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Cam: Explanation of terms

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  • ja70cad
    This was posted by Charles on the MTS message board. Here goes: The comp cam the shop purchased shows gross valve lift (I+E) at .490. What s gross valve
    Message 1 of 1 , Jul 9, 2002
      This was posted by "Charles" on the MTS message board.

      Here goes: The comp cam the shop purchased shows gross valve lift
      (I+E) at .490. What's gross valve lift? That's the total amount of
      lift you get at the valve. It's the amount of lift at the lobe
      (diameter at the lobe minus base circle diameter) multiplied by the
      rocker arm ratio your using. Stock on a caddy is supposed to be about
      1.7:1. Looks like Comp is using 1.72:1. It's a matter of tolerances
      in the machining process. So you take the lift at the lobe (.2853)
      and multiply by the rocker arm (1.72) to get .490 inches of lift at
      the valve. All their saying by E+I is both exhaust and intake are
      ground the same and have the same amount of lift. All things being
      equal, try to use the actual lobe lift of the cam as a gage as to how
      radical it is. With a lot of rocker arm ratio, the same cam can seem
      like it has a lot of lift. That's why manufacturers like to show the
      big "gross lift" number. And I suspect that's why Comp used 1.72:1
      instead of 1.7:1. : Then it says "Duration at .050 : Intake 218,
      Exhaust 218. Lobe Lift: Intake .2853 Exhaust .2853 Duration at .050"
      is a measure of how many degrees of a revolution the valve is held
      open to .050". They do this to help everyone get the same standard
      reading. If the duration were measured from .000" of lift (as some
      are) the measurement could be taken from basically anywhere on the
      base circle. Again, manufacturer's will embelish the numbers to make
      it seem like their cam has more lift or duration than the
      competitor's. Measuring at .050" lift, you can compare the duration
      of different cams all from the same starting and ending points.Lobe
      seperation: 110 Lobe separation is the angle between the centerlines
      of the intake and exhaust lobes of a cam. different centerlines give
      you different characteristics like better idle, more low or top end
      power, etc. One more note you may find interesting. Almost all Caddy
      performance cams are "regrinds". I don't know of any that are not.
      This means they take a new stock caddy cam from GM and regrind it to
      give you more lift and or more duration. The problem is that the only
      way to do this is to grind away more of the base circle (lift is the
      difference between the base circle and lobe) because they can't
      really reliably add material to the top of the lobe. They are limited
      by how much lift they can grind into the cam profile because cams are
      heat treated, and they can't go too small on the base circle without
      grinding through the heat treat and giving you a "soft" cam that
      won't last long at all. The entire lobe also has to be ground during
      this process, so you lose a little bit off the top of the nose as
      well. Cam's can't be re-heat treated after a lobe grind because they
      warp during heat treat and must be ground afterwards. Since the
      bearings are already ground to the correct size when the cam
      manufacturer gets them from GM, they can't go in and re-grind the
      bearings to straighten the cam back out. Special bearings could be
      installed in the block to use undersize cam main bearings, but that's
      a pretty specialized case. So all this means is there are limits to
      how much lift you can grind into a caddy performance cam.
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