Re: Re[2]: Cams, John L.

[jlandry@domain.elided (John A. Landry)]

When he stopped taking his medication, strange voices in
jmbrodsky@domain.elided's head said:

>     Bob (rbrooks@micrelinc) said: that cams are rated for actual 
>     valve lift, not lobe height.
>     
>        Ummm.  Now I really have a question.  Since the factory NEVER 
>     EVER can make each rocker the same, AND since you can maximize 
>     your lift when you rbuild by buying the appropriate tolerance 
>     rocker, you may be able to adjust the lift more or less 
>     accordingly.  About a year or so ago, Petersons 4WOR did an 
>     article on this very practice, and you can get sometimes up to 10 
>     HP by maximizing the lift with the factory spec rockers used at 
>     the limit.

Joel,

A cam manufacturer will typically give specifications for a cam in both
valve and lobe lift.  The key word Bob used was "rated."  When a cam is
said to have .440" lift, normally they *are* referring to gross valve lift
and the cam manufacturer assumes a standard 1.5:1 ratio (and then they
usually round the number off to boot).  In order to actually measure the
cam, the manufacturer will normally provide a specific lobe lift to measure
also... in the case above, say .3026", etc.

When you degree a cam, you *usually* measure the cam directly on the lifter
if the heads aren't yet installed.  However, you can measure the lift at
the valve stem tip and you simply have to take into consideration the
rocker arm ratio in doing so.  Yes there will be variances when measuring
at the valve, but this is taken into consideration when looking at the big
picture.  The lobe center will be the same no matter whether the ratio
differs slightly from one rocket to the next.

Personally I'm going to measure the cam directly at the lifter before the
heads go on and forget about the whole rocker issue.  Just makes it
simpler.

Regards,

John L.
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