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new displacement after overbore

Discussion in 'Members' Lounge' started by reckedracing, Dec 7, 2004.

  1. reckedracing

    reckedracing TTIWWOP VIP

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    how would you calculate the new displacement of a motor if its been bored?

    for example

    xr250 249cc, bored.040 over?
     
  2. Slammed90Lude

    Slammed90Lude Senior Member

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    well, displacement is cylinder volume at BDC minus cylinder volume at TDC

    you really need to know the volume of the combustion chamber to figure it out- if you know the originial bore and stroke you can find the volume of the cylinder at TDC

    [(bore/2)^2*pi*stroke+combustion chamber]-[volume at TDC]

    assuming that everything but TDC volume is known, you'll find a value for that- then just plug the new value for bore into the same formula
     
  3. Battle Pope

    Battle Pope New Member

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    I think the quadratic formula exploded.
     
  4. reckedracing

    reckedracing TTIWWOP VIP

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    how about a simplier way...? lol

    just looking for approx figures here...
     
  5. pissedoffsol

    pissedoffsol RETIRED

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    go to c-speedracing.com and use their caculator
     
  6. reckedracing

    reckedracing TTIWWOP VIP

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    http://www.bgsoflex.com/displacement.html

    1988 xr250

    bore = 73
    stroke = 59.5

    overbore = .040 approx = 1mm

    stock = 249cc
    overbore = 255.9cc

    hmm, i figured it would be a bigger diffrence, guess i was wrong
     
  7. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Those have nothing to do with the volume of a cylinder. :p
     
  8. hcivic.com

    hcivic.com Senior Member

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    just water test it
     
  9. tab

    tab Super Moderator

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    That number sounds about right.

    I just use the desktop dyno for most engines. They calculate that my Olds 455, bored .o60" over, is ~468 ci.

    A normal bore will not gain you all that much displacement.
     
  10. hcivic.com

    hcivic.com Senior Member

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    tab you payd for dt dyno?
     
  11. tab

    tab Super Moderator

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    not really........I DL'd it. It does a pretty good job of calculating what changes will give you the most. If you know the flow rates on your parts, you can input that data. It's all fine and good, but none of us have a flowbench.

    It also does not account for VTEC, or changes in cam profile midway through the rpm range.

    Not good for a Honda, but great for an old Chevy, Ford, Oldsmobile, etc.
     
  12. reckedracing

    reckedracing TTIWWOP VIP

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    hmm, didn't think of that, but searching with google for specs on motor, and then sae to metric, and then a calculator is a bit easier...

    so tab, what else do you use the desktop dyno for?
     
  13. tab

    tab Super Moderator

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    A few things, although I haven't used it in a while.

    I have a 1968 Olds Toronado(FWD V-8 :) ). I bought Arias 12:1 pistons that were bored .060 over. I wanted to know how much extra that would give me, and what would best compliment those parts for the buck. That program told me that my oversize pistons would raise my displacement to 468 ci.

    You can change carb sizes, go single or dual plane manifold, create your own head profile if you have the flow bench results, change compression, engine size, bore, stroke, cam profile, roller rockers, roller cams.

    Basically, there are a ton of things you can easily click to find results.

    I found that my engine would produce over 550 tq and about 570 hp by using a good carb, roller cam, roller rockers, decent heads, and a single plane manifold.

    It really helps if you have specs for your parts. There are templates to get you close on most V-8 applications, but that program isn't the best for imports IMHO.
     
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