[Fot] Kastner F cam

[Tony Drews]

If you are putting in 4 degrees of advance, you want the valve 
opening 4 degrees of crankshaft rotation earlier than whatever spec 
you're following (0.010 lift or 0.050 lift or whatever).  The degrees 
are confusing because you have the "Before" and "After" degrees (BTDC 
- Before Top Dead Center and ATDC - After Top Dead Center) - for one 
you add the 4 degrees, for the other you subtract it.  I always spend 
about 5 minutes figuring out which way to go.  I'm pretty sure that 
an intake normally opens before top dead center, so if it was 
supposed to open to 0.050 lift at 20 degrees before top dead center, 
4 degrees of advance would have it opening at 24 degrees before top 
dead center.  It would also close 4 degrees earlier.  If close is 
supposed to be 50 degrees ATDC, 4 degrees early would be 46 degrees 
ATDC (no one would close the intake that early I'm pretty sure).  I 
guess you add the advance to the "before" and subtract from the 
"after" settings.

When I degree a cam, I measure both the open and close events.  If 
the cam isn't ground PERFECTLY to spec, if you only measure opening 
you may end up with more or less advance than you expected.  So 
average the open / close to get the centerline in the right spot.  If 
close is 4 degrees early when open is on spec, you really have 2 
degrees of advance already.  I have a hard time explaining it so it's 
clear - but it's clear in my head... :)

Tony Drews

At 08:46 AM 12/23/2016, Jerry Van Vlack via Fot wrote:
>Friends, can someone explain installing a cam with xx advance, 
>somewhere I saw a reference to doing a cam installation with a 4 
>degree advance. Does that mean that if the valve opens at xx degrees 
>that it should be set to open at xx minus 4 degrees and what purpose 
>does that serve? Is this the same as using .010 or some other 
>checking clearance. Seems that these two variables are related and 
>one should use one or the other method but not both. Thanks, I'd 
>like to learn more about this stuff. JVV