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Timed (spark) port vs full vacuum port

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steeters View Drop Down
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    Posted: Jun/27/2009 at 7:44pm

When I took my car apart the vacuum advance was to a full vacuum port.  THe new Holley carb/MSD distributor combinations says to connect the vacuum advance to the timed spark port.  I'm okay with using either one...I've read that some people report that their cars ran better with one port instead of the other.

My inclination is to connect the vacuum advance to the timed spark port...but I'm open to opinions/suggestions on where to start.
 
vacuum at idle is at 12...230/236 @.50, 525 lift cam, 401...etc, if that makes a difference.
 
Thanks,
 
Steve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RB401 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/27/2009 at 8:39pm
This is a controversial topic in some circles.  The factory used ported vacuum.  I.e. the factory used the carb's spark port.  In off-idle conditions there is actually no difference between the two.  The difference is just at idle.  At idle you have no vacuum advance with ported vaccum. That's because the spark port is just above the throttle blades.  So when the throttle is closed, the spark port sees no vacuum.  However, as soon as you open the throttle, even just a little, the spark port sees full manifold vaccum.  With a carb manifold port, the port is just below the thottle blades so that port sees full manifold vacuum at all times.  As far as how your engine runs, with ported vacuum the distributor supplies initial mechanical advance only, at idle.  With manifold vacuum, the distributor supplies intial plus full vacuum advance at idle. Which should you use?  Well the factory always used ported vacuum and  Holley and MSD reccomend using ported vacuum too, so if for no other reason I'd say go with ported vacuum at least with a typical street motor that pulls good vacuum at idle.  With a really big cam that doesn't make much vacuum, it probably doesn't make any differnce.
 
Just to be a little more specific. The factory actually used both manifod and ported vacuum (at least during the '70s).  If look at a TSM showing the vacuum hose routing, you will see that the factory used a spark CTO (coolant temperature overide switch) in the thermostat water outlet.  There were two vacuum lines into the CTO.  One supplied manifold vacuum and one supplied ported vacuum.  A third line ran from the CTO to the distributor vacuum advance.  What the spark CTO did was supply full manifold vacuum to the vacuum advance upon startup and until the CTO reached 160 degrees.  Once the CTO reached 160 degrees, the CTO switched from manifold to ported vacuum and supplied ported vacuum to the distributor vacuum advance thereafter.  The factory did this because the extra advance at startup makes the engine run hotter and hence warms it up quicker.  In the '80s, I ran a 401 in a Jeep truck and had to restore all the smog gear (and figure out how it worked--not hard really) to get the motor to pass California smog.


Edited by RB401 - Jun/27/2009 at 9:07pm
'73 Javelin AMX, 401, T5

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Post Options Post Options   Thanks (0) Thanks(0)   Quote poormansMACHINE Quote  Post ReplyReply Direct Link To This Post Posted: Jun/27/2009 at 8:46pm
whichever one works for you.
Some engines need a little crutch so manifold vacuum gets used. As load increases, manifold vacuum drops. Spark port vacuum works on a venturi between the throttle blade and wall so it's more effective at lower throttle blade positions. At full throttle it's the same either way. There is no manifold or timed spark port vacuum.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote whizkidder Quote  Post ReplyReply Direct Link To This Post Posted: Jun/27/2009 at 9:55pm
Originally posted by RB401 RB401 wrote:

if for no other reason I'd say go with ported vacuum at least with a typical street motor that pulls good vacuum at idle.  With a really big cam that doesn't make much vacuum, it probably doesn't make any differnce.
 
I'm going to disagree on this point.  A big cam that produces minimal vacuum at idle may cause the vacuum advance can to dither the timing in and out at idle if connected to manifold vacuum.  Unless your cam makes enough vacuum at idle to pull the can all the way in and hold it there, you'll have trouble getting your idle speed to stabilize.  With a mild cam and good vacuum at idle, you could probably get away with manifold vacuum without affecting the stability of the timing at idle.
 
I fought this problem for a couple of months on my AMX, even tried an adjustable Crane vacuum can, but was only able to get a stable idle after I moved the vacuum advance to the timed vacuum port.
 
Then was pretty straightforward to get the rest of the timing curve "right" by playing with the adjustable can, weights and springs, and initial timing setting.  (I put "right" in quotes because I've got it "good enough," and have no dyno, timeslips, or other hard evidence to prove otherwise -- just good driveability).
 
You can set it up to run with no vacuum advance, but IMHO, you'll give up some economy on the street/highway at the middle rpm ranges/throttle settings.
Ron Frost
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/27/2009 at 11:31pm
The ORIGINAL reason the factory used ported vacuum was to help offset stumble when driving in town.
It's THE reason Chrysler and then Ford went with ported in the late 30's.  (I'm a student of ICE history and development)
And clear up into the 70's, it was a reason. However, to deal with emissions and other little details, it got more complicated........... vacuum is an indicator of load. Timing needs to change with load and speed.
There is ANOTHER, besides ported or timed and that's VENTURI vacuum - some companies used that. It is taken from higher up, in the venturi area. What happens with MANIFOLD vacuum is that it DECREASES with high load and speed. Venturi vacuum INCREASES with speed and doesn't drop with load. It's typically used in combination with other controls but not always.
 
(whizkidder hit on a topic too - unstable vacuum at idle with a big cam.....)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steeters Quote  Post ReplyReply Direct Link To This Post Posted: Jun/28/2009 at 1:14am
Sounds like good advice.  I'll start with the timed vacuum port and go from there.
Thanks,
 
Steve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/28/2009 at 1:23pm
Steve,
You may want to just do away with the vacuum advance all together on your car.  I just put two sets of Pertronix III electronic dizzy inserts in two distributors for the A&B tag team SC/Ramblers and I am setting them both up for a straight 12 degrees initial advance and 36 total so the engine can go up to a total of 38 degrees with a 14 iniitial setting or 34 degrees with a 10 initial setting. 
 
Your engine should run fine without any vacuum advance being used if you decide to go that route. 


Edited by amx39068 - Jun/28/2009 at 1:25pm
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/28/2009 at 4:46pm
IMO, there's too many "that depends".
I think he's on the right path, try things out.....
Mine ain't worth a crap without the VA and things setup the way they are.
I have a mild cam and it's mostly street.
No way I'd run mine sans vacuum advance and those settings. I'd be fighting demons constantly.
 (I know as I tried)


Edited by billd - Jun/28/2009 at 4:47pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/28/2009 at 5:07pm
As I recall, Steve set his up to be a bit more on the performance oriented side.
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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