[DeTomaso] Vacuum Advance - ported or full vacuum source

[Daniel C Jones]

> It can effect your base timing which effects where you end up.

The Mallory Unilite has positive internal stops that will limit
maximum centrifugal advance.  By setting them, you can run the
typical 14 degrees initial advance without over-advancing at
higher RPM.  Depending upon open or closed chamber heads,
displacement and other factors the best maximum advance (less
cruise vacuum advance) can be anywhere from as little as 28 degrees
(e.g. 408C with closed chamber heads) to more than 40 degrees
(e.g. open chamber 351C-4V).  The rate of mechanical advance is
set by the springs and weights.  Mallory makes a tuning kit for
that.  I've included some information from previous posts below.

Dan Jones



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> Using a Snap-On, adjustable, digital timing light, My initial timing is at 27
> degrees. I seem to remember that the Mallory Unilite distridutor (no vaccum)
> has 24 degrees mechanical advance. The engine seems happy and will verify
> when I get it out on the road. Do these numbers seem within reason?

Does your Snap-On read out in distributor advance or crankshaft advance?
Distributor advance is half of crankshaft (measured advance) because the cam
turns at half the speed of the crank.  The crank goes around twice to fire
all eight cylinders.  Unless your cranking compression is low, 27 degrees
initial should kick back on the starter when hot.  Mine does it at 18
degrees initial.

In the absence of a dyno where you can record power versus timing at various
RPM, you'll want to diddle the centrifugal advance until it just barely
pings at WOT, all the way from idle to redline.  Then back the static off
until it doesn't ping any more.  If it's hard to start, you might have
to diddle some more to reduce the static advance and get more timing
when it's running.  It'd be nice to have some sort of scope or readout
to see what the different bits are doing, but we shade-tree types just
have to do it be cut and try.

Once you have the centrifugal curve set, you want as much vacuum advance
as you can get at freeway cruise, without surging.  Most OEM-type canisters
only give 4-8 degrees of advance.  An SBF or SBC can usually take 20 or so.
Though there is a theoretical "Maximum Best Timing", where power falls off
when you go past that point, the point has always been beyond the ping limit
of most American V8's I've played with.

Centrifugal advance is a curve (degrees of advance versus RPM).  Setting
the initial advance only shifts a given curve.  It does not alter the
shape.  At some points, the engine's going to need more advance, at some
points less.  You don't want to just back it off until it's all below the
ping limit or you'll lose a bunch of power, throttle response and fuel
economy (it might even run hotter than desired).  You have to adjust the
curve to what the engine needs at worst case, which is WOT.  To find the
curve you need, keep adding advance until it pings at a given RPM, then
back off.  Note the advance at that point.  Pick another RPM and repeat
the process several times until you have a cut at the curve you need.
Then play with the distributor cam, weights, and springs, until you
approximate the curve.  You're not going to get it perfect; the best you
can do, is cross over the ping line a few times.  Then you back off the
static until it doesn't ping any more.  If you're lucky, the static will
be low enough the engine will start when hot (kick back on the starter).
If not, you need to either tweak the curve until you can back the static
off some or add a start retard device.

It's not a fun job.  On a Unilite, the mechanical advance mechanism
is buried below the optical pick up.  To alter the curve, you need to
change springs, as well as adjust the mechanical stops that limit the
maximum advance.  To set the limit, you remove the Unilite plate and
vacuum advance mechanism. if so equipped.  There are two allen head set
screws that screw into the side of the distributor housing to retain the
Unilite plate.  They are quite small so look closely for them and make
sure not to loose them.  Once the plate is out of the way, there will be
two stops, 180 degrees apart.  You loosen the screws holding the stops
and then insert a the special Mallory stepped plastic "key".  Each step
is marked and corresponds to 2 degree advance increments.  Insert the key
to the desired stop, push the stops in and tighten the retaining screws.
Mallory has a chart that that I've included below that shows the proper
springs to use with the advance limits you've chosen.

Racers operating mostly over, say, 3000 or 4000 RPM can just pick an
arbitrary advance and lock the distributor there.  Some dirt track
guys will add a device to retard the advance to kill power when conditions
get slippery (a very primitive traction control).  These race engines may
detonate horribly at mid-range or below but they're never driven under load
there; just long enough to make it up to racing revs.  Drag racers sometimes
just lock the advance and put up with it, hoping they don't blow a rod
through the side of the block.  Not good for a street engine so you need
a good curve.

