It looks like there are a lot of varations on the 930 distribuitor theem.
75-76. right turning, Vac-advance only.
3.3's all being left turning.
3.3's with just vac-advance --OR -- 3.3's with vac-retard & boost retard.
Calif. may have its own dist or just be advanced 5 deg.
There may be different levels of mechanical advance depending on the distribuitor.
The direction of rotation meens nothing to us exceept that we can not put a 3.0T in a 3.3t without changing the gear on the crank.
Thus:
1) There is a single-conection-pot that has Vacuum-Advance.
2) There is a double-conection-pot that has Vacuum-Retard & Boost Retard.
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The single conection pot (1) would have to work somthing like this:
It would start at idle at somthing like 0 deg.
The pot would be hooked up to a "ported-vacuum" port that is a hole just outside the throtle butterfly so it sees no vaccum at idle.
With acceleration mechanical advance would take it up to about -18 deg.
At cruse or part throtle, the manifold port would then be just past and inside the butterfly so it will see vaccum. This vacuum would creat the vac-advance function and pull timming up to about -29 deg if above 3000rpm.
On boost the vacuum would go away and the vaccum-advance will stop' bringing us back to about -19 for on boost operation.
This is a safer system in that if there is a failure of the pot's conection or a rupture of the diphram it will fail so boost will not exceed a safe level of timming boost (apx -19).
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The double pot (2) distribuitor would have to work somthing like this:
At idle the low pressure port on the distribuitor is seeing vacuum and is pulling back timming about 9 deg in total.
Thus, at idle we should see about 0 deg of timming. With acceleration such vaccum quickly goes away as noted above and timing is no longer in retard an timming jumps to about -9deg. (loss of vac-ret advances timing)
Mechanical-advance then advances timming further to about -26 deg (USA non-CA) for cruse above 3000rpm.
With acceleration boost is then supplyed to the high-pressure side of the pot. At the same time that little solonid that is conected to the low-pressure side closes so the pot only sees the boost signel and we get boost-retard. With this timing drops back to about -16 on boost.
What I do not like about this set up is if there is a failure in the plumbing to the boost retard side we will not see needed boost retard and put the motor at risk.
Furher, if the solinoid that is plumped to the low pressure side fails when it is suposed to close on boost, the can could see boost on both sides of the diphram. This should then cancel each other out and we would not get needed boost-retard and again put the motor at risk and not even know it.
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What dose not add up is the mechanical-advance curve shown above. It is way to short for any of the above.
I can see a way such a short mechanical-advance (about 6 total degrees) might work.
Using a single-pot / vac-advance dist, at cruse and pulling vacuum we might see about -26 deg of timming.
With acceleration vacuum would go away and timming might pull back to somthig like -16 if the rpm is above 3000.
Then at idle before mechanical advance timming would run at about -10.
That would actually be a very good set-up for off throtle response and a set up I think I would prefer over any we have discussed for a weights and springs turbo set up.
I am just trying to figure this out and appricate any correction.
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(Speedy Squirrel, thank you very much for helping me understand the different vac-boost signels available at the throtle body. Very important to understanding how our systems work.)