According to the factory shop manuals, the cams look like this:
for the years '68, '69, '70
IO 18 BTDC
IC 114 ABDC
EO 71 BBDC
EC 58 ATDC
intake lift .440
exhaust lift .454
ln '71 the opening and closing numbers given make no sense. lt
must be a misprint. But the lift numbers match the '68 - '70.
'72 and '73
IO 34 BTDC
IC 100 ABDC
EO 78 BBDC
EC 58 ATDC
intake lift .490
exhaust lift .490
'74 and '75
IO 21 BTDC
IC 111 ABDC
EO 73 BBDC
EC 55 ATDC
intake lift .457
exhaust lift .473
l don't have access to a manual from '76, but l doubt it's any
different than a '75. The shop manuals make no mention of any
difference in cam specs between the 472 and the 500. Nor do they
make mention of any difference in cam specs between the
Eldorado and anything else, much though the dealers liked to
claim the Eldorado was designed for more "performance",
whatever that means. The carburetors were calibrated a little
differently, but not enough to account for that claim.
The shop manuals state that these cam timing numbers were
taken at .001 of lift, which does not compare to anything today. lt
makes the duration numbers look very long. Using a degree wheel
and a dial gauge, l measured a stock '68 cam. At .001 lift my
measurements match the shop manual specs, within production
tolerances. l also measured it at .006 and .050 of lift. Here is what
it looks like:
at .006
IO 12 BTDC
IC 78 ABDC
EO 62 BBDC
EC 20 ATDC
intake dur. 270
exhaust dur. 262
at .050
IO 17 ATDC
IC 36 ABDC
EO 34 BBDC
EC 13 ATDC
intake dur. 199
exhaust dur. 227
The exhaust lobe is about normal for a stock cam. But the intake
lobe is very strange. At the .050 level, it doesn't open until 17
AFTER TDC (l didn't mistype that). That is extremely late. lt badly
chokes the cylinder for air and fuel on the intake stroke. The 36
degree closing figure doesn't look too bad, but at .006 it's 78
degrees ABDC, which is also very late. Late opening and late
closing on an intake lobe really clobber low rpm torque. The cam
specs for all 472/500 engines reflect these same characteristics,
as do the cam specs for the 390/429 engines from '63 to '67. And
the cars these engines were in always felt sluggish and
unresponsive at low speeds under part throttle acceleration. These
stock cams are absolutely terrible. Even if you're rebuilding an
engine stone stock, use Al's MT3 cam; it will really wake up these
engines compared to the way the factory delivered them by letting
them breathe better.
Other seemingly unrelated side effects of this bad cam are that it
screws up the fuel curve in the carburetor. The fuel curve can't be
made correct under all throttle positions and rpms, no matter
what is done to the carburetor. l've tried it, and it's impossible.
Only when a well designed cam is used can the carburetor operate
the way it was designed to and provide a correct fuel curve under
all conditions. The other side effect of the bad stock cam is that it
makes the transmission shift weird, which is really only noticeable
if a shift kit has been put in it. Right before a shift when rpms are
high the power starts to feel pretty good, but after the shift the
power output drops so far that it makes the transmission feel like
it lurches and slams into gear. But put in a well designed cam and
suddenly the transmission shifts completely differently.
Transmissions from Cadillac were calibrated to shift very softly
and it nicely covers up the power output drop from one gear to
the next. But the cars are left feeling very lethargic.
---
Those of you who have been trying to calibrate a carburetor on
top of a stock motor and haven't been able to get the results you
want, and those of you who put a shift kit in a transmission
behind a stock motor and didn't like the way it shifted, you know
what l'm talking about when l describe these strange side effects.
l have a '68 Cadillac convertible that l had for many years thought
was sluggish and unresponsive at part throttle. l'd step down on
the gas, l could hear the engine load up, and not much would
happen. l had the impression the engine couldn't breathe very well
and blamed it on the carburetor. l had been using an air/fuel ratio
gauge and a vacuum gauge to calibrate my carburetor and for two
years l chased around a bad spot in the fuel curve. lt was either
too rich somewhere or too lean somewhere else and no matter
what l did to the carburetor l couldn't get it straightened out.
