I have always heard two version of this story of where to hook up the vacuum advance line. Some folks say hook it up to a manifold port. Others say hook it up to the port on the carb. OK folks, what's the correct answer?

I put my vacuum gauge at both locations to compare them. The port on my Holley does not have any vacuum signal at idle and then rapidly rises to 18" hg as RPM rises. The Manifold on the other hand reads about 15" hg at idle with a monemtary dip when I open the throttle and then a small increase.

These 2 different locations for Vacuum advance are VERY different. Which one is the correct one?
Thanks,
Lee

 

hooking it up to the manifold is a way out of having your distributor curved right. have your distributor recurved by Faron Rhoads (sp?)and he'll probably say hook it up to ported

 

Assuming you dont have a full vaccuum dist

The right place is to the ported vacuum on the carb
ie at idle no vacum, off idle vacuum comes on

You can help big cams by connecting to manifold
But over run has emmisions and back fire risks


In OZ and seeking 427block /428crank

 

Check this out:
http://www.autoweb.lycos.com/garage/subsys/baigni08.htm

 

It all depends on the original design of the system. Some factory designs actually used manifold vacuum but the dizzy was curved for it. Others used either ported or manifold vacuum depending on how hot the engine was running. Personally I like ported except for the lag effect when throttled. On the other hand, running manifold vacuum helped tame the big cam in my CJ and gave me back my power brakes.

Barry

 

From the Century Performance web site

It is our recommendation that when you are using vacuum advance distributors, that you connect the vacuum advance to "full manifold vacuum". There are two schools on where to connect the vacuum advance line. On older applications the connection point was to "ported" vacuum. Ported vacuum means the port is drawing vacuum "above" the throttle blades in the carburetor. This means that as RPM increases, vacuum increases and in turn, vacuum advance increases. This was fine on older applications with high lead fuel and other ancient engine designs. Using this set up today can cause detonation problems, overheating, and other grief.

With our suggestion of using the vacuum connection to full manifold vacuum, the port will be drawing vacuum below the throttle blades. A good running street engine will have a measured vacuum at idle between 14"-20" of manifold vacuum. Now, this will give you a ton of advance at idle, but as load increases (vacuum drops) you will take timing away. This is excellent for the faster burning fuels offered today as well as in the fact that when you put your foot into the throttle and get the RPM building, you DO NOT need or want additional timing. On a RV or tow vehicle, when you put your foot into the throttle and downshift to climb a grade, you DO NOT want added timing that will slow the vehicle and add heat. You want the added timing for subtle throttle response, and low load engine efficiency. So, when you are cruising at freeway speeds or in town traffic, you have the added timing to save fuel, add throttle response, and overall give you a better feel.

See the entire text here

http://www.centuryperformance.com/timing.htm





DSD
69F350
390_4sp

 

So from what I'm getting out of that article they are saying to start out with full advance at idle and decrease it as the RPMs raise. Hmm... I'm going to raise the BS flag here. First of all running massive amounts of advance at idle leads to overheating and detonation. As engine speed increases more advance is REQUIRED to maintain efficiency. I'm not sure what they are saying your initail timing with the vaccum line connected should be but I still think this article is senseless or I just don't understand it. If you stomp the gas down in your RV and you hear pinging you either need to get better gas, have the initial timing retarded, have the distributor recurved or clean the carbon out of the engine. Engines will get the most efficiency when the are at the point right before detonation. By the way sunnysideup, welcome to FTE and I do understand this is not your article so I'm not knockin ya.

 

When you put your foot in it the manifold vacuum goes away and there is no vacuum advance. When you set your initial timing you disconnect and plug the line to the vacuum adv. cannister.
When you accelerate under load there is no vacuum advance so the timing starts at your initial timing setting and advances depending on your initial setting to be (all in) by 2000- 3000
rpm. This is the mechanical advance.
If you experience detonation back off your initial setting.

