Re: Ignition 101
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.
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…..:
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.
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.
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 (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.
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
ed: There is a similiar list for point-type distributors which I have left out as I see little point running points with their increased maintenance requirement on a performance engine.
1968 El Camino, 402 with TKO