Ported Vacuum

[66200i6]

I am planning on building a U-tube Manometer so I can read the ported vacuum pressures more accurately. Does anyone have an idea what level of pressure I will be seeing in inches of water or inches of mercury at the ported vacuum nipple?

Thanks in advance, Ric.

 

[Lazy JW]

I've seen as high as 22 in hg on my cheapo dial guage when attached to the ported vacuum on my Carter YF/300-6
Joe

 

[66200i6]

Well I'm only going to be able to measure around 70 in of water so I guess I'll only be measuring at the lower rpm's.

Thanks for the info.

 

[Lazy JW]

Actually, you will see the highest manifold vacuum at idle. Highest ported vacuum will be just above idle.
Joe

 

[66200i6]

I guess I'm confused. The ported vacuum is created by the increase in flow/velocity thru the venturi. The highest ported vacuum levels should be at the highest rpm levels, shouldn’t they?

Have I been looking at this wrong all this time?

 

[Lazy JW]

Sorry to be confusing, I didn't explain very well Ported vacuum is usually refered to as the vacuum nipple that is located just above the throttle plate at the idle position. This gives no vacuum signal at idle, but as soon as the throttle is opened slightly, the port is exposed to manifold vacuum, and will give a signal almost identical to manifold vacuum. What you are referring to is venturi vacuum, and it does indeed increase with engine speed as the air velocity through the venturi increases. Venturi vacuum is what the older style distributors used that had the vacuum only advance system. If they were hooked to manifold or ported vacuum, it would cause the ignition to advance at idle and retard as the engine speed increased, which is the opposite of what we want. Hope this helps.
Joe

 

[66200i6]

Yes that helps. I didn't realize that manifold and ported vacuum equalized so quickly or equalized at all. I was hoping to use the ported vacuum signal for adjusting multiple carbs. We use to use an analog gauge that sat on top of the Weber carbs for dual VW setups. I guess I will have to rig up a plate that can be used in a similar design with the UTube.

Thanks for the input, Ric.

 

[Lazy JW]

That should work just fine. The distributor on my 68 Bug uses a vacuum only advance setup, it works fine for a stock application.
Joe