Ted That is the first 55 Customline i have seen with a 150 mph speedometer,but knowing how you build horsepower it dont surprise me Carl

Frank has it right.  The ported vacuum and the manifold (direct) vacuum are two completely different vacuum signals.  While the direct vacuum comes from the intake manifold or any place sourced under the carburetor, the ported vacuum signal is being pulled from within the carburetor at a position located just above the throttle plate when it’s in the idle position.  At anything above idle, the port is exposed and gives increasingly more vacuum signal to the distributor as the throttle blades are opened.  Hole sizing on the ported vacuum is also a player on the amount of signal from this port.  After 1/3 throttle position, this particular vacuum signal starts dying away thus keeping the engine away from detonation under the heavier throttle loads.  The separate gauges helps me to monitor exactly how much vacuum signal and aids in modifying carb hole sizing and the tuning of the vacuum pot on the distributor for the appropriate amount of vacuum advance.

 

Steve has elaborated to a more eloquent degree on the vacuum differences yesterday in another post.  Here’s the link.

http://www.y-blocksforever.com/forums/Topic73727-3-1.aspx 

Lorena, Texas (South of Waco)

As he told you it's for ported vacuum which is influenced by the air velocity thru the carb. The other is simply manifold vacuum.

Frank/Rebop

Bristol, In ( by Elkhart) 

Thats a nice setup Ted.I am curious about the manifold vacuum gauge.What does that tell you and why would it be different than the distributor vacuum reading?In other words what would you be looking for with the manifold vacuum gauge?

  

A permanent vacuum gauge where you can see it is a great idea.  Here’s what I did on my ’55 Customline.

 

One gauge for manifold vacuum and the other gauge is used to monitor the vacuum to the distributor.  I’m using ported vacuum as the vacuum source for the distributor and at idle, there is no vacuum going to the distributor.

Lorena, Texas (South of Waco)

I have one next to my other underdash gauges.Tee into vacuum line to distributor.

  

A vac gauge is probably the best $20 investment I made. Most normally will measure pressure(oil, gas) as well.



An extra length of vac hose through the firewall grommet and I will occasionally run on a drive with the gage stuck on the dash. It can give you a lot of info on what is happening in the engine under load and through the throttle range. The engine is basically one big vacuum pump. I have found a lot of issues with timing and carb issues (flat spots, hesitation, rich/lean conditions transmission shift points for AT, etc) by watching what the gauge does during a drive.

I actually thought about mounting one permanently, but only have the 1 spare manifold port for a future PB booster.

Mike Rizzo

1963 F100 "Rudy"

Daniel Island, SC

I have found my vacuum gauge very useful over the years.You can tell a lot about what is going on with the motor by watching the needle.A very inexpensive useful diagnostic tool in your toolbox.

  

Thanks for the tip Mark. I'll try and get a hold of a vacuum gauge.

Old time mechanics use a vacuum gauge to set up timing.Disconnect vacuum hose.Advance the distributor slowly until maximium  reading on gauge,Turn it back slowly until vacuum reading starts to drop.Set the timing halfway between highest reading and where it starts to drop.Road test for ping under load.Back off timing slowly until it doesn't.