Ignition Timing

[JimNolan]

Guys,
   I've got a question. I probably should go to an FE forum but I feel stupid for not knowing and I'm hoping you guys aready know I'm stupid and hopefully won't hurt my feelings over it.
   When I rebuilt my 390 I checked my distributor and I'm using the 13L mechanical weight. This gives 26 degrees mechanical advance. I'm sitting my initial timing on 12 degrees giving me a total of 38 degrees advance. With the vaccume advance disconnected I can see most of it by 3000 rpm.  With the vaccume advance connected I see about 52 degrees total advance at about 3000 rpm. The motor sounds great and it seems to run just fine.
  HERE'S THE QUESTION: Is this what the distributor should be doing. And if so, why does all the information I'm reading say the total advance should be around 38 degrees at most. At higher RPM does the vaccume drop out to establish only 38 degrees or am I really running 52 degrees wide open. Jim

[mrocketscience]

Jim.. you should set your initial timing with the vacuum line connected, for street driving. It should give the correct reading then, since vacuum advance changes with throttle position. For dragstrip runs, pull the vacuum line and reset the timing to 38, or maybe 40 total. The FE I used to have liked 40 on the strip.

[bagboy]

The question is ; is your vacume 'manifold' or ported. The former would give full advance at idle.Most likely hooked to 'ported'-open with throtle. So, depending on source I would limit total to 40-42.

[JimNolan]

Guys,
   The vacuum line is ported. ( no vaccum at idle ) I read some information on the vacuum advance on the internet. It suppose to work like I thought it did. With a Holley carburator it doesn't matter if you plug the vaccum line while setting initial timing at idle or not ( no vacuum sucking on advance pot ). A distributor that you can control the advance curve on is apparently what is needed. It looks like the vacuum advance on my car is pulling the plate around too far. I've got an initial of 12 degrees, 26 degrees centrifical degrees and I'm ending up with about 52 degrees at @ 3000 rpm. ( Does anyone know when the vacuum advance suppose to start dropping off.) That means the vacuum advance is giving me at least 14 degrees. Isn't that too much. Jim

[JimNolan]

Guys, UPDATE
   I just got through reading how to adjust the centrifical timing rate and limit of my stock autolite distributor. I also learned how to adjust the vacuum advance limit and what part no. to get if the one I've got isn't adjustable. I should be set. I hated getting rid of my distributor ( it's a rebuilt autolite from NAPA that isn't very old and the shaft is tight). I'm also running a Pertronix ignitor that seems to be working OK. Thanks guys, Jim

[Ford Blue blood]

Jim, the last two Z codes I built liked about 8 to 10 for initial.  Both had NAPA units and gave right at 40 total on manifold vac.  If I pushed past the 10 mark they both did a lot of rattling under light to moderate throttle.  Just my experiance.  Bill

[JimNolan]

Guys,
   I researched my vacuum advance and I researched my distributor. I found information I didn't know existed. Now I'm a certified genious when it comes to vacuum advance distributors ( ya right). All that information didn't help me didly-squat. The distributor wasn't doing what I wanted it to do. Knowledge or no knowledge. Soooooo, what I did was make it so I could control the amount of vacuum advance. I already had an adjustment for RATE on the vacuum cannister. What I did was make an adjustment for AMOUNT of advance. I installed an 8-32 screw and locknut to limit the amount of advance. Controllable from the outside during tune-up. I set my initial timing (vacuum disconnected) to 12 degrees. I then adjusted the idle to 2000 rpm and observed 26 degrees on the harmonic balancer. I then hooked up the vacuum advance and observed @ 52 degrees on the harmonic balancer. I screwed the limit screw in until I read 36 degrees advance on the harmonic balancer. Now (theoretically) I can adjust the centrifical advance anywhere I want it and know the vacuum advance will only be 10 degrees. I can see now why I had to have the distributor adjusted for 6-8 degrees advance to keep the engine from pinging during accelleration. The engine seems to rev just as smoothly as before with a whole lot less advance. As soon as I get a chance I'm going to do the same thing to the distributor on my 57. Jim