Chapter 12

3/5/01: I think that the relationship between boost and vacuum is much clearer now!  I have been checking into what would be necessary to come up with a boost retard vacuum control on my old 1947 Autolite distributor.  I've been trying to find a parts store with a batch of these which I could blow or suck into, but, alas, I came up empty.  The next step was to drag myself through aisles of snow covered cars in the junkyard, chipping away ice and peering in at the distributors of smog-mobiles of the late 70's-early 80's.  You may have to do this too.  One good guess would be 85'-86' Caravans.  Anyhow, I got to looking carefully at the vacuum control I had.  In those days, the spring was removable, and the control was mounted outside the distributor in such a way as to move the whole body of the device, not just the inner breaker plate.  It seemed possible to mount a second spring opposing the inner spring which would push the diaphragm to a "some" advance position.  This position could be changed by altering the springs or adding spacers on either side.  I then welded on a bracket which would hold the spring.  I can't imagine why this won't work!

  In monkeying with the distributor, I finally noticed that the mechanical advance was COMPLETELY SEIZED in the retarded position.   Suddenly, all the tuning issues which have been present since TurboStude first breathed again became clear.  I have been obsessed with the transition circuit in the little carb.  "It can't be the timing, It's spot on when I use the timing light!"  Well, if there is no rpm at idle where it is timed, and the engine vacuum is high with no boost, then the mechanical advance WOULD be closed (where it was stuck) and timing would be fine.  I noticed the distributor twisting as I revved the motor, so I figured there wasn't much that could be wrong there!  The points were carefully gapped.  Here's how it is supposed to work:

At idle, there is vacuum pulling on the vacuum advance, advancing the timing much of the way,additive to its static timing which was set by rotating the whole distributor .  Depending where the vacuum line is connected to the carb, the vacuum sensed by the vacuum control will be different.  Above the throttle plate, at idle, the vacuum is different than below it, since the plate is closed almost completely. 

As the engine accelerates, the vacuum goes down, but the mechanical weights start to swing outward, in turn twisting the breaker cam and advancing the timing proportional to the rpm.  The weight of the bob-weights and the strength of the springs holding them back (and the amount of rust and grime on the pivots....) will determine how rapidly the advance occurs.  With your foot in it, the vacuum contribution to advance will be minimal.

Now at speed, coasting along, the engine again develops a max vacuum advance.  The rpm have drifted down (mechanical is down) and the throttle is again barely open, just enough to maintain the speed (ever eco-drive with a vacuum gauge?).

The normally aspirated engine is optimized for max advance at all times except starting using the yin-yang of mechanical and vacuum conditions.

The boosted engine will not tolerate the same amount of advance as the normally aspirated one when under boost conditions.  The double life it must lead is the issue!

Boost negates the vacuum advance just fine, but the problem is on acceleration when there isn't any of that to speak of anyway.  If mechanical brings the boosted engine too far into advance, detonation occurs.  The trick will be to get a fat  mechanical advance to be good for initial acceleration off idle but before boost, but not too much total advance when boost comes in.  Then it would be nice to get some of that mechanical to disappear.  That is what a boost retard vacuum control will do.  As boost increases, the diaphragm moves towards the distributor, effectively retarding it!  Three more places to read about distributor curving:   1 2

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