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Tech Tips :: C3 Related Tech Tips :: Distributor Vacuum Advance Control units

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Distributor Vacuum Advance
Control units
Specs and facts for GM Distributors

by Lars
Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic
Auto Repair & Restoration
Broomfield, CO
Rev. B 8-19-02


I’ve been seeing a lot of discussion and
questions regarding distributor vacuum advance control units; what do they
do, which ones are best, what was used on what, etc., etc. To clarify
some of this, I thought I’d summarize a few facts and definitions, and
provide a complete part number and specification listing for all vacuum
advance control units used by Chevrolet on the points-style distributors.
I’m also providing a listing of the specs for all other GM (non-Chevrolet)
control units, but without the specific application listed for each (it
would take me a bit too much time to research each part number by
application across each of the GM Motor Divisions – it took me long enough
to compile just the Chevy stuff…!). This latest revision to this paper
also includes the HEI listings (the HEI distributors use a longer control
unit, so the non-HEI and HEI vacuum advance control units CANNOT be
interchanged).

As always, I’m going to include the disclaimer that
many of these are my own comments and opinions based on my personal tuning
experience. Others may have differing opinions & tuning techniques from
those presented here. I have made every attempt to present factual,
technically accurate data wherever possible. If you find factual errors
in this information, please let me know so I can correct
it.

Background
The vacuum advance control unit on the
distributor is intended to advance the ignition timing above and beyond
the limits of the mechanical advance (mechanical advance consists of the
initial timing plus the centrifugal advance that the distributor adds as
rpm comes up) under light to medium throttle settings. When the load on
the engine is light or moderate, the timing can be advanced to improve
fuel economy and throttle response. Once the engine load increases, this
“over-advance” condition must be eliminated to produce peak power and to
eliminate the possibility of detonation (“engine knock”). A control unit
that responds to engine vacuum performs this job remarkably
well.

Most GM V8 engines (not including “fast-burn” style heads),
and specifically Chevys, will produce peak torque and power at wide open
throttle with a total timing advance of 36 degrees (some will take 38).
Also, a GM V8 engine, under light load and steady-state cruise, will
accept a maximum timing advance of about 52 degrees. Some will take up to
54 degrees advance under these conditions. Once you advance the timing
beyond this, the engine/car will start to “chug” or “jerk” at cruise due
to the over-advanced timing condition. Anything less than 52 degrees
produces less than optimum fuel economy at cruise speed.

The
additional timing produced by the vacuum advance control unit must be
tailored and matched to the engine and the distributor’s mechanical
advance curve. The following considerations must be made when selecting a
vacuum advance spec:

How much engine vacuum is produced at cruise?
If max vacuum at cruise, on a car with a radical cam, is only 15 inches
Hg, a vacuum advance control unit that needs 18 inches to peg out would be
a poor selection.

How much centrifugal advance (“total timing”) is
in effect at cruise rpm? If the distributor has very stiff centrifugal
advance springs in it that allow maximum timing to only come in near
red-line rpm, the vacuum advance control unit can be allowed to pull in
more advance without the risk of exceeding the 52-degree maximum limit.
If the engine has an advance curve that allows a full 36-degree mechanical
advance at cruise rpm, the vacuum advance unit can only be allowed to pull
in 16 more degrees of advance.

Are you using “ported” or “manifold”
vacuum to the distributor? “Ported” vacuum allows little or no vacuum to
the distributor at idle. “Manifold” vacuum allows actual manifold vacuum
to the distributor at all times.

Does your engine require
additional timing advance at idle in order to idle properly? Radical cams
will often require over 16 degrees of timing advance at idle in order to
produce acceptable idle characteristics. If all of this initial advance
is created by advancing the mechanical timing, the total mechanical
advance may exceed the 36-degree limit by a significant margin. An
appropriately selected vacuum advance unit, plugged into manifold vacuum,
can provide the needed extra timing at idle to allow a fair idle, while
maintaining maximum mechanical timing at 36. A tuning note on this: If
you choose to run straight manifold vacuum to your vacuum advance in order
to gain the additional timing advance at idle, you must select a vacuum
advance control unit that pulls in all of the advance at a vacuum level 2”
below (numerically less than) the manifold vacuum present at idle. If the
vacuum advance control unit is not fully pulled in at idle, it will be
somewhere in its mid-range, and it will fluctuate and vary the timing
while the engine is idling. This will cause erratic timing with
associated unstable idle rpm. A second tuning note on this: Advancing
the timing at idle can assist in lowering engine temperatures. If you
have an overheating problem at idle, and you have verified proper
operation of your cooling system components, you can try running manifold
vacuum to an appropriately selected vacuum advance unit as noted above.
This will lower engine temps, but it will also increase hydrocarbon
emissions on emission-controlled vehicles.

