Dynamic comp on 390

Small Block Mopar Engine

  1. OLFOOL

    OLFOOL OLFOOL

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    Just wondering if anyone could work out the dynamic comp on the 318 stroker we are about to build using a Comp Cams Xtreme Energy 242/248 Hydraulic Roller I normally use the UEM calculator but it asks for inlet ABDC @.050" and the roller is at .006". Any help appreciated.
    upload_2021-7-28_10-25-6.png


    View attachment 1715772244
     
  2. rumblefish360

    rumblefish360 So close, yet so far away FABO Gold Member

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    Comps web site should have all of that information for you. They post the cam card on there site when you look up the cam itself.
     
  3. OLFOOL

    OLFOOL OLFOOL

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    not @ 050" only at .006" as I posted.
     
  4. Max1196

    Max1196 Well-Known Member

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    Use the Wallace Calculator. My ASSumptions include an elavation of 366 feet.

    Camshaft, Rod Length, Boost and Altitude Correction to Compression
    Static compression ratio of 11.7:1.
    Effective stroke is 2.92 inches.
    Your dynamic compression ratio is 8.81:1 .
    Your dynamic cranking pressure is 183.51
    PSI.
    Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0
    PSI is 8.74 :1.
    V/P (Volume to Pressure Index) is 157.

    Edit , I see Comp Cams aren't being clear on their timing figures on the cam card. Those #'s are not at .006 there at .050 on their illustration.
    Is there not a "value" given for piston valve reliefs?
     
    Last edited: Jul 28, 2021
  5. rumblefish360

    rumblefish360 So close, yet so far away FABO Gold Member

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    Sorry, my bad. Thought I read different.

    The advertised numbers are 292/300 but at what exact lift point Comp uses for that XE Hyd roller shaft is IDK. You could try those numbers. I’m not 100% on this, but *I Think* Comp uses the .006 on there other cams along the similar lines. As in not there race cams.

    Xtreme Energy 242/248 Hydraulic Roller Cam for Chrysler 273-360

    I see your location. Perhaps a phone call would work? I know the time difference is huge.
     
  6. mopowers

    mopowers Well-Known Member

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    I don't get the numbers they post on their site. The intake timing events at .006" lift are shown, but actually add up to their duration at .050" (15+180+47 = 242*). I'm guessing it must be a misprint and those are actually the events are .050"??

    upload_2021-7-28_6-33-59.png
     
  7. rumblefish360

    rumblefish360 So close, yet so far away FABO Gold Member

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    I initially started to write what you have discovered but decided to bite my counts and figured a call to Comp would clear things up faster and more accurately since it’s there cam and web site. LMAO!

    Expert time!
     
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    • 33IMP

      33IMP Well-Known Member

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      Are you trying to get compression ratio, or DYNAMIC comp ratio? Two very different numbers and calculations!
       
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