Vacuum advance?

[46flattie]

My vacuum advance failed and while trying to order a replacement the parts store book list 2 different ones for the V8's. One is rated at 8-11 inches if Hg (73-74 304/360) and one is listed at 5-7 inches (72-74 all V8's).

I'm running a 401 w/a Delco points dist. 9:5 CR and summit 8600 cam. Seems that the 8-11 inches advance would allow the mechanical advance to come in quicker and the 5-7 inch advance would be slower. If I remeber correctly the vacuum advance actually retards timing under high vacuum for easier starting and cruise drivability and then when there is no or little vacuum (WOT, etc) let's the mechanical advance fully advance for more power.

Any advice?

[1980_Cj7]

Someone posted a VERY good write-up in here not too long ago about distributer advance, vacuum and mechanical. It really opened my eyes. I just searched, but haven't been able to find it yet.

One of the things it explained is that vacuum advance advances your timing to give a better idle, but then as you mash the pedal, you lose vacuum, and vacuum advance, and the mechanical advance takes over. It said you need advance at idle, then less off idle, but increasing amounts then as rpms go up, and explained whiy.

My understanding is that the vacuum advance mechanism can only advance timing, and can't retard it. I believe the mechanical advance works independently of the vacuum advance and is strictly a function of how fast the distributor is spinning. The weights fly out due to centrifugal force.

Try to find that other post though, it is really good reading. Maybe someone else can remember where it was.

[AMX69PHATTY]

It was here

section: Engines topic: High Flow Water Pump

http://www.bulltear.com/forums/viewtopic.php?t=7322&postdays=0&postorder=asc&start=0

post was:

Some very very good Vacuum Advance Reading here:

http://www.stevesnovasite.com/forums/showthread.php?t=11689

[1980_Cj7]

Yep, that's the write-up I was talking about. No wonder I couldn't find it. I would have never looked under water pump.

I'm not supposed to have a vacuum advance on my water pump that I didn't know about am I???

[46flattie]

Thanks for the link to the great article. It was just what I was looking for.

Scott