Thus, we see that there
are many variables in the selection of an appropriate control unit. Yet,
we should keep in mind that the control unit is somewhat of a “finesse” or
“final tuning” aid to obtain a final, refined state of tune; we use it to
just “tweak” the car a little bit to provide that last little bit of
optimization for drivability and economy. The vacuum advance unit is not
used for primary tuning, nor does it have an effect on power or
performance at wide open throttle.

With these general (and a little
bit vague, I know…) concepts in mind, let’s review a few concepts and
terms. Then it’s on to the master listing of specs and
parts…..:

Part Number
There are many different sources for these
control units. Borg Warner, Echlin, Wells, and others all sell them in
their own boxes and with their own part numbers. Actually, there are very
few manufacturers of the actual units: Dana Engine Controls in
Connecticut manufactures the units for all three of the brands just
mentioned, so it doesn’t make much difference who you buy from: They’re
made by the same manufacturer. The part numbers I have listed here are
the NAPA/Echlin part numbers, simply because they are available in any
part of the country.

ID#
Every vacuum advance control unit built
by Dana, and sold under virtually any brand name (including GM), has a
stamped ID number right on top of the mounting plate extension. This ID,
cross referenced below, will give you all specifications for the unit. So
now, when you’re shopping in a junkyard, you’ll be able to quickly
identify the “good” vs. the “bad” control units.

Starts @
“Hg
Vacuum is measured in “inches of Mercury.” Mercury has the
chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to
as “Hg. The “Start” spec for the control unit is a range of the minimum
vacuum required to get the control unit to just barely start moving. When
selecting this specification, consideration should be made to the amount
of vacuum that a given engine produces, and what the load is on the engine
at this specification. For example, an engine with a very radical cam may
be under very light load at 7 inches Hg, and can tolerate a little vacuum
advance at this load level. Your mom’s Caprice, on the other hand, has
such a mild cam that you don’t want the vacuum to start coming in until 9
– 10 inches Hg. For most street driven vehicle performance applications,
starting the vacuum advance at about 8” Hg produces good
results.

Max Advance
Since the vacuum advance control unit is a
part of the distributor, the number of degrees of vacuum advance is
specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking
about these control units, it is important that you know whether the
person you’re talking to is referring to the distributor degrees, or if
he’s talking crankshaft degrees. All of the listings shown in the
following chart, and in any shop manual & technical spec sheet, will refer
to distributor degrees of vacuum advance. You must DOUBLE this number to
obtain crankshaft degrees (which is what you “see” with your timing
light). Thus, a vacuum advance control unit with 8 degrees of maximum
advance produces 16 degrees of ignition advance in relationship to the
crankshaft. When selecting a unit for max advance spec, the total
centrifugal timing at cruise must be considered. Thus, a car set up to
produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum
advance control unit producing 16 degrees of crankshaft advance. This
would be an 8-degree vacuum advance control unit.

Max Advance @
“Hg
This is the range of manifold vacuum at which the maximum vacuum
advance is pegged out. In selecting this specification, you must consider
the vacuum produced at cruise speed and light throttle application. If
your engine never produces 20” Hg, you better not select a control unit
requiring 21” Hg to work.

The following listing (Non-HEI) is as
follows: The first two part number listings are the two numbers that are
most commonly used in a Chevrolet performance application. The “B1” can
is the most versatile and user-friendly unit for a good performance street
engine. As you can see, it was selected by GM for use in most high
performance engines due to its ideal specs. The “B28” can was used on
fuel injected engines and a few select engines that produced very poor
vacuum at idle. The advance comes in very quick on this unit – too quick
for many performance engines. Do not use this very quick unit unless you
have a cam/engine combination that really needs an advance like this. It
can be used as a tuning aid for problem engines that do not respond well
to other timing combinations, and can be successfully used in applications
where direct manifold vacuum is applied to the can (see paragraph and
discussion on this above)

After this, the listing is by Echlin part
number. The Chevrolet applications are listed first by application,
followed by a complete listing of all of the units used on any GM product
(all GM units are interchangeable, so you can use a Cadillac or GMC Truck
unit on your Vette, if that’s what you want to do).

