While the 'hot rodders" might consider this question mere "mental masturbation", those of us married to the original, obsolete system might be interested:

Recently, someone mentioned that a longer duration camshaft would reduce manifold vaccum, and implied a resulting loss of spark advance on a Loadomatic distributor/carb combo. If the signal to the diaphram is a mixture of manifold/venturi vaccum, then the more important performance factor (acceleration) would depend on the venturi signal, as manifold vaccum would seem inconsequential at WOT. In fact, one of the problems with the LOADOMATIC system when initial advance is set by ear is too much advance at cruising (no load). If an early distributor was "race tuned" on a SUN machine (weaker springs), wouldn't the reduction in manifold vaccum actually cure the rough-running-at-cruise problem without affecting the enhanced, venturi-driven performance curve?

6 VOLTS/POS. GRD. NW INDIANA

Arent WE getting a little deep?  Seems like the pre 57 performance distributors had either larger diaphragms, or double diaphragms to compensate for the lower manifold/venturi pressures.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

Dan, the early distributors don't have springs. As Mike mentioned, they had two diaphragms. I'll bet the dual diaphragm design worked beautifully in a new, stock engine, but the advance rate was not tunable the way the centrifugal advance system is.



As I'm sure you know, the centrifugal system used weights, springs, and slots to adjust the timing, rate, and limit of spark advance. This tuning was further modified by ported manifold vacuum to a diaphragm that could add advance under light loads.



I've never studied the dual-diaphragm system, but I'd guess it can be tweaked. Is somebody supplying new dual-diaphragms for the early distributor? I have some early distributors but the diaphragms leak.


Best regards,



Paul Menten

Meridian, Idaho

I have read the installation instructions for the installation of the '56 dual four kit supplied by Ford.  It included instructions for installing the different vacuum advance and return springs, so obviously the system can be tweaked.  I would guess the new diaphragm had at least one bigger chamber and the springs weaker to compensate for the reduced vacuum signal.  Also, they connected one diaphragm to the front carb and the other to the rear carb.

John

John - "The Hoosier Hurricane"

I guess I should study the '56 dual-diaphragm Loadomatic system for a clue to the relationship between manifold vaccum and advance. It could have been Ford's attempt to correct the rough-running at low-load limitations and improve the drivability of a performance-tuned pre-56 single setup? (The increasing availability of high-octane gas and demands of T-bird/police customers must have tempted Ford mechanics to crank-up the ultra-conservative factory initial advance settings). Also, I don't think the '56 312 camshaft duration changed from my late '55's 292.



FYI: The '55 distributor DOES have springs. The tension is adjustable by turning eccentric attaching posts. I believe one spring is weaker than the other, which allows fine tuning. A performance advance curve would require a considerable reduction in tension from new springs, but weaker ones only MIGHT be required (depending on the adjustment range). As the ONLY other variable in the factory Loadomatic setup, the delicate ballance between manifold and venturi vaccum would have been critical in dialing in an RPM advance CURVE (inscrutable). I assume mid-50s shade-tree rodders installed some sort of stop-screw to limit total advance.



Easy to see why Mallory centrifical distributors were so popular back then.

6 VOLTS/POS. GRD. NW INDIANA

Rather than me coughing up $30 for a '56 service manual (my car is a '55), maybe someone out there who has one would scan & email me (dantinh62@kconline.com) the page(s) that explain the operation of the dual diaphragm advance mechanism? Or, if you know that the '56 Holley teapot manual supplement (CM56) has the info, let me know and I will send for it. Apparently, there is no '56 supplement available for my '55 Holley distributor manual.

6 VOLTS/POS. GRD. NW INDIANA

I think you are right Daniel, regarding the mechanical stop used by 50's guys.  It has been suggested I solder in a mechanical stop on my 57 and later distributor, so that the amount of vacuum advance is reduced.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

46yblock,



I MIGHT have an old HRM article (with photos) with details on how to do that. I'll look for it.

6 VOLTS/POS. GRD. NW INDIANA

Daniel I would appreciate it.

When Bill the Super Mechanic was helping me with my distributor problem the other day, he felt the vacuum advance was going to far. 

Mike

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

Mike,

           You can (or could) get different cams that had differing amounts of travel. they simply install in the distributor over the pins. Something in the range of 8 -12 deg initial with a total in the mid thirties seems to work out best.

Frank/Rebop

Bristol, In ( by Elkhart)