Distributor advance

One 'reason' or 'scenario' that I have run manifold vacuum to vacuum advance

69 ambassador 390 wrote: Phat is 100% correct. AMC did not use manifold vac to the distributor at idle EVER. The later cars had a TCVS that connected the distributor to ported vac under normal conditions and would switch to manifold vac if coolant temps got above a set high limit. This was done to reduce further heating by advancing the timing, thus raising the idle and also reducing EGTs to help cool the heads. Running manifold vac under light load and lean cruise can actually raise NOX emissions by increasing combustion pressures and temps in the chamber. In short, You would normally run ported, just like AMC did.

... on my '73 and '74 AMCs, it used Manifold vacuum at idle when cold (to improve driveability when cold, according to the TSM) and ported when warm, switched by CTO valve. Also had TCS that limited vacuum advance in high gear / above certain rpm. All emissions related.




Ref:http://www.oldcarmanualproject.com/manuals/AMC/1973/Service/Chapter4Ahtml/browser.htm

Some engines like vacuum advance connected to manifold vacuum to get advance at idle ... but can cause timing instability without the right vacuum advance unit. Once throttle blades are opened enough (just off idle), manifold and ported vacuum is the same. Use what the engine likes.

Hope this helps,RD

Thank you. I will play around with it to find the sweet spot.
Jeff

The engine will idle and run much better with 5+ degrees of advance (no vacuum to distributor when you set initial timing). The TDC setting was for emissions. As said, limit the mechanical and vacuum advance as necessary to prevent pinging. if you run too much initial advance the engine may be slow cranking depending on how good your starter is; so that's another variable

Not to dispute others posts.
Limit the Distributor to only 24 Degrees total potential Mechanical Advance at whatever rpm.