Most
of you will not be affected by this due to a cam
replacement; but may be suffering as a result of
geometry problems anyway.
This
is not intended to be a thorough discussion of cam
geometry; just some things floating around in my head
I've told people before about dealing with the U20
cylinder head in emails and while leaning over a car
fender. I'm just banging this stuff out on this
keyboard; I have little time for these tasks; however
much I sometimes enjoy them;-- I'm lucky if I take
time to proofread it!
CAM
GEOMETRY refers to the relationship (location) between
the center of the camshaft and the tips of the valves;
and how this changes how the cam, rockers and valves
interact together.
All
kinds of things can affect it; rockers, cam, length of
valve stems and valve lock position of any aftermarket
valves have been installed ever, how far valve is sunk
into head during previous work, head being surfaced on
the top for warpage, HOW it was surfaced, if it was
shimmed, HOW it was shimmed, thickness of lashpads; if
cam towers were switched... First you have to get cam
to turn with two fingers without rockers in place; if
it doesn't you have a problem with tower alignment
either from head warpage or them just not being
installed correctly. On day one they were installed
and THEN the towers were drilled for the cam. Even
taking the towers off a new head and bolting them back
on can cause misalignment enough to snap a cam. This
is why Nissan says never remove them; but it is
necessary to do so if the top needs to be surfaced. If
heads were available and $150 you never would mess
with it. I used the singular form of "valve" above
instead of valves because EVERY lobe on the cam may
require a different thickness lashpad in radical
situations or at least be running on a different
section of the rocker arm. Altering the height of the
center of the cam in relation to the top of the
installed valve stem either from milling/valve
work/base circle changes to cam etc etc etc can make
each lobe run in a different position on its rocker
arm. The critical thing to remember is if the cam lobe
runs OFF the roadway on the rocker arm; the cam is now
TOAST, JUNK, not even good as a prybar since it is a
casting. The position the cam is going to run can be
determined using "blueing" like for setting
differentials. Again; this is the field your head
rebuilder will deal with.
When
heads warp they can also sink and twist; all of this
can be surfaced out and then each valve can be
different in its new valve/cam geometry world. A shop
that deals with overhead cam engines may be able to
take on this task for you. There is more to it than
appears on the surface. Even a new cam is not a bolt
in unless your head has never been run before;
otherwise a new camshaft can present the same problems
as a regrind as to geometry problems. Sometimes a
reground cam with an altered base circle can work
BETTER than a new cam. So many variables. You can
always weld up every single journal and recut them to
bring back stock base circle; $$$$ or use new camshaft
$$$$ :-(
Read
up on overhead camshaft base circle info on the
internet; I've found some great discussions; the best
by far is a long paper that "Racer Brown" wrote that
is probably the bible of camshaft design; operation
and problems. I don't know if that was really his
first name; but it should have been! It is a must read
for anyone who ever gives the lumpy stick a second
look. Back in time when I thought I knew something
about camshafts I started reading his writings and I
soon realized I knew NOTHING...NOTHING. And I STILL
DON'T comparatively. I've had a few people far more
involved with cams than I am admit they too were blown
away by his writings.
BASE
CIRCLE CHANGES.
With a stock cam; even a reground one; base circle
problems are at a minimum. This usually comes into
play with a big change in the grind over what the cam
was designed for. Base circle refers to the diameter
of the shaft "under" the lobe; so to measure it on the
U20 cams you measure the cam diameter with the lobe
pointed straight up.
The
only thing we are dealing with here is the camshaft
portion of the equation; this would assume your head
was correct in all other aspects.
Keep
in mind there is a fairly large range of change that a
head can take before the cam lobe is in danger of
running off the rocker "pad". But just for
fun:
For
every .002 thousandths a cam's base circle has been
reduced below 1.300 or 1.305 maximum; add .0014 to the
button thickness to retain the position it was in.
This is kind of a crude method as any number of things
are probably already affecting the geometry as it is
(more than the cam); I am assuming a rough figure of
1.4 for the rocker ratio; but it is a starting point.
Different rockers will make different numbers; as will
using the old style rockers (which you shouldn't be
doing). It is possible to have 8 different thicknesses
needed to make all of them the same. Forget about head
problems; just the fact that many valve jobs are done
with any though to valve height is enough to throw
everything off. Still a lot of pushrod machinists out
there waiting for the OHC's to go away... ??? Assuming
you are striking for a middle of the patch wear
pattern; you can also adjust the valve clearance
(lash) up or down to get to that position (to check)
and start with that change in adjustment as a figure
to add on to your original pad to figure a likely
thicker pad (or sometimes thinner). I instead usually
use a set of feeler gauges I have chopped down to fit
in under the rocker arm end to get an idea of what I'd
need. You have to just go with what will work unless
you have time or a pleasant machinist who doesn't mind
taking thicker ones and milling them to different
thicknesses!
I've
also been in discussions where someone has said just
start with half the base circle reduction; but this
makes no sense to me because of the rocker arm ratio.
Everybody's got an opinion! I've never had it work out
that easy when playing with one of these. And of
course the rocker arm ratio of "1.4" may be correct
for the overall lift; but when discussing it in
relation to geometry; the ratio varies as the rocker
moves; probably from 1.25-1.75:1? Speaking of Bob
Sharp; his engine team had some ideas that seem almost
opposed to what so many people accept as how it is.
You can watch a couple of hours disappear staring at
their manual with a U20 head in front of you to see
how it all goes together. I'm just saying once you go
away from near stock conditions it gets complicated;
and once you want more than average horsepower and
torque it gets complicated.
At
one time I had a set of springs; I think they were
from some fuel pump; that worked perfectly as fake
valve springs; I could install them on a head and
install the rockers but still easily turn the cam to
do preliminary checking while generating enough
pressure to be able to check the position pattern of
the cam as it traveled over the rocker
arms.
Although
"correct" is having it run in the middle of the
rocker; having it more towards the adjuster side can
increase the lift a bit and I think that way the valve
timing is advanced a skosh. I'm going off of old
memories here; I'd have to have a cylinder head in
front of me to verify what I'm mumbling about. I don't
let the wipe pattern get within .025 of the end of the
available run area; 1/32 would be safer.
Like
I have said; the cam is usually the least of the
problem; we didn't have that many inquiries about it
the past decades; all kinds of nonsense gets spread
now with the internet; maybe it is just that. For
those of you that are just curious or of the black
helicopter club; we are going to begin listing the
base circle measurements of cams we have for
sale.
There
is "right" and there is "wrong"; but I have seen many
engines WAY off from being "right" that produce
amazing power and have shown to be durable.
Even
adjusting the lashpads and checking wipe patterns is
not as far as it could go to maximizing power and
minimizing friction; that is reached so geometry is
adjusted so that the center tip of the rocker arm
engages the lashpad center and 1/2 valve lift. Details
like this are why engines can make 15-20% more power
in the right hands; all other items being unchanged.
If
you are going to be having or have rocker arm geometry
problems I do not suggest reground rockers. We
typically have both new and reground rockers. Reground
rockers can sometimes aggravate the problem; sometimes
not. Even the NEW rockers can vary in size and shape.
The reground rockers are cut the bare minimum to make
them serviceable though...and that $400 for new
rockers really cuts into the bowling money.
Actually
solving cam geometry problems can be a very lengthy
task and discussion. The causes are myriad and
different head / cam conditions can make it tough!
Your cam supplier should be going over this with you.
We have enough trouble keeping up with taking care of
our own customers to be doing after sale work on
someone else's job. A shop that deals with overhead
cam engines may be able to take on this task for you.
There is a lot to it.
