[327]

Vacuum ?\'s-ported, manifold and AMC dist.

What is the difference between ported and manifold vacuum? Does ported come through the carb? does vacuum (both kinds) change appreciably through the rpm range? What type does my motorcraft distributor need to be hooked up to?

Trying to figure out where to hook the vacuum line from the distributor on my 401. A freind told me that AMC dist need full manifold vacuum. I looked at a 360 in a waggy today and the vacuum line came off the distributor and went to a CTO switch in the thermostat neck. Does that mean that the dist gets manifold vacuum, but only when the engine is a certain temp?

Thanks
Max

 

[RRich]

Re: Vacuum ?\'s-ported, manifold and AMC dist.

Simply put:

1. The engine needs more timing advance the faster it goes.

2. The vacuum advance diaphram advances the timing as more and more vacuum is applied to it. Full vacuum gives full advance.

3. Manifold vacuum is the highest at idle, stays high until about 1/2 throttle, then drops to near 0 under full load and high speed. Manifold vacuum can be found on the vacuum tree on the manifold or on the carb.

4. Ported vacuum is near 0 at idle, becomes stronger the faster the engine goes. Ported is generated by a small hole or slot in the carb's venturi, near the base, yet the connecting tube can be almost anywhere on the carb. Ya gotta check to see when it has vacuum.

Some systems use a solenoid to let the distributor get vacuum only in top gear, or only at certain times. Those usually use manifold vacuum, since the vacuum is only used when all conditions are met.

Some systems use a temperature switch to keep it from advancing until the temp's up. Some only let vacuum in when the engine's overheated.

Best way, though not usually legal, find the ported vacuum, connect directly to distributor.

Set base timing without vacuum connected.
Check total advance with vacuum connected at 2500-3000 RPM - should NOT EXCEED 35 DEGREES.

 

[webjeep]

Re: Vacuum ?\'s-ported, manifold and AMC dist.

The CTO switched vacuum was, if hooked up correctly, running from one end to the dist advance and the other tube coming from the CTO should have been going to a ported vacuum port on the carb. that is to prevent too much advance while the engine is warming up, choke engaged, etc

 

[327]

Re: Vacuum ?\'s-ported, manifold and AMC dist.

I am swapping a GM TBI unit onto an AMC 401. So I should utilize the waterneck CTO for the distributor advance? Ported vacuum from the TBI unit to the switch then line from the switch to the distributor? Any idea which port on the switch gets which line?

And what should I set the static timing to? Ignintion is TFI upgrade with an MSD 6A module.

thanks
Max

 

[webjeep]

Re: Vacuum ?\'s-ported, manifold and AMC dist.

on my 360 swap i went ahead and eliminated the CTO switches, they were pretty much dry rotted anyway. i just ran my advance straight from the ported vacuum on the q-jet, hooked up the PCV to manifold vacuum and called it a day. those are all of the vaccum lines that i am running besides the secondary pull off on the q-jet. i am running the TFI coil, w/ ford cap and rotor, and MSD 6. haven't fired it yet so i haven't screwed with the timing, i think it's probably about 8 right now, once i prime it and get it ready to fire we'll see.

-web