Thread: Timing

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Hi, this is my first post. I usually get the info I need by reading previous posts but need help. I have a 383 stroker with a Mallory Unilite with vacuum advance. I unplugged the vacuum advance and set the total timing to 38 degrees at 3000 rpm. This caused the initial timing to be 16 degrees. When I plug the vacuum advance back in the initial timing jumped to 39 degrees. Is the vacuum advance can adjustable on these distributors? What should I set it to? How do I set it? I only intend to drag race this engine. Would mechanical advance only be better? Also what would be better, the vacuum port above or below the throttle blades? I have an Edelbrock 750 cfm on it.

Thanks in advance for any help.

Moving the vacuum advance line to ported vacuum above the throttle blades will drop the advance at idle back to reality. If this is a race-only motor, you won't need vacuum advance, it's for fuel economy at high manifold vacuum (cruising).

Make a pass with it in both carb positions and without it at all. That'll tell the tale.

Thanks for the reply. I had it below the blades and it has 15 psi at idle, which shocked me. I moved it up to the port above the blades since then. Is it ok to not hook up the vacuum can? I am considering a different distibutor.

"Is it ok to not hook up the vacuum can?"

Yep, just plug the ported and manifold connections at the carb and use a rubber boot on the can to keep dirt and trash out in case you want to use it later. Like I said, though, if street-driven, mileage will suffer.

Originally posted by techinspector1
Moving the vacuum advance line to ported vacuum above the throttle blades will drop the advance at idle back to reality. If this is a race-only motor, you won't need vacuum advance, it's for fuel economy at high manifold vacuum (cruising).

Make a pass with it in both carb positions and without it at all. That'll tell the tale.

Since vacuum levels drop to zero or near zero at full throttle, it doesn't make any difference where vacuum is sourced from.

The dragstrip passes should be virtually identical regardless of where the vacuum can gets it's vacuum.

C9x.....

wrong answer......

hook a vacuum gauge to full then ported vacuum and see what you get...

One will have vacuum at idle then decrease when quickly opening the throttle....

while the other will have no vacuum at idle and increase while opening the throttle....

Originally posted by dr_bowtie
C9x.....

wrong answer......

hook a vacuum gauge to full then ported vacuum and see what you get...

One will have vacuum at idle then decrease when quickly opening the throttle....

while the other will have no vacuum at idle and increase while opening the throttle....

The answer I gave is correct.

Running a fairly well matched pair of vacuum gauges - which I've done - on an engine shows zero or near zero vacuum from both ported and manifold vacuum when the throttle is wide open.

Part throttle operation does show differences, but wide open throttle shows zero regardless of vacuum source.