carb size

[hot_roder09]

can 1 run a 750 vacum secondaries on my f250 360 or a 390 and can i tie the vacum for the secondaries into the vacum line going to my brake booster that line is coming off the back of the intake

[hazelnut]

i thought the vacume for the secondaries was built into the carb. been a long time since ive had a vacum secondaries carb though. A 750 is a little big for a 360-390, being that a 360 is a lowend power motor you would be better off with a 600cfm

[knightfire83]

I might put a 750 cfm on a rarely driven 460 but that's about it. That much carb on a 360-390 is not necessary.. but the gas station owners will love you if you did.

Vacuum secondaries get their vacuum source from within the carb body itself.

[hot_roder09]

so what is the little port on the back of the carb for all carbs have it looks like a 3/8 hose will fit it

[george worley]

I use it for the PCV valve hose.

[hot_roder09]

thanx but i don't have too hook anything specila up too make the vacum secondaries on my carbtoo work right i can just plug the port in the back can't i

[cdeal28078]

Plug the port. It is not for the secondaries. It is for the power brakes OR the PCV valve.
clint

[fomocoguy]

And a 750 is waaay too big. A 600 would suit your engine much better if it's pretty much stock.

[Redcap]

And a 750 is waaay too big. A 600 would suit your engine much better if it's pretty much stock.

[Dragon]

A 390 truck engine at 4400 rpm uses 421 cfm. I have a car engine and it goes to 4800 but I never shift above 4000.

390/2*(max rpm you use my case 3800/1728) * .85 for stock or near stock engines Above a Performer RPM manifold with headers use .9
A 360 at 440 is 389 cfm
Mine at 3800 is 363. Once I add my Streetmaster I will be at .9 and then 386 CFM is all it would need. So the Holley 465 is my choice for a little growth. I have a 750 VS and unless I am revving above 4000 all it does is slows my truck down if I floor it in 3rd and 4th until my engine can draw more than the primary size of 375 cfm.

[hot_roder09]

thanx guys for the info

[m-mman]

Ther are two ports because the brakes, A/C and PCV work on MANIFOLD vacuum and the secondaries work on VENTURI vacuum.
They are both mutally exclusive - meaning that if the carb throtle plates are shut (idling) the manifold vacuum is high and the venturi vacuum is nil.
When the throttle is wide open the manifold vacuum is 0 and the venturi vacuum is max.

IF you could connect your secondaries to the manifold vacuum source they would TRY to open at idle and then close at wide open throtle. Not a good thing.