Frustrated, l put the carburetor on another car on which l also had
an air/fuel ratio gauge installed and suddenly the carburetor was
fine. l put the other car's carburetor, which had been fine, on this
one, and suddenly it had the same problem. l was amazed to
realize the problem was not with the carburetor but it still took a
while to figure out what it was. l had also noticed a habit of
stepping down farther on the gas just after each upshift of the
transmission to compensate for the car feeling like it was slowing
down, but l still blamed this on the carburetor not flowing the
right amount of fuel when the air velocity through it changed.
Some 15 years ago l tried a shift kit in the transmission. l used
B&Ms kit at the stage one level. The transmission shifted so rough
and harsh that it wasn't long before l took it out.
About four years ago l finally got around to putting together a 472
with an MT10 cam l had bought from MTS about 10 years ago.
When the block was on the stand was when l measured the specs
of the stock cam and also the MTS cam. When l got it together and
in the car, l realized how much differently the engine ran, even
with an as yet unmodified carburetor and transmission. The
engine didn't feel sluggish at all; it felt like it had unlimited
energy. l could see on the air/fuel ratio gauge that the fuel curve
was operating completely differently, and it didn't take very long
to calibrate the carburetor and get what l wanted out of it. l
modified the transmission to shift even more firmly than B&Ms
stage one. lt's still comfortable and l may yet go even farther with
that. Just after the shift at the lower rpms the engine still pulls just
as hard as it did right before the shift, rather than feeling like it
slows down, and this completely changes the feel of the shift. The
difference is incredible. l realized that it was getting rid of the
stock cam that made all the difference. l don't know what the idea
was behind the intake lobe design. lt's very strange. lt doesn't
make any sense even from an emission controls standpoint. The
stock cam does nothing but cripple the motor badly.
One might think at first that a late opening and closing intake lobe
just means that the cam is ground several degrees retarded, but
that's not the case here. The point of peak valve lift is in a fairly
normal place, and it's not anywhere near the midpoint of the valve
opening and closing numbers. The opening ramp is late and fast
and the closing ramp is late and slow, which makes a lopsided
cam lobe, about like an egg that's flat on one side. lf the opening
and closing ramps had been designed correctly, the stock cam
probably wouldn't be too bad. The cams in the 390/429 motors
from '63 to '67 are the same way, which is why l asked Al in an
earlier post if he had considered making cams for that engine.
Give those poor engines some help!
Any well designed aftermarket cam from any manufacturer will fix
the problems created by the stock cam. l like Al's MT series the
best because they have appropriate valve timing figures, and they
have the most lift for a given amount of duration compared to
some others l've looked at. Even the small MT3 cam will run better
than the stock one and it will idle as smooth as glass because the
duration is short and valve overlap area is so small.
l wrote all that to say that those of you who have been driving a
'68 to '76 Cadillac (or even a '63 to '67) with a stock motor can
relate to the car feeling like this, even if you haven't realized there
was really a problem, and even if you have never tinkered with the
carburetor or the transmission. lt's easy to blame the slow feel of
the car on its weight. Those who talk about this engine having
good performance even in stock form, at the risk of being
insulting, don't know much about performance and were probably
used to the power levels of a smallblock Chevy. The only reason
the Cadillac engine had any performance at all stock is because of
its massive displacement. To those who have never driven one of
these engines in stock form, all this information is fairly worthless.
You put in an aftermarket cam and get all the benefits without
even realizing all the problems you eliminated in the process.
My suggestion is to pull the stock cam out, tie it in a knot (cartoon
style), and toss it in a creek, even if you don't like modifying
Cadillacs and want leave the rest of the car stone stock.
l also did all the other usual stuff to warm up the engine. Next
summer l want to put it on a chassis dyno and see just what it's
making. We have one in town where l live. Yes, l have been
rewarded by having a rather fast '82 Coupe. ln the stoplight wars
no one ever even inches ahead of me unless l'm not trying. The
worst that ever happens is that someone keeps up with me, but
even that is uncommon. Usually they get a good look at the
taillights of a Cadillac.