Crankshaft degrees After Bottom Dead Center.
See: “SEAT DURATION.”
Crankshaft degrees After Top Dead Center.
AREA UNDER THE CAM LIFT CURVE:
The area under the bell-shaped lift curve is
depicted with lift drawn on the vertical axis and degrees of crankshaft rotation
on the horizontal axis. The greater
the area under the lift curve, the greater is the lift and/or duration at some
point on the cam lobe profile.
The round portion of the cam lobe, concentric with
the cam axis, where lobe lift is zero and valve lash adjustments must be made.
This portion of a lobe is also called the heel.
Crankshaft degrees Before Bottom Dead Center.
Crankshaft degrees Before Top Dead Center.
This is the maximum distance that the cam pushes
the follower when the valve is fully open.
Cam lift differs from valve lift.
See “GROSS VALVE LIFT.”
After the design of a new cam is computed, its
dimensions are transferred to a precision template called a master.
The master is then installed in the cam- grinding machine to generate the
shape of the lobes onto the finished cam.
The finished shape of a cam lobe.
Its segments are the base circle (or heel), a short opening clearance
ramp, the opening flank, the nose, the closing flank, and a short closing
clearance ramp onto the heel again.
See further entries for each segment, and also “LOBE TAPER.”
The rate of change in lobe lift per degree of cam
rotation, either positive (while opening) or negative (closing).
The highest velocities occur on cam flanks, the slowest velocities on the
ramps. Velocity is rendered visual
on a lift graph, as the slope of the curve at any point.
A shaft containing many cams (or lobes) that
convert rotary motion to reciprocating (lifting) motion in an internal
combustion engine. For every two
revolutions of the crankshaft, the camshaft(s) rotates one revolution.
The lobes on the camshaft actuate the valve train in a phased relation to
piston movement. The camshaft
determines when the valves open and close, how long they stay open, and how far
they open. A complete camshaft also
contains journals (mains) that the shaft rotates on and a fixture at one end for
the drive gear, plus (depending on the engine) perhaps a distributor/oil pump
drive gear, fuel pump eccentric, tachometer drive, and/or oiling holes.
Gas carburizing is a heat-treatment process for
steel camshaft billets. In this
procedure, the camshaft is placed in a furnace with a carbon-gas atmosphere and
heated to a specific temperature.
After the camshaft surface has absorbed a desired amount of additional carbon,
it is removed from the furnace and quenched to attain the proper temper.
A term to describe a camshaft that is made from a
casting. The material for the
casting is a special grade of iron alloy called “Proferal”, which is used
primarily for non-roller camshafts because of its excellent anti-wear
See: “IMPROVED STOCK CAMS.”
CHILLED IRON LIFTER:
A cam tappet/lifter made from high-quality iron
alloy that is heat-treated during its casting.
Molten iron is poured into a honeycomb mold with a chilled steel plate at
the bottom, to quench and so heat-treat the face of the lifter.
This type of tappet is compatible only with steel and hardface overlay
Two short segments of the cam lobe, between the
base circle and either flank. Ramps
change the lift at a constant, slow speed, in theory to compensate for small
deflections and slack in the valve train.
The opening ramp takes up all clearance in the valve train and brings the
valve to the verge of opening. The
closing ramp sets the valve on the seat, and ends when the tappet returns to the
base circle. Ramp designs have
extraordinary effects on power output and valve train reliability.
A valve spring that has been compressed to the
point where there is no space between the coils, where the coils are stacked
solid, is in coil bind. The valve
cannot open any further from this point
In machining, having the same center, or running
true. I camshaft terminology, the
cam bearing journals and lobes are concentric with each other when the camshaft
is straight, and there is .001” or less runout between all the cam lobes and
DURATION AT .050”
The degrees of crankshaft rotation from when the
valve is lifted open .050” until it is .050” from closing.
A heat-treating process in which a camshaft is
exposed to an open flame and then quenched (cooled) in oil.
The two sides of each cam lobe face, the segments
that lie between the nose and the clearance ramps before the base circle.
Specifically, the process of shaping cam lobes on a
specialized cam-grinding machine. In
gearhead slang, the general profile of a cam independent of application, as in
“drag race grind” or “Street Hemi grind.”
GROSS VALVE LIFT:
A nominal total valve lift measurement estimated by
multiplying the highest cam lift by the rocker arm ratio.
