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Re: [GMCnet] Vacuum Advance [message #66172 is a reply to message #66031] Mon, 30 November 2009 19:07 Go to previous messageGo to previous message
ljdavick is currently offline  ljdavick   United States
Messages: 3548
Registered: March 2007
Location: Fremont, CA
Karma:
Senior Member
Advance curves. Don't they all become moot with an electronic distributer tied to a knock sensor (and fuel injection)?

Larry Davick
The Mystery Machine

----- Original Message -----
From: "Ken Burton" <n9cv@comcast.net>
To: gmclist@temp.gmcnet.org
Sent: Sunday, November 29, 2009 9:29:16 PM GMT -08:00 US/Canada Pacific
Subject: Re: [GMCnet] Vacuum Advance



roy1 wrote on Sun, 29 November 2009 19&#58;04
> Ken
> I used to use my wife for that chore also but her hearing went south too. MSD makes a nice knock sense that has that chore now.
> Roy

This is always a problem when people try to mess around with their spark advance without understanding the whole picture of what they have installed.

I finally found a listing of the various advances used on the GMC. The numbers are all over the place depending on the year of the coach, HEI vs. points ignition, 455 vs. 403 engines and federal vs. California.

Until you know the advance curve of your particular distributor and which advance you have, I would not screw around with it.

Here are some of the numbers I found:

Distributor # 1112172 used on 455's with points (1973-1975)
GM vacuum advance # 1974308
advance start 8 = 10"
advance maximum 19 - 20" (24 degrees)
Maximum vacuum advance 24 degrees
Static timing 8 degrees BTDC
Maximum mechanical advance 16 degrees at 3400 RPM



Distributor # 1103267 used on Federal 403's (1977 and 1978)
GM vacuum advance # 1973609
advance start 4-5"
advance maximum 7-8" (8 degrees)
Maximum vacuum advance 8 degrees
Static timing 12 degrees BTDC
Maximum Mechanical advance 16 degrees at 3400 RPM


There were three other HEI distributors and advances used and their values fell in the middle between those shown above. I'm a terrible typist and too lazy to type all of them in here.


The Dick Paterson rebuilt distributor numbers are even different. Here are the numbers off of my distributor that I got from Dick.

BTW, he says to use "PORTED" vacuum as the vacuum source.

vacuum advance start 8" (2 degrees)
vacuum advance maximum 12" (10 degrees)
Maximum vacuum advance 10 degrees
Static timing 12 degrees BTDC - mine is set at 13
Maximum Mechanical advance 18 degrees at 2750 RPM
Note here: mine is 16 degrees at 2250 RPM

My previous point is that if you move your vacuum source to the manifold to provide additional vacuum you stand a good chance of applying vacuum to the advance outside of the range the it was designed for. This will provide maximum advance most of the time and require you to reduce the static timing to make up for the vacuum advance difference.

HTH

Ken B.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy

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