Non-HEI
Distributors:

P/N ID# Application
Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680
B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18

1964 Corvette exc. FI
1964 Impala, Chevy II

1965 396 High Perf.
1965-67 283, 409

1966-68 327 exc. Powerglide
1967-68 All 396

1969 Corvette 427 High Perf.
1969 396 Exc. High
Perf.
1969 Corvette 350 TI

1969-70 302 Camaro
1970 400 4-bbl

1970 396 High Perf.
1970 Corvette 350 High Perf.

1973-74 454 Exc. HEI

VC1810
B28 1965 409 High Perf. 3-5 8 @ 5.75-8

1965 327 High Perf.
1966 327 High Perf.

1964-67 Corvette High Perf.
FI

--------------------------------------------------------------------------------------------------------------------------------------------


VC1605 B9 1965 impala 396 Exc. High Perf.
7-9 10.3 @ 16-18
1965 327 All Exc.
FI
1969 327 Camaro, Chevelle, Impala

1969-70 Corvette 350 Exc. High Perf.
1969-70
350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle,
Impala High Perf.
1971-72 350 2-bbl AT

1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide
9-11 8 @ 16-18
1968 327 Impala AT

1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy
II
1970 350 Camaro, Chevelle Exc. High Perf.


VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl
5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765
B20 1965 396 Impala High Perf 5-7 8 @
11-13
1966-67 Corvette Exc. High Perf.

1966-67 Impala 427 Exc. High Perf.
1966-68 327
Powerglide Exc. High Perf.
1969 307 All

1969-70 396, 427 Camaro, Chevelle High Perf.

1970 400 2-bbl
1970 307 MT

1973 Camaro 350 High Perf.

VC1801
B21 1971 350 2-bbl 7-9 10 @
16-18
1971-72 400, 402

1971-72 307 AT

VC1802 B22
1971-72 350 4-bbl 7-9 8 @
14-16


Other Part Numbers & Specs:

VC700 B3
8-10 11.5 @ 19-21
VC1415 M1
6-8 10 @ 13-15
VC1420 M2
5-7 11 @ 16-17
VC1650 B12
8-10 10 @ 15-17
VC1725 B18
8-10 12 @ 13-16
VC1740 A5
6-8 12 @ 15-17.5
VC1755 A7
8-10 12.5 @ 18-20.5
VC1804 B24
6.5-8.5 10 @ 12-14
VC1805 M13
6-8 12 @ 14.5-15.5
VC1807 B25
5-7 8 @ 13-15
VC1808 B26
5-7 8 @ 11-13
VC1809 B27
5-7 9 @ 10-12
VC1812 B30
5-7 12 @ 11.75-14



The following listing (HEI)
is as follows: The first four part number listings are the 4 numbers that
are most commonly used in a Chevrolet performance application. The “AR12”
can is the most versatile and user-friendly unit for a good performance
street engine. The AR 15 and AR23 are almost identical, with only slight
variations in their “start-stop” specs. The “AR31” can is the HEI
equivalent to the “B28” Hi-Perf can used on the early engines: The advance
comes in very quick on this unit – too quick for many performance engines.
Do not use this very quick unit unless you have a cam/engine combination
that really needs an advance like this. It can be used as a tuning aid
for problem engines that do not respond well to other timing combinations,
and can be successfully used in applications where direct manifold vacuum
is applied to the can (see paragraph and discussion on this
above)

After this, the listing is by Echlin part number. All GM
HEI vacuum advance units are interchangeable, so you can use a Cadillac or
GMC Truck unit on your Vette, if that’s what you want to do.

HEI
Distributors:

P/N ID# Application
Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC1838
AR12 1975 350 Buick 7-9 7 @
10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc,
Calif. 3-5 7.5 @ 9-11
1974 400 All
w/2-bbl
1977 305 El Camino

1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf,
Incl. Calif
1975 350 Z-28

1977 305 Buick Skylark

VC1853 AR23 1976 350 All
Calif. 5-7 7.5 @ 11-12.5

1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc.
Calif
1975 350 4-bbl

1974 350 All w/1112528 Distr.
1978 350/400 Heavy
Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31
2-4 8 @
6-8

--------------------------------------------------------------------------------------------------------------------------------------------

