RE: Ported Vacuum vs Manifold Vacuum and Ventiri vacuum

[rll <rlipinski3@xxxxxxx>]

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Hello list

My late dad and I had a running argument about this issue.

Also an old Hot Rod with "ask MacFarland" also missed the mark....

My Dad was a ford man for years but finally bought mopar..a 73 318 sat 
and then an 85 K car.

The old fords...55..56...had a venturi advance distributor system..in 
fact the top hat four barrels had
a double advance can on their distributors

As the flow thru the carb increased the timing advance increased.

The only venturi related devices since then (to my limited knowledge)  
are holley carbs....4 barrel and 6 pack seccondaries
and EGR associated monstrosities.

A bad vacuum advance should manifest itself as a decrease in fuel 
economy(sp) and perhaps minor
drivability issuses. Now if any older mopar had a venturi advance only 
distributor please enlighten me...

I ran a 340 in my 69 charger for a number of years...hooking the vacuum 
advance to RAW manifold vacuum
seemed to make it smoother off idle transision than connecting it to 
PORTED vacuum.

The only difference between RAW and PORTED vacuum is that the ported 
connection is above the throttle plate.

At a properly adjusted idle it reads Zero while the manifold vacuum 
reads...13-18...or what ever your cam allows.

Unless there is a restriction in the intake airflow both RAW and PORTED 
vacuum will read Zero under wide open
throttle. Manifold vacuum more than zero says you carb is too small...or 
clogged air cleaner..or whatever

Under wide open throttle , the venturi port will increase for EGR 
signaling and the secondaries of 4 barrel and six packs....and old 
fords.....

We adapted a 500 2 barrel holley to be our center carb on our  six pack 
challenger....we drilled through the carb to locate the signal port for the
other two carbs.

We copied the location at the necked down area of the throat as the 
original was.

The ported port is down just above the throttle plate.....no where near 
the venturi....

If one has any problem with this theory ...connect a vacuum gauge to 
ported port and any manifold vacuum port and see the difference,
while under wide open throttle.

rob lipinski
65 sat

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