vacuum reference port

[diymirage]

this year i am finally going to install a return line in my 71 dusters fuel system
i just bought a new regulator, and it comes with a vacuum port

is there any advantage to hooking that up?
in theory, it would give me a higher fuel pressure at the carb at WOT but i dont know if i need it

i guess it could run a lower fuel pressure at idle or part throttle, but again, no idea if there is a benefit to that

im running a quickfuel 750 on top of a 408
in the past i have had issues outrunning my fuel supply during the 1/4 but i believe that might have been due to the "jumper" hose from the pickup to the hardline


from the pump manual:
VACUUM/BOOST COMPENSATION PORT
While it is not necessary to connect a hose to the vacuum/boost compensation port, it may be beneficial in some cases. The vacuum/boost compensation port can be used to momentarily decrease fuel pressure (vacuum compensation) at idle and part throttle, or increase fuel pressure under blower boost (boost compensation). Connect a hose between the vacuum/boost compensation port and a carburetor or intake manifold vacuum port to vacuum index the regulator. The set pressure of the regulator decreases about 1 PSI for every 2 inches Hg (2 inches on the vacuum gauge). Connect a hose between the vacuum/boost compensation port and an intake manifold boost port to boost index the regulator. The set pressure of the regulator increases 1 PSI for every 1 PSI of boost.


if i understand this right, if i use a baseline of 7 PSI and run 14 inches of vacuum, that means it drops to zero at idle, that cant be good
on the other hand, if i set it at, say 4 at idle (while pulling 14 inches of vacuum) then stepping on it will spike the fuel pressure to 10 PSI
again, doesnt sound good

does this make sense? or am i missing something?

 

[67Dart273]

You lost me here..........You are NOT boosted, right? If not you do NOT want pressure compensation

So far as a return, don't make the mistake I did.....you want the return at least as large as the supply. This is because the return is at lower pressure than the supply, and needs to be larger to flow volume

 

[pishta]

Don't run it. Not needed.

 

[67Dart273]

So, somebody disagreed, but doesn't want to explain why??

 

[diymirage]

You lost me here..........You are NOT boosted, right? If not you do NOT want pressure compensation

So far as a return, don't make the mistake I did.....you want the return at least as large as the supply. This is because the return is at lower pressure than the supply, and needs to be larger to flow volume

nope, i am not boosted (i wish i was)

i just thought i would ask, because the manual also mentioned the possibility to run it off vacuum but i guess that is still only for a boosted engine then

as for the lines, my plan was to add a new 3/8th fuel line, and plum the stock 1/4 as the return line

 

[67Dart273]

I would not do that. Use a larger return, and you really should use at least same size as supply. My 67 of course had no 1/4 vapor line. I brazed a 1/4 line into the tank which was fine for a vapor return, then later converted to EFI. Even with a "low" output TBI, the 1/4" had lots of back pressure. I managed to 'get by' using 3/8 all the way to the tank, than adapted to the 1/4" nipple. There is not enough restriction in a few inches of 1/4" tube to matter at this time. If I get more power in the thing, I might have to rework that fitting.

Also are you aware that your 1/4 factory line IS THE VENT. You will have to provide something else for the vent

 

[diymirage]

Also are you aware that your 1/4 factory line IS THE VENT. You will have to provide something else for the vent

This I did not know
Are you saying the fuel line vents the entire tank?
That makes no sense
(Specially since I got a line from the tank running to my vapor chanister)

 

[67Dart273]

On the cars with the 1/4" line coming up front, which originally hooked to the vapor carbon can system, THAT IS the vent. There are some ways around this depending on the year. One of the best is to pull the filler tube out and braze a vent line into it to mimick the 69/ earlier system.