[vitamin c]

Originally Posted by RocktimusPryme

Cliff I'm probably misinterpreting something your saying there. But I don't understand your math. 8-12 base I understand. But your saying the distributor for your ideal setups is only adding 10-12 to the base? So at WOT with minimal vacuum signal your making leak power at 20-24* total timing? And you said most of your engines with vacuum at part throttle end up 42-48.

But even using the largest numbers your giving there it only adds up to 38*.

8-12 base degrees at the crank, + 10-12 "distributor" degrees = 28-36 at the crank. Distributor degrees are half of crank degrees. The distributor turns twice to each turn of the crank

[Formulajones]

Correct, usually setting this up on a distributor machine is how it's done, so you would double the number for the engine at the crank.

However, All these recommendations are a very broad range so don't take anything as concrete. Every single engine is going to require it's own timing parameters. As mentioned there are just too many variables that affect the outcome.

I am also an advocate of running vacuum advance on any street engine. There is no reason not to.

[TedRamAirII]

Originally Posted by vitamin c

8-12 base degrees at the crank, + 10-12 "distributor" degrees = 28-36 at the crank. Distributor degrees are half of crank degrees. The distributor turns twice to each turn of the crank

Distributor turns HALF SPEED. But I know what you meant. LOL It does get confusing, watching Distributor Degrees and RPM.

[RocktimusPryme]

Okay. I knew it had to be a question of terminology. I'm aware how a 4 stroke engine works. I just assumed when we were mentioning timing degrees we were talking about on the timing tape.

[Cliff R]

There are ways to determine how much timing is being added by the distributor w/o having timing tape. You can do some math and mark the balancer for the total timing number. For example using the diameter of the balancer determine in inches how much equals 36 degrees and make a mark on it with a white or yellow paint marker that distance from the TDC mark. You can then use a timing light and rev the engine until the mechanical curve is all in, then line the 36 degree mark up with "0" on the timing tab.

Vacuum advance units come in many flavors, with different "cut-in" points in inches of vacuum and the travel of the arm varies considerably in how much timing they add.

We stock the good VA units here that have light spring tension and short travel. I prefer to use them vs adjustable models and custom tune the travel of the advance by measuring it and adjusting it if/as needed by opening up the slot with a round file, even if I have to use the MIG to reduce travel some.

Some folks use the Crane unit where you add the "goofy" cam to the screw to shorten travel. This moves initial timing but works OK if that is what you have to work with.......Cliff

[Kenth]

With a dial back timing light just remove springs from distributor weights and set timing were you want it to be.
If the initial gets too low this way push a piece of 1/4" vakuum hose or 1/4" brake pipe on the stop pin at the advance plate and repeat the setting w/o the springs.
Next time, with the springs, the initial should come in at 8-12° which is fine.

When the timing is done, the carb calibration work starts......

[geeteeohguy]

Originally Posted by Cliff R

One needs to consider here that the more things you do to increase combustion efficiency, such as high compression, tighter quench, well chosen camshaft, etc, the LESS timing is needed at every rpm to best power and least fuel consumption.

I've seen very, very few of these engines that like over 50 degrees at light throttle cruise. Most of the ones that we do here end up around 42-48 degrees total when you add up initial, mechanical and what is added by the vacuum unit. Engine that need more timing than that will typically be relatively "low" compression and lots of quench in them, and/or having cams that are basically too big for what they are doing.

For most very well thought out engines base timing will end up around 8-12 degrees, about 10-12 degrees from the distributor (20-24) at the crank, and another 10-14 or so degrees from the vacuum advance. There is also no need or benefit to have a super-quick mechanical curve with those engines either, all in by 2800-3200rpms or so is fine.

We ALWAYS recommend using a vacuum advance on anything street driven, even pretty high HP stuff still benefits from a few additional degrees at light throttle for less throttle angle and increased fuel economy.

I still see PLENTY of folks buying "race" type distributors w/o vacuum advance, then running a super-quick timing curve, all in by 1500-2000rpm's or so......and they still believe they are accomplishing the same thing as running vacuum advance. Those set-ups are "old school" thinking, left over from the days when you absolutely had to have a double roller timing chain, high volume oil pump, "camel hump" "202" heads, 3/4 cam, some sort of "high rise" intake, gas guzzling aftermarket carb, and dual point distributor w/o vacuum advance.....Cliff

THIS^^^^. And I'll add, that most of these cars had manifold plumbed vacuum advance from the factory. In my experience, too much vacuum advance at cruise (over 50 degrees) results in 'trailer-hitching' or surging that is often mistaken as a fuel problem.

