Originally Posted by polyspheric
For engines close to stock tune (any size, but mild cam) full vacuum to the distributor works well.
As the cam approaches 240 degrees @ .050", full vacuum works at idle, but as you open the throttle vacuum quickly dies, which reduces RPM, etc.
Ported fixes this, since idle is based solely on initial advance it's completely stable until a later throttle position.
You can even fab a new port position by inserting a small tube right through the casting to a point (generally slightly higher to delay vacuum) than the original, which will not add vacuum until slightly higher throttle disc angle. Hint: doesn't need to be horizontal.
Try both, keep the one that works best, cap the other (don't blank it), it may help the next time you change your tune-up.


Good point, but mostly applicable to stock initial timing settings. If you run your initial timing at 15 to 20°, where it should be for a performance motor, there's still plenty of advance to prevent the motor from bogging or stumbling when the vacuum advance goes away upon throttle tip in.

If you run ported vacuum to your advance can, you still have the same ignition timing at low vacuum conditions that you would with manifold vacuum, except you give up the clean idle.


[img]http://i.imgur.com/boeexFms.jpg[/img]
31 Plymouth Coupe, 392 Hemi, T56 magnum
RS23J71
RS27J77
RP23J71
RO23J71
WM21J8A
I don't regret the things I've done. I only regret the things I didn't do.
"Wise men talk because they have something to say; fools because they have to say something. ~ Plato"