Distributor advance

The engine tag says to set the initial at TDC with the vacuum off. I assume they
using a ported vacuum port to advance the initial timing after TDC is set. 390 motor. I did a search and found posts that recommended the initial timing being set at 10-12 degrees advance at idle with and the vacuum port comes into play a WOT.
Jeff

Edited by gnrand - Jan/15/2020 at 8:55pm

The original set up from the factory was to use manifold vacuum. If they say "with the hoses disconnected" that is why, since ported won't do much at idle. They did that to give it extra timing at idle, for better smoothness and to be easier on the starter.

Your distributor is likely around 32 degrees of mechanical advance. Set at 0 or tdc, that will give a conservative advance at WOT. The vac advance will add around 12 degrees at both idle and at a steady cruise, giving a smoother idle and better mileage on the highway.

All of this only applies if you are still using the original camshaft and other major parts. If so, then the factory settings will work, though most people back in the day found that around 4-5 degrees advance (giving a total of around 36-37 mechanical advance at WOT) worked better. If you use the vacuum advance and the stock distributor settings, then I would not use ported vacuum because your idle timing will be too retarded for a good idle.

Chris

PS: If you are interested in re-curving the distributor, you may get better performance even with everything else stock.


Edited by ccowx - Jan/16/2020 at 12:30am

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

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

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