[AdamIsAdam]

Originally Posted by Tom Vaught

There was a post in the street section a bit ago where a guy was using a vac distributor can from a later GM engine that seemed to work well for him.

...

I have used the old 427-435 HP and the 454-469 hp type Corvette Vac advance cans as those engines had low vacuum due to the camshaft used.

Tom V.

That was me! (at least I think you're referring to my threads since they had thousands of posts). These are two of the threads:
http://forums.maxperformanceinc.com/...vacuum+advance

And this one: http://forums.maxperformanceinc.com/...&postcount=157

I was trying to pull together all the great advice on here into one thread. Note that I reposted A LOT of Cliff's advice. (although some of it was on cooling too)


I DID A LOT of research and bought MANY different vacuum advance cans that each required various amounts of vacuum to start advancing, and the range of advance they provided ranged. I also used the Crane adjustable unit which I found not to be consistent. It is important to understand how each vacuum advance unit works, how much Hg it takes to activate them and how much total advance you'll get.

What worked for me was EXACTLY what cliff said to do. The only difference between my setup and what Cliff usually suggests is that for my engine it clearly liked MVA over ported. But as Cliff and the other experts say, to each (engine) his own. If it likes MVA over ported, go with it.

SOLUTION: I modified a vacuum advance unit, P/N VC1849/AR19. Stock, it begins advancing at 4-6" of Hg and is all in by 7-10" providing 12* of distributor timing (24* crank timing) (so for my engine I get just about all of the advance at idle). I then had the travel limited by welding in metal so that total travel is .12" which provides 10* of advance at the crank, exactly as Cliff said it would. And it works as promised.

Results: Initial timing is 15* and now +10* of vacuum advance added, the cold start-up and cold driving was wonderful. Warmed up driving is all around excellent. I used to have a surge at 3,000 RPM which is now gone as well. I tuned the carb per Cliff's advice (the APT screws). I have not adjusted the tune in nearly two years and it's been terrific ever since switching to the fixed travel, modified vacuum advance unit.

If you look closely in this pic, you'll notice some weld marks. Basically, the rectangular hole for the travel used to be much longer.




Oh, and for the record, I also cured my temp issues by removing my electric fan and going to a manual 7 blade 18" fan with a modified clutch so that it engages earlier than stock. The car usually runs at the 160* thermostat level, if it's very hot out and I'm on the highway steady for a long time it'll go to about 185*.

Wow, I read my old posts and realize the car's come a long way with all these details.

Anyway, I hope this helps.

Oh, and here's the article many people reference:

Date Published: 2001-10-01

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:





Echlin#, Standard#, BW# ID# Adv Starts @ “Hg” Max Adv (Distr. Degrees @ “Hg”)

VC 680, VC24A, V280 B1 8-11” 8 degrees @ 16-18”

1959 – 63 All Chevrolet
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, VC177, V329 B28 3-5” 8 degrees @ 5.75-8”

1965 409 High Perf
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI



VC1605, ?, V353 B9 7-9” 10.3 degrees @ 16-18”
1965 Impala 396 Exc. High Perf.
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, ?, V446 B13 9-11” 8 degrees @ 16-18”
1968 328 Camaro Powerglide
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

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

VC1765, VC181, V375 B20 5-7” 8 degrees @ 11-13”
1965 396 Impala High Perf.
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, ?, V420 B21 7-9” 10 degrees @ 16-18”
1971 350 2-bbl.
1971-72 400, 402
1971-72 307 AT

VC1802, VC 171, V325 B22 7-9” 8 degrees @ 14-16”
1971 – 72 350 4-bbl.

