On Sun, 21 Sep 1997 12:07:22, -0500 GDWF22A(at)prodigy.com (MR RICHARD T
TRENK SR) writes:
>---------------------
>Re. camshafts: I need your clarification on the term " l/C 108 " which
>puzzles. Tim posted a thought on this but that is not the answer, as
>cam
>grinders do NOT quote such a figure for lobe center to center. Reason
>being
>that the grinder MUST make the valves open and close in relationship
>to
>ONLY the crankshaft angles. No concern is made for the relationship
>between lobes nor is anyone interested in such a measurement.
The 108 info would be the Lobe center SEPERATION. It is a relative
term, and actually has nothing to do with the true peak lift point on
the cam in realtion to the crank. This is because the grinder generally
has no knowledge of what the cams keyway has to do with its
index to the crank TDC.
The assumption that is made by the cam grinder is that the int/exh cam
lobes are symetrical to one another realtive to TDC. They grind the cam
in this manner, and ignore the true realtion to the crankshaft because it
gives more area to grind from, when the lobe centers are altered.
This gives the proper overlap and list and duration, but leaves an
unknown timing offset in the cam.
It is ultimately the installers responsibility to "degree" the camshaft
with a degree wheel, and offset the cam in relation to the crank.
This accomplished via offset keys, and adjustable timing wheels.
This is an elaborate procedure, that fixes the above problem.
The grinder will specify the as intalled degree on both intake and
exhaust, for which will occurr at .050 lifter lift. After the offset
is tweeked in on the cam in relation to the crank, all other
specs should also work out, else the grinder screwed up.
>Jarrid writes about a Series II and SII Rally cams but ran the numbers
>all together making it at first hard to understand but once I noticed he
>also ran the series names together I saw the numbers were done the
>same.The SV stock grind IS hotter than the Series II Rally by a small
>amount.Unfortunately, Rootes data does NOT verify lift height in old
>books but ofcourse with a cam in your hand you can simply measure
>the base circle diaand subtract it from lobe height = lobe lift.
On all Alpine factory cams this is between .290 an .300.
Easy to measure, just subtract the diameter between the narrow
side of the cam, from the peak of the cam to the base circle.
This is a good approx, but not exact due to errors imposed
by the quieting ramps.
My potential expalnation for the huge cam that is specified as SV stock
is this...
All roots cams were specified from lash to lash.
This is not the norm in automotive cam lingo.
It totally ignors the acceleration of the lobe itself, and has the
fundemental problem of ignoring the effect of the quieting ramps
on the opening, closing and overall duration.
What is the norm, is to specify the cam opening and closing point
in degrees (at).050 lifter lift.
This tells a much more complete story as to the action of the
camshaft in relation to another.
Also, higher lift with a given duration gives additional acceleration
of the valve, thus giving more area under the flow curve.
I believe that the SV stock specs were probably as the result
of a longer quieting ramp on the SV motor, that would make
for quieter valve opperation, but totally hoses the lash
to lash specs in comparison to earlier spec'd cams.
The Rally SII cam was a great cam, but had no quieting ramps,
as lack many performance cam profiles. It, if installed in a SV
motor, would have produced about 120 BHP, 25 more than the
spec'd 95BHP on the SV motor. The SII rally cam puts power
into the 4000 to 6000 RPM range. If the SV had that kind of
power range, these stockers, would have been monsters
right off the showroom floor.
Jarrid Gross
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