber volume, and/or deck clearance, you are playing darts in a dark room with a blind fold after being spun on a bar stool 20 times, after you drank 6 rum and cokes. That might work for picking a camshaft, but not for figuring compression ratio.



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    Last edited by gn7; 01-15-2014 at 11:16 PM.

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    Quote Originally Posted by gn7 View Post
    you are playing darts in a dark room with a blind fold after being spun on a bar stool 20 times, after you drank 6 rum and cokes.
    Have you been following me into the bar?
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    Senior Member 74glencoe's Avatar
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    Quote Originally Posted by gn7 View Post

    If you are guessing at the combustion chamber volume, and/or deck clearance, you are playing darts in a dark room with a blind fold after being spun on a bar stool 20 times, after you drank 6 rum and cokes. That might work for picking a camshaft, but not for figuring compression ratio.
    So that is where I went wrong, been drinking crown and coke!

    Ok, I will get all the real numbers together, I was guessing on head cc of course. deck clearance, bore, head gasket thickness, and head gasket hole diameter were all measured with caliper. Good to know I was at least a little wrong, number was pretty high.
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    gn7
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    Quote Originally Posted by 74glencoe View Post
    So that is where I went wrong, been drinking crown and coke!

    Ok, I will get all the real numbers together, I was guessing on head cc of course. deck clearance, bore, head gasket thickness, and head gasket hole diameter were all measured with caliper. Good to know I was at least a little wrong, number was pretty high.
    If this is the same 30 CC dome, closed chambered head deal from the other thread, it will be high. With virtually any sized closed chamber head, it will be higher than I would run a pump gas deal. Sold Honda may run it, but has mystical powers most humans don't possess, and he sure as hell isn't doing any water skiing and tubing with that thing.

    Camshaft plays a huge part in how much compression you can get away with, and I am thinking you are playing with a little less camshaft than SHBH is using.



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    Last edited by gn7; 01-16-2014 at 12:18 AM.

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    Quote Originally Posted by gn7 View Post
    If this is the same 30 CC dome, closed chambered head deal from the other thread, it will be high. With virtually any sized closed chamber head, it will be higher than I would run a pump gas deal. Sold Honda may run it, but has mystical powers most humans don't possess, and he sure as hell isn't doing any water skiing and tubing with that thing.

    Camshaft plays a huge part in how much compression you can get away with, and I am thinking you are playing with a little less camshaft than SHBH is using.
    So we made full circle back to where I commented on what the compression was with that large dome in a closed chamber head with pump gas, rather than trying to suggest a cam to go with something I would not try and run.

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    Quote Originally Posted by obnoxious001 View Post
    So we made full circle back to where I commented on what the compression was with that large dome in a closed chamber head with pump gas, rather than trying to suggest a cam to go with something I would not try and run.
    I agree. 100%. I don't think open heads would lower it enough for a ski/pleasure boat, and he would gain near nothing with open heads.
    Don't know enough about those pistons to know if they will tolerate cutting the dome.



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    Quote Originally Posted by gn7 View Post
    If this is the same 30 CC dome, closed chambered head deal from the other thread, it will be high. With virtually any sized closed chamber head, it will be higher than I would run a pump gas deal. Sold Honda may run it, but has mystical powers most humans don't possess, and he sure as hell isn't doing any water skiing and tubing with that thing.

    Camshaft plays a huge part in how much compression you can get away with, and I am thinking you are playing with a little less camshaft than SHBH is using.
    Yep less cam and less stroke. Had 215-220 cranking with last cam, down to 150-160 now.
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    Quote Originally Posted by SoldHondaBoughtHondo View Post
    Yep less cam and less stroke. Had 215-220 cranking with last cam, down to 150-160 now.
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    You don't figure less stroke may have lowered the compression with those pistons just a wee bit?? And your telling the OP yours runs fine one pump gas? No shit? Does it really. Who knew?



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    Last edited by gn7; 01-16-2014 at 12:29 PM.

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