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Thread: Rod Length????

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    Rod Length????

    I'm trying to calculate dynamic compression.

    Can anyone tell me connecting rod length center to center???

    My head came back from machine work. They didn't have a maximum spec for the the 3y / 4y. I am not sure if the head has been machined before.
    They took off -0.012".

    I am curious if i need to run higher octane or change my base timing. My 3y had been running great untill the head gasket started leaking. I was getting 22mpg at 70mph on 30" tires. One of the ways i got there was 16* advance.

    One calculator suggests i am at 10.7:1
    http://www.csgnetwork.com/compcalc.html

    Another suggests I am at 12.3:1
    http://members.uia.net/pkelley2/DynamicCR.html
    I'm using the visual basic program at the bottom of the page.

    I put the same numbers in both calculators.

    But all of that is neither here, nor there. Fuel does not care what static compression is. Only how much compression is generated after the intake valve closes.

    Any who?

    Rod length any one?????

  2. #2
    Administrator timsrv's Avatar
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    Re: Rod Length????

    I can't imagine .012 having much effect on compression (I'm thinking about .2 increase max). I don't have any 3y parts laying around, but I've got lots of 4y stuff. I grabbed a rod (piston still attached) and took a crude measurement of 5.35" (pretty sure I'm within .05" +/-). This is the distance from center of wrist pin to center of bearing bore

    Stock compression ratio isn't discussed in the service manuals, but I found a few references that state both engines are 8.8 : 1. I also found the stroke of both engines is the same (86 mm). The difference in size comes from the bore. 3Y bore is 86 mm & the 4y is 91mm. Assuming your engine was previously 8.8 : 1, I think now it will be closer to 9 : 1. Hope this helps. Tim

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    Re: Rod Length????

    thanks tim!

    This article suggests -0.010" off the head gets you to 10:1 compression.
    http://brianlawson59.blogspot.com/p/efi-conversion.html

    I found this rule of thumb last night:
    There is another factor which corresponds to DCR and that is cranking pressure. I have found it to be generally agreed that a cranking pressure under 180psi will keep you on pump gas. There is a very good article in “Hot Rod” Aug 2005 on all of these issues which continues to back up all of the data.
    per:
    http://www.forabodiesonly.com/mopar/showthread.php?t=46600
    I was at 155-160 psi on all cylinders before the rebuild so its only going to go up. So its only going to go up.

    The bottom of the article discusses pulling timing back to compensate for knock.
    My concern is tuning the motor durring an 80*F day and it knocking on a 110*F Day.

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