[David Jones]

Given the broad selection available and given that I have found what seems to be my engines sweet spot, does one of the following vacuum advances fit my engines needs? How does one pick the right one?
Forget about the Crane Adjustable for a minute.

14 degrees initial
32 degrees total
15" vacuum at 800 rpm out of gear.

All the talk of crankshaft degrees/distributor degrees, starts at this, total by this, has my head swimming.



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

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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

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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

[Rocky Rotella]

Do you have a points-type or HEI distributor? And do you plan on running manifold or ported vacuum advance?

An B-20 or AR-12 would probably be a decent starting point.

[johnsma22]

I agree that for your motor with 15" of vacuum at idle that a B-20 would be a good place to start. Just to clarify what this (8-11 8 @ 16-18) means. The "8-11" is the vacuum level in "Hg that will start to move the diaphragm. The "8" refers to the distributor degrees, which needs to be doubled to see what the advance provided by the can will be in crank degrees. The "@ 16-18" is the vacuum level in "Hg that the advance provided by the can will be fully deployed.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will "dither" in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1", preferably 2" of vacuum less than idle
vacuum level so idle advance is solid and stable.

John

[David Jones]

Rocky, I have a Dave Ray HEI. Points style distributor converted to HEI.
Ported or manifold? I've tried them both and at this time I run ported.

Question
The way I figure it I would need 9 crank degrees advance (18) to get me where I think I need to be. 14 initial to 32 total = 18. Correct?

I had selected the B27 as the most likely candidate since it would give me the mechanical advance # I calculated. How would it do?

How do the vacuum #'s work? I have 15" vacuum at 800 rpm in park. The B27starts activating at 5-7 and is fully deployed at 10-12". When does my engine see 5-7 and 10-12" in normal operation?

[johnsma22]

Your 32˚ total advance figure has nothing to do with vacuum advance. The total advance comes from your initial setting of 14˚ and 18˚ of mechanical advance from the weights, springs and stop in the distributor. The vacuum advance would be added to the 32˚, but would only be present at idle and part throttle cruise. When you get into the throttle your engine vacuum drops and takes the vacuum advance out of the picture. It is no longer needed because of the enriched fuel condition.

You need to see what your vacuum level is when at idle and in drive. You will want a can that is fully deployed at 1-2" Hg. below that to avoid hunting. 15-16 crank degrees (8 distributor degrees) would be a good starting point for the max advance parameter. The point of when the vacuum advance starts to come in is better expressed in reverse. What I mean is, when would you, under what throttle/engine load would you want the vacuum advance to be out of the picture?


Again I say that the B-20 would be a good place to start. It will provide 16 crank degrees of advance at idle and part throttle cruise and will be fully deployed at 2"Hg below your stated 15"Hg vacuum at idle. It would be out of the picture when your engine vacuum drops below 5-7"Hg.

You now need to determine how much mechanical (centrifugal) advance your distributor has been set up to provide and at what rpm it is all in at.

As far as ported versus manifold vacuum is concerned, it is whatever your engine likes the best. Manifold will provide vacuum to the can constantly during idle conditions, and ported will only provide vacuum when the throttle blades start to move and expose the port to engine vacuum. Once you get things set up, try both and see what your engine prefers.


John

[David Jones]

Duh?
Thanks John for the explaination. I'm really struggling with this part of my engine puzzle. (as you can see!) I'll look into the B20 and begin a quest to understand the slot in the distributor that controls the mechanical part of the advance.

[johnsma22]

My pleasure David. Give the guy who converted your distributor a call and ask him how he set it up. How many degrees of mechanical advance and at what rpm he has them all in at. These are figures you will need to know.

John

[David Jones]

I've "read" my balancer while the car was running from Idle to 3500 rpm. There is an 18 degree spread from idle to all in. Vacuum advance disconnected. I have about decided that my vacuum advance is caput. Connected to a vacuum source with 15" of vacuum the timing doesn't change.

[Tom Vaught]

The old AK-12 HO Vacuum can was actually the 1383 Vac can.

It was for the High Perf BB Chebby engines. It had about 6 distributor degrees (12 crank) in the can. I forget what the trigger point was.

Tom V.

[David Jones]

Mine is a Crane unit. Pretty sure it is dead.

[Rocky Rotella]

David, the B20 unit should still provide the desired results, especially if you're running ported vacuum advance.

[David Jones]

What would you recommend with manifold vacuum? Why the difference?

[Rocky Rotella]

Well, what's your vacuum reading in gear? The B20 may still work ok, but at that point the Crane adjustable unit may be a better choice. I will give you the adjustability to pick and choose the start/end vacuum points and tailor them to your combo. Keep in mind though, that the Crane unit is dual-adjustable. You adjust the amount of advance and the vacuum actuation points separately.

[Rocky Rotella]

BTW, as to why a different cam. When you quickly put a load on the engine, like dropping it into gear (or engaging the clutch with a manual), the vacuum can drop off quickly causing the vacuum advance unit to retract. That can lead to lost idle rpm and idle strength causing the engine to stall, sputter, or die.

[David Jones]

Thanks. Many variables in a correctly set up vacuum advance. Many different cans will work OK, but it is possible to really fine tune one. I am going to try a B20 and see how that works out. At least with it as a baseline, I'll possibly know which way to go if needed. Maybe. Chances are. With some luck. Good possibility.............

[Rocky Rotella]

Yes, there a number of variables that must be considered. That's why so many people give up on it! But the benefits a matched vacuum advance unit offers are definitely worth the effort.

BTW, the Borg Warner part number for the B20 can is V375.

[Rocky Rotella]

The limiting plate is a good piece to use since it is adjustable, but I have followed Dave Ray's suggestion and relocated it to the pull side of the vacuum advance unit. But either was is acceptable.