VC1703
N/A 1978-79 Vette Special Hi Perf N/A N/A

1979 305 El Camino Calif.
1978-79 350 Blazer &
Suburban
1979 Buick 305/350

VC1825
AR1 1976 454 Caprice, Impala 3-5 9
@ 6-8
1975 454 Caprice, Chevelle, Monte,
Suburban

VC1826 AR2
5-7 12 @ 10-13

VC1827 AR3
5-7 9 @ 9-11

VC1828 AR4 1975-76 350 Buick
& Olds 6-9 10 @ 12-14
1976 350
Pontiac

VC1831 AR7
6-8 12 @ 14-16

VC1832 AR8 1975-76
455 Buick Electra 4-6 12 @ 12-14

VC1833
AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14
@ 15-16

VC1834 AR9
4-6 13 @ 13-16

VC1835 AS2 1975-76
350 Olds 5.5-7.5 12 @ 15-17

VC1836
AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9
@ 11-13
1977 350 All exc. Calif.

1977 350 Vette Exc. Calif, Exc. Hi Perf
1976 305
All Exc. Calif
1976 350 All Exc. Vette, Exc.
Calif
1976 350 Vette Exc. Calif., Exc. Hi
Perf
1975 262, 350 All w/2-bbl carb

1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.

1977 305 Chev Truck Light Duty
1975-76 350 El
Camino 2-bbl

VC1837 AR11 1976 305 Blazer, Exc.
Calif 6-8 12.5 @ 10.5-13.5

1976 350/400/455 Pontiac 4-bbl

VC1839 AR13
4-6 12 @ 11-13

VC1840 AR14 1975-76
350/400/455 Pontiac Firebird 6-8 10 @ 9-12

VC1841
AS3 1975-76 500 Cadillac Calif. 5-7 10 @
13-14

VC1842 AS4 1976 350 Olds Cutlass
5-7 12 @ 13-15

VC1844 AR16
3-5 12 @ 13.5-15.5

VC1845 AS5 1978-79
425 Cadillac w/F.I. 4-6 14 @ 14-16

1977 425 Cadillac

VC1846 AR17 1977 301 Buick Skylark
3-6 13 @ 10-13
1977 301
Pontiac

VC1847 AS6 1978 403 Motor Home
4-6 12 @ 12-14
1977-79 350/403 Buick
LeSabre Hi Alt, Riviera, Olds
1977-79 350/403
Pontiac Hi Alt

VC1848 AR18
4-6 12 @ 9-12

VC1849 AR19
4-6 12 @ 7-10

VC1850 AR20 1977 350/400
Pontiac 4-6 10 @ 8-11

VC1851
AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @
11-13
1978-79 350 Pontiac

VC1852
AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5
@ 12-14
1975-76 350/400 Chev Truck Heavy
Duty

VC1854 AR24
3-5 13 @ 10-13

VC1855 AS7 1977-79
260 Olds Cutlass 3-5 15 @
10-12

VC1856 AR25
3-6 15 @ 10-14

VC1857 AR26
3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All
3-6 9 @ 11-13
1978 350 Camaro

1978 305 Chev Truck, M/T, Light Duty
1978 350
Chev Truck Hi Alt
1978 305/350 Buick & Olds

1978-79 305 Pontiac

VC1859 AR28 1979 350 Vette Exc
Hi Perf 3-6 10 @ 9-12
1978-79
305 w/1103282 Distr., Incl. El Camino A/T
1979 350
Camaro, Impala, Nova, Malibu, Monte
1979 350
Suburban
1979 350 Buick Century

1978 305/350 Buick & Olds
1978-79 305 Pontiac Hi
Alt.

VC1860 AR29
3-6 12 @ 10-13

VC1861 AR30 1978-79
301Buick 3-5 13 @ 11-13

1979 301 Olds
1978-79 301 Pontiac

VC1863
AR32 2-4 10 @
11-13

VC1864 AR33 1978 305 Chev Truck, A/T, Light
Duty 4.5-6.5 13 @ 11-13

VC1865 AR34
1973-74 350 Vette Special Hi Perf 3-5 15 @
8.5-11.5

VC1866 AS8 1978-79 425 Cadillac w/carb
3-5 14 @ 13-15

VC1867 AS9
2-4 10 @ 8-10

VC1868 AR35 1979 305 Chev
Truck & El Camino 2-4 10 @ 6-9

1979 305 Buick & Olds
1979 305 Pontiac
A/T

VC1869 AS10
2-4 12 @ 8-11



 

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