2011 - Crane Cams

VALVE TRAIN
Cam Timing Explained
Cam Timing Explained
Cam advance, lobe separation, lobe centerline, intake
lobe centerline, etc. are all terms being used for comparing
and devising camshaft specifications. With so
many similar terms being used, there can be a bit of
confusion when folks from different backgrounds start
talking about them.
Lobe separation is the measurement in CAM degrees
between the maximum lift point of the exhaust lobe to
the maximum lift point of the intake lobe on any cylinder.
Some also refer to this as lobe centerline. This
dimension is ground into the camshaft and can not be
changed by advancing or retarding the camshaft
(unless it’s an engine with separate intake and exhaust
cams.)
Intake lobe centerline, or intake maximum lift, refers to
the distance in crankshaft degrees from the cylinder’s
Top Dead Center point to the maximum lift point of the
intake lobe on any one cylinder. This is usually measured
as degrees After Top Dead Center. This figure
WILL change when the cam is advanced or retarded. As
you advance the cam, this number will get smaller, as
you are opening it fewer degrees AFTER Top Dead
Center. Retarding the cam will make this number larger,
as you are opening it more degrees AFTER Top Dead
Center.
Exhaust lobe centerline, or exhaust maximum lift, is
usually expressed in crankshaft degrees Before Top
Dead Center. As you advance the cam, this number will
get larger, since you are opening it more degrees
BEFORE Top Dead Center. Retarding the cam will make
this number smaller.
The average of the intake lobe centerline and the
exhaust lobe centerline should equal your lobe separation.
The cam timing figures (as measured at a specific lobe
lift: .004”, .020”, .050”, etc.) may show the maximum lift
point to be distorted when you’re dealing with nonsymmetrical
camshaft lobes (the opening side has a different
shape than the closing side). If you split the difference
between the opening and closing figures at
.020” or .050” lobe lift, this figure will not coincide with
the actual maximum lift point of the lobe. There are
instances where a non-symmetrical intake lobe is paired
with a symmetrical exhaust lobe (or vice-versa), or lobes
with varying amounts of non-symmetry may be used as
intake and exhaust. We believe that where the opening
and closing events actually occur are the most important
figures to pay attention to when degreeing your
camshaft. Just finding the maximum lift points doesn’t
really tell you anything about the camshaft, or it’s even
the correct camshaft! By documenting the opening
and closing numbers as you tune, you will gain more
knowledge as to what actually helps or hinders your
performance. This is also a good time to emphasize
keeping track of your cranking compression whenever
you change valve lash, cam timing, rocker arm ratio,
and especially when changing camshafts.
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CRANECAMS.COM
You may have noticed that most Crane Cams have a
certain amount of advance ground into them when you
check out the cam specification card. This is primarily
done to insure that you have adequate torque to establish
a good performance baseline. We have also found
over the years, that the correct camshaft for most applications
will run best with some amount of advance in it.
We believe that it’s certainly better to begin with too
much bottom end and mid-range torque, and tune
from there, than to have a shortage of torque, and try
to figure out how to compensate for that.
The following is a general rule for how we grind most of
our camshafts:
Lobe Separation Degrees Advance
Up to 102 0
103-104 2
105 3
106-107 4
108 or more 5
This has certainly not been a list of all of the terms and
philosophies we use when producing our camshafts,
but it will hopefully provide a bit of insight as to some
of our methods of camshaft recommendation and production.
We invite any questions or comments that you
may have.