This cannot actually be attained in a running engine, due to valve lash
or hydraulic lifter bleed-down, plus clearances and flex in the valvetrain.
Production tolerances in rocker arms further vary this figure by as much
as .015” plus or minus. See: “NET
HARDENABLE IRON LIFTERS:
A cam follower made from a special high-quality
iron alloy that is compatible with cast iron billet camshafts.
The entire cylindrical body of a hardenable iron lifter is hard, in
contrast to a chilled iron lifter with only its base hardened.
By engineering definition, resistance against
penetration. For cams and lifters,
hardness is directly related to resistance against wear.
A typical iron cam registers in mid- to high-40s on the Rockwell C scale
(Rc). Compatible lifters register
five to ten points higher. The
smaller part, running hotter, must be harder to equalize wear between parts.
A number of heat-cycle processes to alter the
surface hardness of a metal. The
temperature of the part is raised to less than its melting point, held there for
a specified time, then allowed to cool at a specified rate to nearly room
temperature. This heat cycle alters
the crystal and grain structures of the metal, which affect its hardness.
Some processes also diffuse carbon or nitrogen atoms into iron surfaces.
All flat and roller tappets are heat treated.
Some iron cams (mostly British) are heat-treated after re-grinding, to
regain surface hardness. See:
“CARBURIZING, CHILLED IRON LIFTER, FLAME HARDENING, HARDNESS, INDUCTION
HARDENING, NITRIDING, and QUENCHING.”
See: “BASE CIRCLE.”
HYDRAULIC VALVE LIFTERS:
Lifters with an inner mechanism fed by engine oil,
designed to maintain constant zero lash in the entire valve train.
Their advantages include quieter engine operation and elimination of
periodic adjustmentS to maintain proper lash, as required with solid valve
lifters. Hydraulic lifters do,
however, maintain a constant pressure against the camshaft, which solid lifters
do not. Therefore the anti-scuff
additives in lubricating oils are more essential with hydraulic lifters.
IMPROVED STOCK CAMS (CHEATER CAMS):
An improved stock cam has stock lift and duration,
but the flanks are modified so that they are faster acting.
Such a process increases the area under the lift curve by about a 5%.
That means that there will be a power increase across the entire rpm
range of the engine. This type of
custom grind works very well in engines with fuel injection systems that
calibrate off manifold vacuum, which are therefore very sensitive to changes in
An electrical heat-treating process in which a
ferrous part is placed inside a coil of heavy wire through which a
high-frequency current is passed.
Through the electro-magnetic phenomenon of induction, energy transfers from the
coil to the part. The part inside
the coil becomes cherry red almost instantly, and is then quenched.
The quench medium is either water (for large parts, like cam billets) or
oil (for small parts, like needle bearing rollers, to forestall cracking).
There is a slight press fit between dual valve
springs, if the outside diameter of the inner spring and inside diameter of the
outer spring approximate each other.
The slight friction between them produces a damping effect on spring vibration
and surge. See: “SPRING SURGE.”
LASH (VALVE LASH):
Valve train clearance, usually measured at the
valve tip, opens up necessary clearance between the base circle of the camshaft
lobe and a solid camshaft follower or tappet, so that the valve is certain of
closing and staying closed when the tappet is on the lobe heel.
After installing a camshaft in a block or head, a
mechanic can plot the lift of the cam in relation to each degree of camshaft
rotation. Install a dial indicator
on the cam follower or tappet and a degree wheel on the crankshaft.
Rotate the crankshaft in five-degree steps, and take a lift reading from
the dial indicator at each interval.
Then plot the readings on graph paper, with cam lift on the vertical axis and
degrees of crankshaft rotation on the horizontal axis.
LIFTER PRINT (CAM PRINT):
The amount of travel that a rotating cam lobe makes
across the lifter face. Since the
cam lobe must not run off the edge of the lifter, lifter diameter determines the
maximum usable flank velocity for a cam.
See: “CAM VELOCITY.”
Each lobe face is eccentric to the cam journals and
transmits a lifting motion through the valve train to operate the valves.
The design of the lobe determines the usage of the camshaft, e.g. street
use or all-out competition.
The difference measured in cam degrees between the
centerline of the intake lobe (its point of highest lift) and the centerline of
the exhaust lobe in the same cylinder.
The point on each lobe, measured in crank degrees
from TDC, at which the valve is most fully open.