 

Lee, vacuum advance should be hooked into full manifold vacuum. If hooked into ported vacuum it won't do it's job properly. You might have to reset the static timing to get it to work right, but ported vacuum on a distributer is a mistake. DF

 

Hey Ratsmoker, thanks for the words of welcome. Is it true them things taste like chicken ;~).

As the article says there two schools of thought on this. I run mine set up as described and it runs fine. I may just try it the other way just for the hell of it.


DSD
69F350
390_4sp

 

Sorry DF I must disagree with you here. Almost every vac. advance I have seen was originally connected to ported vacuum for two reasons - one, there will be full (i.e. excessive) advance at idle. For example, say your engine has 6 deg initial timing, 28 additional centrifugal advance by 2000 rpm for a total of 34 deg. Vacuum advance starts at about 5" and adds about another 12 deg. by 10" vac. or greater, at light throttle cruise. (I "averaged" these numbers from several FE specs).

Now if you have the vac. advance connected to the manifold and it's idling at 18", the additional 12 degrees of vacuum advance is fully in already, so your idle timing is now 6+12 = 18 deg BTDC. This is probably excessive unless you have a lot of cam overlap and duration. Certainly it will increase idle emissions and heat.

And there's another and possibly more important problem. As soon as you tip in the throttle the manifold vacuum drops, say to 4" on a moderately firm acceleration. That will cause all the 12 deg. of vac advance to suddenly go away, remember it starts coming in at 5" - pulling out 12 deg.timing every time you touch the gas will cause a major bog.

Best advice I can give is, hook it up like the manufacturer recommends. Of course with modified engines do what your cam and distributor people recommend...

 

Lee, the simplest answer is do whatever works best for your personal situation. Since it is a very easy job to swap the vacuum line between ports, try it both ways and go with whatever feels best to you.

 

Either method will work as long as the distributor is curved correctly for the vacuum source. It has been my experience that most Ford distributors from the mid to late 70's are curved for ported vacuum. If you hook them up to manifold vacuum without restricting the movement of the vacuum pot you can add up to 40 degrees of vacuum advance. I made a little widget with a set screw to clamp on to the actuator arm of the vacuum advance pot of my distributors to limit their total contribution to about 15 degrees. This seems to work well with the initial timing set at 8 degrees. So at idle I have around 23 degrees advance and if I put my foot in it the vacuum goes away and my timing drops back to about 8 but then the mechanical starts adding advance as the rpms rise topping out at about 38 degrees at 2800 rpm. I posted a rather lengthy article about a year ago on this subject that I borrowed from a Chevy engine building book.
Yeah, it was for a Chevy but they are all internal combustion working on the same principles. It basicly stated that contrary to popular belief it is not necessary to keep adding advance as the rpms go up because the efficiency of combustion chambers increase to a certain extent as rpms increase, thus more advance isn't needed.
Anywho.....If your engine and all it's related emission plumbing is intact and you want to keep it that way, then you should keep it plugged into wherever the factory had it plugged into. If you are an adventurer and tinkerer and aren't afraid to experiment, by all means do so!!!

DannyP
89 F-150 4x4 former EFI I-6 now carbed 351W, Edelbrock heads,cam,intake,carb.
MSD 6A, ZF, Sterling 10.25 with 3.55L's.

 

 

As the article says there two schools of thought on this. I run mine set up as described and it runs fine. I may just try it the other way just for the hell of it.
=====

Well I done it. No changes except moved vacumn source from manifold to ported. Seems to run the same. did seem to take a very slight low speed surge out of it and soften the exhaust notes.

This a 69 F350 chassismount motorhome ...390 4sp with Ebok performer, 650 Holley, heddman headers and dual 3" exhaust w/FlowMaster 50's. Weights 10,000 lbs. tows a 3,000 lb boat get about 7mpg. Now I'm gonna have to go fishing one more time this year to complete the comparison, damm. Gonna leave it ported for a while.

Hey Ratsmoker you never did comment on the taste of them smoked rats :~)

DSD
69F350
390_4sp