As an addendum to the excellent John Washburn article on Ignition Advance and Vacuum Advance, I compiled Ignition Advance information for the 1963 & 1964 Cadillacs specifically and included in our March 2014 1963/64 Cadillac Chapter Newsletter. I have saved that "Tech Tip" as a separate standalone PDF document which you can see by clicking on
I have also copied it in here:
We have periodically discussed ignition timing and vacuum advance and the idea that vacuum advance at high engine vacuum (low rpm and cruising), and the mechanical centrifugal advance (at higher RPM and wide open throttle) all play into setting the engines "total advance".
Understanding the basics of max vacuum and centrifugal advance, total advance, manifold vs ported vacuum, and how it all works to make your engine run smoothly and efficiently is a complex topic, but have to say that John Washburn's 2 parts look at this subject in the January and February 2014 Issues of the Self Starter provides an excellent summary.
In Part 1 in the January 2014 Self Starter, he covers vacuum advance includes the part # for the 1956 to 1963, and 1965 to 1966 Vacuum Advance (part # 111 6168 in groups 2.410) but skips over the 1964 one year only vacuum advance (part # 111 6237).
In Part 2 in the February 2014 Self Starter he covers all 3 elements of engine advance including engine timing, vacuum advance and centrifugal/mechanical advance and how they affect performance. Both Part 1 and Part 2 include charts and stats and illustrations up through 1962 which is applicable to our cars since the 1963 distributor and advance where used for previous years.
A quick look at the Shop manuals reveals the difference in vacuum advances from 63 to 64:
For 1963 the shop manual indicates:
Vacuum Advance: Start 8" - 10", Full 13.25" - 15.25", Max Vacuum advance 11.25 degrees
Centrifugal Advance: Start 0 to 1.5 degree advance at 400 rpm, Max 7 to 9 degrees at 2000rpm
Timing advance 5 Degrees BTDC
For 1964 the shop manual indicates:
Vacuum Advance: Start 5 - 7", Full 20", Max Vacuum Advance 10.75 degrees.
Centrifugal Advance: Start 0 to 1 degree advance at 400 rpm, Max 8 to 10 degrees at 2000rpm
Timing advance 5 Degrees BTDC
In 1965, Cadillac went back to the same distributor and vacuum advance used in 1963.
--> FIRST: The vacuum advance at high vacuum conditions (low rpm and cruising speeds) provides a LOT of advance. That's 10.75 to 11.25 degrees advance if you have sufficient manifold vacuum, and your vacuum advance is in good working order. If in doubt have your distributor checked on a Sun Distributor machine using the specs on the previous page and replace and rebuild if needed.
--> SECOND: The 1964 requires a full 20" of vacuum to hit its max advance, so if you drive at a high altitude (as referenced in Mr. Washburn’s article) or have other issues causing low engine vacuum and do not reach the 20" of vacuum, you will never reach the max vacuum advance. The article references adjustment using alternate parts to compensate for the lack of obtaining maximum advance at high altitudes.
--> THIRD: You can compensate for defective vacuum advance by physically changing the engine timing setting; however, you will be adversely affecting performance somewhere else as you alter the total engine vacuum. Mr. Washburn references the fact that at cruising speeds your engine is running lean and does not need a lot of horsepower.
--> FOURTH AND FINALLY: Read the excellent article by John Washburn on vacuum advance in the January and February 2014 Self Starter
--> Also. Check out a good description of ported vs manifold vacuum for those running aftermarket carbs, by a GM tech by Clicking Here.