liquify33

I'm working on a '77 Vista Cruiser and we found the SAVM module in 2 pieces the glove box on arrival. Right now the distributor vacuum is split with the transmission directly off the carb.
Is this okay?

Sugar Bear

What is the vacuum source, direct intake or ported from above the throttle plate? If direct from the manifold it's perfect for the trans. I'd rather have the trans on its own/dedicated source in the event of a vacuum advance failure/leak. In short it's probably fine absent any leaks spurring from the other connection in the line.

Good luck!!!

joe_padavano

All manifold vacuum has a common source. It doesn't matter if the lines are teed together or use separate ports in the intake. A leak in one will disrupt the other (and the engine itself).

liquify33

Thanks guys, Here's my real question I guess:
The stock setup ran through the SAVM which determined ported/vs manifold under 3 load options.
Currently it's manifold right out of the carb, what am I gaining or losing by bypassing the mechanical logic of the stock system?
Does the distributor retard or advance w/vacuum?
Per a different site:

• Heavy throttle: Low vacuum at both inputs: manifold vacuum controls distributor
• Idle: High vacuum at "mani" input but low vacuum at "carb" input: distributor gets 7 inches
• Part throttle: High vacuum at both inputs: ported vacuum controls distributor

The EVAP is gone, the EGR is gone, AIR pump gone if it ever existed, FWIW and manifold holds ~21inches at idle so vacuum is good just making sure I'm keeping a bypassed system as ideal as possible if that makes sense. The other things are "block off and ignore" types for now.

joe_padavano

The vacuum advance unit will, well, advance the timing when vacuum is applied. More (as in higher) vacuum, more advance in degrees until the unit hits the stops. Typically at idle and small throttle openings, more advance will increase engine speed and lower coolant temps, up until you start to get pinging. As you open the throttle and manifold vacuum drops, vacuum advance goes back to zero, but mechanical advance takes over as engine (and thus distributor) RPMs increase. Ported vacuum is the same as manifold vacuum except that it's zero at idle. Once the throttle blades open enough to uncover the port in the throttle body, then it's exactly the same as manifold vacuum. Whether your engine runs better with ported or manifold vacuum depends on the specifics of your combination and the details of which vacuum advance can you have in your distributor.

liquify33

Thanks @joe_padavano that clears it all up quite well. She died on us while idling, had no spark and when we went to replace the coil noticed the distributor wasn’t moving. Pulled distributor and it’s fine so I’m hoping it’s just a timing chain at this point. The distributor gear on the cam ain’t moving, we know that much!