MSD makes a timing computer for this sort of thing and Crane makes a
distributor with a number of pre-programmed curves that can be selected
externally.

Below I've attached a previous post on Mallory Unilte distributors that
may of some use.

Dan Jones

> Car has Mallory Unilite and big Mallory coil

Does the Unilite have a vacuum can?

> It was wired so that the power came thru the stock ballast and then to 2 more
> ballasts in series.

Too much ballast.  Mallory supplies a misleading instruction document that
can lead you to want to do that.  If you download the more detailed info from
their website or call them, they tell you different.  With multiple ballast
it will have a weak spark and may not keep the plugs clean.  A quick test
is to use one of the spark testers that look like a clear spark plug with a
grounding clamp.

> Mallory site says to use the stock ballast if the car came with one. Nasty
> wiring job with crimp on connectors. I rewired and soldered all the ring
> terminals. Only used the stock ballast. Seems to run fine.

There's a maximum voltage at the photoelectic eye (7 to 9 volts) that's safe.
Above that, the Unilite will work just fine but it will eventually fail.
Also, don't run solid core wires with the Unilite.  They will also eventually
kill the module.  Apparently the coil wire is the worst culprit and some coils
can leak back to the module when the ignition is switched off.  The module
will pick up noise and produce a poor trigger signal if the wires aren't
suppressed.  Also, the module is cooled by heat transfer to the aluminum
housing.  There is a thin layer of white dielectric grease between the module
and the housing that helps heat transfer.  The grease will eventually dry out
so it's a good idea to put in a fresh layer after a number of years.

The Unilite uses a three wire hook up.  The brown wire is grounded to the
engine. Make sure you also have a good ground cable/strap between the engine
and the body.  You can look at my car or I think Mike Daily has a write up
on panteraplace.com. Do not ground the brown wire from the module directly
to the frame or battery.  If you do, the brown wire will become a ground for
the starter during cranking which will melt the brown wire and take out the
module. The green wire connects to the negative side of the coil.  The red
wire can connect to the positive side of the coil but it's better to connect
it to the high side (12 volt) of the ballast resistor.  The ballast or
resistor wire is there to limit the current through the coil and the green
wire.  The red wire prefers to see full battery voltage.  If you connect
the red wire to the positive side of the coil (which is also the low side of
the ballast) the engine may not start as easy especially if the battery is
low.  If your car has a resistor wire rather than a ballast, you can either
hook the red wire to the positive side of the coil, or better yet, find a
place to hook it that will feed the red wire full battery voltage whenever
the key is on.

BTW, I run an MSD 6AL multi-spark box with my Unilite.  The Unilite is
basically just a points replacement module so don't try to run too wide a
gap unless you run an external amplifier.  If you run a spark box, the
wiring is different but Mallory has detail diagrams for those.  A ballast
or resistor wire is not needed when using an external box since all the
current for the coil goes through the box rather than the module.  When
used with an external box, the Unilite modules will last almost forever
because very little current goes through it.  MSD suggests you leave the
ballast or resistor wire in place so it can be hooked back up should the
MSD box fail.  I had that happen once and just used the MSD supplied
jumper and fired it up and drove home.  Now I carry a spare MSD.

Some additional info from a web source on how to properly check the Mallory
wiring:

I often hear people ask what voltage they should have at the coil when running
a Mallory module without an external box. The standard answer is about 7 to 10
volts measured at the coil with the engine running. However, while this is easy
to measure, it is not the best thing to measure since the voltage will vary
depending on the voltage output of the alternator. What really matters is the
total resistance of the primary side of the ignition. This includes the ballast
(or resistor wire) and the primary resistance in the coil (measured with an ohm
meter between the positive and negative terminals on the coil). The ideal total
resistance is a minimum of 1.4 ohms and a maximum of about 3.5 ohms. More than
3.5 ohms is safe for the module but the spark may be too weak for optimum
performance. Use an ohm meter to measure the resistance across the ballast and
add that to the amount of primary resistance in the coil. The total should be
between 1.4 and 3.5 ohms. You may need to measure the resistance of the ballast
after it has warmed up since some ballast resistors increase in resistance with
temperature. If your car has a resistor wire rather than a ballast it can be
difficult to measure the resistance in the wire as the wire gets lost in the
harness. If so, assume that the resistance in the wire is about 1.0 ohms which
is typical.

Some coils have so much primary resistance that a ballast resistor or resistor
wire is not necessary. Mallory has two new Promaster coils that have all the
resistance built into the coil (29450 and 30450). Using these coils, no
additional resistance is needed.