For one example, take a cam that measures full intake lobe lift at 110
degrees ATDC and full exhaust lobe lift at 110 degrees BTDC.
This camshaft was ground with 110-degree lobe centers and is ground timed
“straight up”, neither advanced nor retarded.
As another example, a different cam measures full intake lobe lift at 105
degrees ATDC and full exhaust lobe lift at 115 degrees BTDC.
This camshaft was also ground on 110-degree lobe centers, but it is
advanced by five crankshaft degrees.
The small amount by which one side of a flat-tappet
lobe is larger than the other, even though both follow the same profile.
Cams carry taper left or taper right and zero to .003” taper, depending
on the engine. The direction and amount of taper is measured best across the
diameter of the base circle. Hold
the front of the cam to your left (as a cam-grinding machine does).
If the forward (left) side of the lobe is larger, that is taper-left
(TL). If the rearward (right) side
of the lobe is larger, that is taper-right (TR).
All lobes should measure with the same amount of taper, but not
necessarily the same direction. TR
pushes cams into the block, off the angled pressure from tappets.
Engines with TL, mixed tapers, or roller tappets (with no taper) require
a cam thrust plate. Lobe taper works
with tappet crown and tappet bores offset from lobes bores to drive flat tappets
into rotation. See: “Tappet
Crown, Tappet Rotation.”
Cylindrical surfaces on a camshaft, concentric with
the camshaft axis, which ride in bearing surfaces to support the camshaft in an
engine block or head(s).
NET VALVE LIFT:
The probable lift of the valve, determined by
subtracting the valve lash dimension from the gross valve lift figure.
But production tolerances in rocker arms can vary this figure by as much
as .015” plus or minus. So can poor
geometric designs of rocker arms, e.g. in Chrysler Slant 6, Ford FE, and many
aftermarket rockers. So does flex in
the valve train, especially pushrods.
See: “GROSS VALVE LIFT.”
Gas nitriding is a surface heat treatment that
leaves a hard case on the surface of an iron cam.
The hard case is up to .010” deep and is typically twice the hardness of
the core material. The process is
accomplished by placing the cam into a sealed chamber filled with ammonia gas,
and heating it to approximately 950 degrees F (510 degrees C).
At this temperature, a chemical reaction occurs between the ammonia and
iron of the cam to form ferrous nitride on the surface of the cam.
As the reaction progresses, ferrous nitride diffuses into the cam core to
a case depth of approximately .010”.
The nitriding process is done at relatively low heat (as heat treatments go), so
the core material loses no hardness.
Also, the chamber temperature is raised and lowered slowly, so that the cam is
not thermally shocked, which would create internal strains.
Gas nitriding was originally intended for where sliding motion between
two parts occurs repeatedly, so it is therefore directly applicable to solving
camshaft wear problems. Ferrous
nitride is a ceramic compound, which accounts for its hardness.
It also has some lubricity when sliding against other parts.
NOSE OF THE LOBE:
The portion of a cam lobe highest from its base
circle, activating full lift position.
Overhead Cam engines.
In this type of engine the cam is positioned in the head, above the
valves. (e.g. Porsche 944 engine)
OHV (PUSHROD ENGINES):
Overhead Valve engines.
In this type of engine, the camshaft is positioned in the block, beneath
the valves in the head. (e.g.
Chevrolet 350 c.i.d. V8 engine)
The angle in crankshaft degrees when both the
intake and exhaust valves are open at the same time.
This occurs when the piston is near Top Dead Center on the exhaust
stroke. The greater the seat-to-seat
duration on the intake and exhaust lobes and the less their lobe centers-cam,
the greater the overlap will be in crankshaft degrees.
See “LOBE CENTERS-CAM and SEAT DURATION.”
A thermo-chemical surfacing process, whereby a
nonmetallic, oil-absorptive coating is applied to the outside surface of the
camshaft except on the mains (journals).
The lubricity of this coating permits rapid break-in of cam lobes without
scuffing. (The Parker family
developed this process in NJ in the 1920s.)
See “CAST BILLET.”
The final stage in ferrous heat-treating processes
is cooling the workpiece nearly to room temperature.
The rate of cooling affects the resulting microstructure in metals, and
from that either hardness or toughness.
Quenching cools the fastest and generates the hardest surface.
