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Help Needed with C. R.

Discussion in 'Under the Hood' started by imp, June 12, 2019.

^^Searches ExplorerForum.com^^





  1. imp

    imp Well-Known Member

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    Compression Ratio is defined as the ratio of combustion chamber volume to the SUM of C.C. volume + the swept cylinder volume. Here's my problem:

    Guy has a 4.6, stock bore and stroke, 43cc combustion chamber volume. 4.6 is 4600cc, take 1 cylinder,
    4600/8 = 575cc swept volume. C.R. works out to 575 + 43 divided by 43 = 14.4. His head supplier claims 10.1.

    Where am I going wrong? imp
     
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  3. CDW6212R

    CDW6212R Hauls the mail. Elite Explorer

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    That 43cc combustion chamber sounds small, but I know nothing about modular Fords. I would have expected those CC sizes to be more like 50cc's or so. Do the math for a SBF like a 302, and a 58cc chamber size.

    Do remember that the chamber dimension for the compression should include piston dish, gasket and deck height volume. Those won't make up that difference from 10 to 14:1, but some of it.
     
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  4. Dono

    Dono 347 V8 Limited turbo Elite Explorer

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    You need more information.
    upload_2019-6-13_14-49-16.png


    There is all kinds of space that still isn't accounted for. If you just look at bore/stroke and combustion chamber volume you are absolutely correct.
     
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  5. imp

    imp Well-Known Member

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    @Dono
    Good points, which I neglected. Given a gasket thickness of 0.06" and bore 3.5", that volume contained = 0.58 cu. in, = 9.5cc. Deck height, normally less than 0.06" I think, maybe 0.02", so another 0.19 cu. in. = 3.2cc. Assuming flat-top piston, comb. chamber 43cc + gasket 9.5cc + deck height 3.2cc = 55.7cc.

    575 / 55.7 = 10.3 C.R.! Damn! Guesswork on the added volumes, real close to advertised 10.1!

    Thank you for alerting me to this; getting rusty in my old age! imp

    As an aside, regarding deck height, long ago an article published in Hot Rod Magazine claimed a 283 cu. in Chevy V-8 connecting rod had been dynamically measured to stretch 0.034" at 6000 rpm! Believe it?
     
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  6. CDW6212R

    CDW6212R Hauls the mail. Elite Explorer

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    That's just another weak rod Chevy.
     
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  7. imp

    imp Well-Known Member

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    @CDW6212R
    This afternoon, relaxing on the couch after eating too much at a buffet, I got to thinking: one could calculate that rod stretch given a few dimensions, and knowing a bit about the operating characteristics. Saw that I needed the maximum acceleration as well as velocity, the easy one, assume max. velocity is @90` (it really ain't), velocity at TDC = zero, distance = 1/2 a X t X t,
    a = v / 2s..........

    Then I fell asleep. imp
     
  8. CDW6212R

    CDW6212R Hauls the mail. Elite Explorer

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    Back when you and I would have been early at building engines, aluminum rods were exotic race only parts. I read a couple years ago that some people have run AL rods for several years in street and strip cars.

    Things always change, and with car technology if you aren't paying attention all the time, you get surprised by lots of new things. I always wanted to build a high compression Cleveland and hit 7500+ rpm, that was the top dream back in the 70's and 80's. Now all pushrod Fords are behind the times, a near stock newer 5.0 modular engine will make more power for less money than what everyone's been building(SBF's) for years.
     
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  9. blakshukvw

    blakshukvw Active Member

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    The ZR1 Vette has titanium rods from the factory. Interesting as titanium has a life expectancy.
     
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  10. CDW6212R

    CDW6212R Hauls the mail. Elite Explorer

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    Yeah, we'd all love to rob the JY engines of the exotic parts Chevy has put in them. Too bad the dimensions are all wrong.
     
    Last edited: July 10, 2019
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