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Distributor Vacuum Advance Control units
Specs and facts for GM Point-Style 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.
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Inhaltsverzeichnis

Seite 1

Distributor Vacuum Advance Control units Specs and facts for GM Point-Style Distributors by Lars Grimsrud SVE Automotive Restoration Musclecar, Coll

Seite 2

Are you using "ported" or "manifold" vacuum to the distributor? "Ported" vacuum allows little or no vacuum to the dist

Seite 3

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 d

Seite 4

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18 1965 327 All Exc. FI 1969 327 Camaro, Chevelle, Impala 19

Seite 5

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 perform

Seite 6

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. Cal

Seite 7

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

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