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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,289
The outer pot on the '87 distributor is vacuum advance when vacuum is applied to it and boost retard when air pressure from the turbo is applied to it. You'll see that when you hook a mighty vac to it and test it.
The inner pot is vacuum retard when vacuum is applied to it and when air pressure is applied to it nothing happens, it does nothing. You'll see that with the mighty vac test.

Mark and I have a similar timing setup with the distributor slot machined over so we can advance the distributor static timing farther to 12 -15 degrees btdc which is farther than the slot allowed origonally. I had to also remove a little aluminum from part of the housing near the fan housing area to turn the distributor that far because it was hitting something limiting the travel back there.
We both use the MSD 8762 boost timing retard box to retard timing along with the origonal boost retard working.
This way timing is around 12 to 15 degrees btdc at idle and with centrifical advance and vacuum advance working timing increases to around 35* btdc by 4000rpms.
Then when boost comes up both the inner vacuum pot and the MSD 8762 pull the timing back to around 18* btdc in response to the air pressure.

I don't have a good timing light that has the dial back funtion on it that is compatable with the multiple sparks from MSD ignition and the pulley only has 0 and 26* btdc marks on it so I have to guess or borrow a timing light with the dial back retard function to know what timing is above idle. I should file some marks into the pulley someday so I can see what timing is with my regular old timing light.

It's kinda hard to describe how all this stuff works in text with a keyboard but once you've done it and learn it all it makes sense.
The purpose is to have a little more power and drivability around town below 3000rpms.

If you were rebuilding the motor and wanted better low speed drivability it would be good to do the timing curve modifications and increase compression to around 8:1, enlarge the intake ports, and use a smaller and more efficiant turbo that comes on earlier and doesn't boost over .8 or .9 bar with SC cams and headers with a free fow muffler.
Henry at Supertech does something like that.

Last edited by JFairman; 04-21-2013 at 06:31 AM..
Old 04-20-2013, 12:33 PM
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