The quenching liquid is often water for large parts, but must be oil for
very small parts (like needle bearing rollers) to prevent them cracking from
ROCKER ARM GEOMETRY:
Any OHV rocker arm must comply with geometric law
to transfer the lift curve of the cam properly into the valves and valve
springs. The rocker’s three
centroids (centers of motion) must lie in a straight line: its contact cone with
the pushrod tip, its axis of rotation, and either the axle of a roller tip or
the center of a circle extended from a
curved plain tip. Centroids not in
line transfer cam action non-linearly, with unpredictable effects on ranges and
motions of valves and springs.
A roller tappet performs the same end function as a
mechanical or hydraulic flat tappet.
But instead of sliding on the cam face, this lifter contains a roller bearing at
its cam face that rolls over the cam surface.
Moderate mechanical wear between two metallic
surfaces sliding on each other, if and when they touch because of insufficient
oil to keep them fully separated.
SEAT DURATION (SEAT-TO-SEAT DURATION):
The total time in degrees of crankshaft rotation
that a valve is off its valve seat, from when it opens to when it closes.
This is usually termed by production cam vendors “advertised duration”,
to distinguish it from duration at .050” lift.
The point near TDC on the exhaust stroke when both
the intake valve and exhaust valve are off their seats at the same time by the
Valve springs have a tendency to lose part of their
compressive strength after being run in an engine for certain periods of time,
because of the huge cyclic stresses they are under.
At 6000 rpm, for example, each spring must cycle 50 times per second.
The immense heat generated by such stresses eventually counteracts some
of the heat-treating of the spring wire, causing the springs to take a slight
drop in pressure (a "set”).
A dynamic reaction that causes unpredictable valve
spring behavior at high rpms.
At rapid reciprocating frequencies, forces put a
spring coil in motion, but then its own inertia KEEPS the coil in motion.
So waves of compression and rarefaction pass rapidly up and down the
stack of spring coils. Moreover, at
certain rpms the cam activates the spring at the spring’s natural vibration
frequency, and the waves intensify.
At such critical speeds, this surge effect substantially reduces the static
(rated) spring strength available to operate the valve train.
Clearances between parts widen.
Valve motion becomes uncontrolled.
A cylindrical component, either (nearly) flat-faced
or roller-equipped, that rides on each cam lobe to transfer the lift action of
the lobe to the rest of the valve train.
One alternate name is lifter, and another term is follower.
Follower is an all-inclusive term for tappets, rocker-type or finger
followers on some OHC engines, and cup-type followers on most OHC engines.
Rocker-type (or finger) followers slide across cam lobes, flat tappets
slide across cam lobes and must also rotate in their bores, and roller tappets
roll over lobes but do not rotate in their bores.
Because of geometric differences in how each type of follower rides a cam
lobe, each uses a different cam profile to generate the same
A small amount of spherical crown ground on the
faces of most nominally “flat” tappets, to prevent the edge of the tappet from
riding off the edge of a tapered cam lobe.
The amount of crown is determined by the amount of lobe taper, both being
set by the engine manufacturer.
Normal tappet wear occurs as a “donut” partway off-centered on the face.
Wear near the edge indicates a tappet with too little crown for that cam.
Loss of specified tappet crown indicates a worn cam.
Flat tappets must rotate in their bores, to
continually present a fresh part of their face against a rotating cam, and thus
equalize and minimize wear. (In
contrast, roller tappets must NOT rotate at all.)
Rotation is driven by offsetting the lifter bore from the center of the
lobe face, lobe taper in the same direction as lifter offset, and lifter crown
to match lobe taper. Before
installing tappets, check that each lifter is free to rotate in its bore, and
apply only engine oil (not sticky “cam lube’) on the sides of lifters.
If a flat tappet does not rotate sufficiently, or at all, that lifter and
cam lobe will wear out prematurely, perhaps as soon as break-in.
A man-made motor oil additive essential for dry
lubrication between camshafts and flat tappets, where extreme pressure squeezes
away all the oil molecules. ZDDP*
circulates with oil in 800-1200 parts-per-million, until it is crushed within a
cam-lifter interface and plates itself into the iron surfaces.
Progressive reduction in percentages of ZDDP in motor oil since 2001 is
causing premature wear in many high-performance street cams.
(*Zinc dialkyl dithiophosphate.)
See: “HYDRAULIC VALVE LIFTERS.”