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Old 02-08-2003, 09:50 PM   #1
RMS73
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Default Distributor Vacuum Advance Motor

What is the correct/best vacuum advance motor for my distributor. The distributor is used on a stock 1973 L-82. According to the service manual the advance should start at 6" Hg, and have a maximum of 15deg at 12"Hg. The engine has a vacuum of 13" Hg at an idle speed of 700 rpm. I have the Accel adjustable motor but its response range appears too short. Just wondering if someone might have a GM part number or a NAPA part number for a vacuum advance motor with the above specification.
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Old 02-08-2003, 10:27 PM   #2
redwingvette
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Default Re: Distributor Vacuum Advance Motor (RMS73)

15 degrees is good for vacuum advance. What you are looking for is total mechanical and vacuum advance to be no more than 50 degrees. To adjust for max performance you want to disconect the vacuum and set the mechanical to advance about 36 degrees. You will need one of Lars kits for that. Once you get that right you can hook up the vacuum and be good to go.
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Old 02-08-2003, 11:45 PM   #3
Vetterodder
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Default Re: Distributor Vacuum Advance Motor (RMS73)

The following is a post from Lar's several months ago. As usual for him, great advice! The VC #'s are Napa, the B #'s are general reference and are usually stamped on the vacuum advance shaft.

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. New 4-19-01


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…!). I have not included the HEI listings at this time (the HEI distributors use a longer control unit, so these part numbers CANNOT be used on an HEI) in the interest of time, but I can certainly compile those specs, too, if there’s interest.

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.

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 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 most 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.

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).

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

VC680........B1.......59–63 All Chev......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......65 409 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....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......68 327 Camaro A/T..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.....69 350 Camaro4-bbl..5.5-8............12 @ 14-18
..............................1969-70 350 2-bbl

VC1765.......B20.....65 396 Impala Hi Per..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......................... ......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........................ .......8 @ 11-13
VC1809.......B27..........5-7................9 @ 10-12
VC1812.......B30..........5-7................12 @ 11.75-14


_______

Lars in Denver
'85 Coupe, no MAF screen, K&N, Energy Suspension, Bilstein, glass top, 200,000 miles & daily driven... hard.
'71 GTO Judge 455 H.O. 4-spd (12.6 on slicks at sea level)
'68 GTO Convert 455 4-spd - the CorvetteForum Q-Jet Testbed
'74 Ram Air Formula 455
'80 Ford Pinto (this classic collector was just donated for charity to the National Kidney Foundation)

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Old 02-08-2003, 11:45 PM
 
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1964, 1971, 1974, advance, advances, corvette, distributor, echlin, grimsrud, gto, idle, lars, napa, timing, type, vacuum, vc1765, vc1802


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