Having all the resistance built into the coil has drawbacks however. The coil
will run hotter which will reduce the life of the coil somewhat. Also, high
resistance coils are not recommended for use with external boxes as the coils
will overheat and fail (sometimes even explode!).

Don't use solid core plug wires with any electronic ignition.

If you weld on the car, disconnect the module before doing so.

Still with me?  Once we have the wiring verified, we should probably check
the advance curve with a dial back timing light (I've got one if you need it).
Do you have the stepped Mallory key that's used to set the stops?  If not, I
should have one of those too.  You may want to pick up a kit to curve the
distributor but that might wait 'til later.  The mechanical advance mechanism
is buried below the optical pick up.  To alter the curve, you need to change
springs, as well as adjust the mechanical stops that limit the maximum
advance.  To set the limit, you remove the Unilite plate and vacuum advance
mechanism, if so equipped.  There are two allen head set screws that screw
into the side of the distributor housing to retain the Unilite plate.  They
are quite small so look closely for them and make sure not to lose them.
Once the plate is out of the way, there will be two stops, 180 degrees apart.
You loosen the screws holding the stops and then insert the Mallory stepped
plastic key.  Each step is marked and corresponds to 2 degree advance
increments.  Insert the key to the desired stop, push the stops in and
tighten the retaining screws.  Mallory has a chart that that I've included
below that shows the proper springs to use with the advance limits chosen.

Advance spring selection instructions (thankfully typed in by Kirby Schraeder)

There are two basic advance curve configurations; the Straight-Line Curve
and the Hop-Out Curve. The Straight-Line Curve is used when a smooth linear
advance is required throughout the engine operating RPM. The Hop-Out Curve
is used when a lot of advance is required early and a more gradual advance
through the rest of the engine operating RPM.

1. The first step is to decide which curve will best suit your particular
application. This will all be determined by engine RPM operating range, type
of use and engine components used. For figuring Straight-Line Curve, use
steps 2 through 5. For figuring Hop-Out Curve, use steps 6 through 11.

2. In order to plot a Straight-Line Curve you must decide at what RPM you
want your advance to start and at what RPM you want your total advance to
finish. The curve in Figure 1 is an example, it starts at 1000 RPM and is
fully advanced at 2400 RPM. Plot your curve on the graph in Figure 3.

(Figure 1, 2 and 3 are all line graphs with the x-axis being engine rpm and
the y axis being degrees advance at the crankshaft.)

3. Mark off a 1000 RPM interval and count the number of degrees in that
interval. (The curve in Figure 1 of their chart has 19 degrees per 1000 rpm
interval because it starts at 1000 and ends up at 26 degrees advance at 2400
rpm. A little straight line y=mx+b math will give you the solution between
any other points.)

4. Using the Straight-Line Curve Chart, go down the Degree Column until you
come to the same number of degrees as you figured in step 3. The letters in
the Spring Column of what I sent previously will tell you which springs to
use. The springs they use in their example are A-B for 19 degrees.

The 'Hop-Out stuff' is a bunch more typing....

With quench heads, you'll not want to exceed 20 degrees total advance.
Initial will be around 16 degrees with a hot cam, with 10 at the distributor
(10 distributor = 20 crank degrees since the cam and distributor run at half
the crank speed) for a total advance of 36 degrees.

Mallory Unilite advance spring selection Table 1 shows:

Degrees             Springs

7 degrees or less     E-F
8 1/2                 E-E
9                     A-F
9 1/2                 B-F
10                    C-F
11                    D-D
12 1/2                C-D
13 1/2                C-C
14                    A-D
15                    B-C
16                    A-C
18                    B-B
19                    A-B
20                    A-A
20-26                 B
28 up                 A

Springs
A Red
B Blue
C Dark Green
D Bronze
E Black
F Light Green
G Orange
H Light blue
I Yellow
J Pink
K Silver
L White

If all this looks like too much work, there's always the MSD timing computer.
The timing computer locks out the distributor's mechanical advance and
replaces it with whatever you dial in.  To specify the curve (a line really),
you enter a start point, a maximum value, and a slope.  Added features are
20 degrees start up retard (for easy cranking on high compression engines)
and selectable high speed retard.  Can also be used with MSD's adjustable
initial advance knob doohickey, too.  It would make it very easy to dial in
the curve, then you could remove the unit and have the distributor re-curved
on a machine to match the computer curve.  You would lose the start-up and
high speed retard, though.

Dan Jones