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Thread: compression,regarding cam lobe separation

  1. #1
    Senior Member SENIOR BUILDER
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    compression,regarding cam lobe separation

    I`m running a BBC 588 small 305 heads,and a 264/272 @ .050 w/.715 lift lobe separation of 108, flat top piston CR 10.1-1, I`m getting 170# crank compression, (I`d like to keep the compression),, and go to a 110 L/S cam with a 271/274, 750 lift cam and 357 heads, what do you guys think my compression will go down to with the new cam???also how much difference would it make if I kept the 715 lift as opposed to going to 750 lift???and take the 271/274 duration @.050

  2. #2
    It will probably drop down a bit. You are running more duration @ .050. Hard to tell until you do it. Also I could be wrong but doesn't cranking compression change when you change the valve lash?

  3. #3
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    I know it will drop a little from the extra duration, but how much with the difference in the lobe separation

  4. #4
    Member JOURNEYMAN
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    Use a dynamic compression calculator, it will show the difference using intake closing points between the two cams.

  5. #5
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    sure??? that sounds a little over my head!

  6. #6
    Member JOURNEYMAN
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    There are dynamic calculators on the net, http://www.kb-silvolite.com/calc.php?action=comp2, this will let you see the difference. Shoot for 7.5 to 8:1 for dynamic, you can run higher on pump gas but tuning gets very important.
    "Good people sleep peacefully in their beds at night, only because rough men stand ready to do violence on their behalf."

  7. #7
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    Quote Originally Posted by badrad
    There are dynamic calculators on the net, http://www.kb-silvolite.com/calc.php?action=comp2, this will let you see the difference. Shoot for 7.5 to 8:1 for dynamic, you can run higher on pump gas but tuning gets very important.
    OK Badrad I think I got the dynamic figured out, But I installed the Cam at 2 degrees retarded,it didn`t change it much,all my figures show between 7.72 and 7.9, So I guess I`m ok,,I`ve never seen that method before,,Just out of curiousity,what does a longer connecting rod do for the ratio ? Whats the advantage of longer or shorter,other than it raises and lowers the Dynamic

  8. #8
    Member JOURNEYMAN
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    The only other advantage I know of from running longer connecting rods is a better crank/rod ratio. As far as your question about the lift I would stick with the lower of the two. The 305 AFR only had one cfm difference between .600-.700 in stock form, therefore the lower lift should extend valve train life. It is only a guide line but I have plugged in the figures for a few build ups I read and the one with best power fell within the 7.5-8.5, usually ~8.
    "Good people sleep peacefully in their beds at night, only because rough men stand ready to do violence on their behalf."


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