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Old 05-06-2010, 07:43 PM
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Centrifugal advance

How many degrees does a centrifugal advance normally offer in a sbc distributor? 20 degrees?

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Old 05-06-2010, 08:19 PM
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Quote:
Originally Posted by DENCOUCH
How many degrees does a centrifugal advance normally offer in a sbc distributor? 20 degrees?
It varies by application. The HEI distributors are all capable of the same total amount of mechanical advance, AFAIK, BUT the cams and weights themselves- by their shape- sets or limits the exact amount of advance available.
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Old 05-06-2010, 08:30 PM
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I have seen them clear up to 32° added by the centrifugal
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Old 05-06-2010, 08:37 PM
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If I had to make a WAG, I'd say the majority are somewhere between 15º and 25º.

The other end of the spectrum from the 32º Ericnova72 cites is an HEI from a 1981 229 cid V6 that was in a base Camaro. It had maybe 12º total mechanical advance. The vacuum advance was around 20º.

So like I said, it varies by application! lol
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Old 05-06-2010, 09:36 PM
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If you are speaking about an MSD the weights are the same, the curve kit offers 3 stop bushings, the approximate crank degrees are as follows. It comes with the blue bushing installed and that is 21 degrees.

other choices :
red smallest 28 degrees
silver 25 degrees
black largest 18 degrees .

if the initial timing is 10 degrees and the red bushing is installed you will have 38 degrees total .

The bushings give the tuner the option of low initial timing such as shown by using red bushing or a high amount of initial timing if the black bushing is used .

The spring rate controls the rpm at which total timing is all in (curve), SBC typically run best 36 -38 total timing.
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Old 05-07-2010, 09:12 AM
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Quote:
Originally Posted by DENCOUCH
How many degrees does a centrifugal advance normally offer in a sbc distributor? 20 degrees?
To the best of my knowledge, Most GM non-computer controlled HEI distributors have 20 degrees of built in mechanical (centrifugal) advance. I believe there may be some that will allow 24 degrees of mechanical advance but I have not been able to verify that.

Several aftermarket distributors have 24 degrees of mechanical advance and some (such as MSD) come with bushings to be able to modify and/or limit the amount of mechanical advance.

The rate of how quickly the mechanical advance comes into play is controlled by a combination of the return springs and the actual weight of the flyweights. For ease of setting the timing and actual street performance, the mechanical timing should not begin to appear until slightly above 800 RPM and should be "all in" in the 2500-3000 RPM range. The upper end is somewhat variable depending on what type of performance is desired.

Total Mechanical Timing consists of the initial plus the built in mechanical with NO vacuum advance added. This total should fall in the 32-38 degree range and should not exceed 40 degrees. On small block Chevy engines with Vortec heads, the total should be in the 32-34 degree range.
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Old 05-07-2010, 09:27 AM
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You didn't ask about vacuum advance and the vacuum cannisters available for GM distributors and there limitations. I thought this information would be of some interest in this thread as well. It has been posted before, but I think should be re-visited.

Here is a portion of some info concerning GM vacuum advance cannisters and their part numbers. I have only included the ones applicable to the GM HEI distributors. These are fixed and non-adjustable units.

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. 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
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Old 05-07-2010, 07:39 PM
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Quote:
Originally Posted by Frisco
To the best of my knowledge, Most GM non-computer controlled HEI distributors have 20 degrees of built in mechanical (centrifugal) advance. I believe there may be some that will allow 24 degrees of mechanical advance but I have not been able to verify that.
Really, there are many different amounts of mechanical advance that may be found in HEI non-feedback distributors.

The weights and cams not only control the total mechanical advance, but the curve 'shape' as well.

If you look at the weights/cams below, you'll see they have a multitude of total advance amounts.


Last edited by cobalt327; 05-07-2010 at 08:31 PM. Reason: Add image.
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