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Old 10/13/2006, 09:20 AM   #10
p51
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Location: NorCal
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Quote:
Originally Posted by Mosesatm
Also, does anyone have a diagram that shows where to run the vacuum lines to the distributor? On the forward port I have a line that comes from the forward vacuum distribution block but is also teed into the rear block. The rear port on the distributor runs straight to the forward block. With that setup the rear port is receiving much more vacuum at idle than the forward port, which seems correct to me since the vacuum should only kick in under acceleration.
Regarding how the vacuum should be hooked to the distributor, here's a general article on setting timing that I found useful...

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

The interesting part about hooking up the vacuum is in the following...

"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."...

From this I gather that depending on whether you hook to the vacuum above or below the throttle blades you either advance or retard the timing when you open the throttle up. On my distributor there is only one vac hook point. Sounds like on yours there's two points - one to hook an "average vac" (teed pt) and one for full manifold vac.




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