## Cam: Explanation of terms

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• 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
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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