Other Part Numbers & Specs:

VC700 B3 8-10” 11.5 degrees @ 19-21”
VC1415 M1 6-8” 10 degrees @ 13-15”
VC1420 M2 5-7” 11 degrees @ 16-17”
VC1650 B12 8-10” 10 degrees @ 15-17”
VC1725 B18 8-10” 12 degrees @ 13-16”
VC1740 A5 6-8” 12 degrees @ 15-17.5”
VC1755 A7 8-10” 12.5 degrees @ 18-20.5”
VC1804 B24 6.5-8.5” 10 degrees @ 12-14”
VC1805 M13 6-8” 12 degrees @ 14.5-15.5”
VC1807, VC183, ? B25 5-7” 8 degrees @ 13-15”
VC1808, VC181, ? B26 5-7” 8 degrees @ 11-13”
VC1809 B27 5-7” 9 degrees @ 10-12”
VC1812 B30 5-7” 12 degrees @ 11.75-14”

Submitter's Name: Lars Grimsrud
Email Address:
www.Co 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

[AdamIsAdam]

Originally Posted by geeteeohguy

THIS^^^^. And I'll add, that most of these cars had manifold plumbed vacuum advance from the factory. In my experience, too much vacuum advance at cruise (over 50 degrees) results in 'trailer-hitching' or surging that is often mistaken as a fuel problem.

THAT WAS EXACTLY MY ISSUE. And it was corrected as stated above, less vacuum advance.

"Trailer-hitching." That's funny, not to mention a very accurate description of what I had going on.

[Cliff R]

Just a couple of notes on vacuum advance units. When you read thru the list and select one, then attempt to purchase, you will find many of them are either factory order, shown as on the shelf in another store in another State, or NLA. The factory order numbers and those on the shelf in a warehouse or another store someplace may require than you pay the shipping on them.

I've also noticed than many of the ones still available are not USA manufacture, and when tested may deviate some from the posted specifications on them........Cliff

[nmoreilly]

Great thread but having difficulty finding a lot of these VA cans, particularly the ones that activate at lower vacuum hg. Adding this Hinckley piece which also very informative

http://rockridgefarm.com/vettdoc/Timing_101.pdf

[AdamIsAdam]

I ordered a bunch. I still have a few left over free to good home, you pay shipping. Not sure which ones I have but can check numbers

[nmoreilly]

Originally Posted by AdamIsAdam

I ordered a bunch. I still have a few left over free to good home, you pay shipping. Not sure which ones I have but can check numbers

Thanks Adam...sending a PM

[Kenth]

Using a vacuum advance activated at lower vacuum will in most cases make part throttle ping, and with a faster than factory centrifugal advance it´s guaranteed to ping.

Factory knew this and used a ported source for the vacuum advance with specs suited for the engine.
They also added idle bypass air to have the throttle blades in correct position to the off-idle slots due to the lack of vacuum advance at idle speed.
With this followed recalibrated idle circuits, better cooling radiators and sensor controlled vacuum advance.
At driving speeds, no difference in vacuum advance action, function is the same as with a manifold vacuum source.
This works for the RAIV an S.D. engines and will work for any engine wit a hotter than std camshaft.

[goatman65]

I have the B28 can in my points dist. It is tripower so it has to run manifold vac. Engine only makes 8-9'' vac @ idle. The B28 starts at 4'' and is all in at 7'' at idle.

[Formulajones]

I have one of mine set up almost identical Goatman. It only makes 9 inches at idle up here at 5,000 ft. My vacuum can starts around 4 inches and is all in around 7 inches. I run it on manifold vacuum. I have a quick curve, 36 degrees all in at 2400 rpm. It's 11:1 compression and the car is running fantastic on 91 octane pump gas.

[shaker455]

I see Crane offers these units that have adjustable vacuum canisters and limiter plate so you don't have to collect every advance can made.

https://www.summitracing.com/search/...rder=Ascending

[Formulajones]

Originally Posted by shaker455

I see Crane offers these units that have adjustable vacuum canisters and limiter plate so you don't have to collect every advance can made.

https://www.summitracing.com/search/...rder=Ascending

That's what I use most of the time. Matter of fact, the car I mentioned that I have setup similar to what Goatman posted, I'm using the Crane adjustable unit to achieve those numbers.
Another reason I like the adjustable is that I live at 5,000 feet, and drive to the valley often. Engine makes 3-4 more inches of vacuum near sea level so it gives me the ability to pop the hose off and adjust the sensitivity if need be.

[AdamIsAdam]

I found mine to be inconsistent. Maybe it was operator error