Vacuum Testing....

[Larry S.]

I have a rebuilt 427 - 425HP engine that is in my frame-off 1966 coupe and I have had the Holley carb (original) rebuilt too. I want to set up the carb and was told to use a vacuum gauge for setup. I hooked it on the vacuum line from the vac advance (advance was then disconected) and adjusted the idle to about 850-900 rpm's.
With the engine relatively cold, the vacuum at 900 rpm's was around 10 in. As the engine warmed up (10-15 min.) the vacuum at 900 rpms dropped a bit to about 7.5-8 in. The engine sounded and idled smooth, the vacuum gauge needle was steady, and if I accelerated the engine to about 1500 rpm the vacuum went up smoothly to around 15 in. The needle was not jumping around . I tried to adjust the 2 idle needle valves for maximum vacuum in. but this didnt change the vacuum reading at all. I have not checked the timing yet, but didnt think that would affect the vacuum reading.
1) Is the vacuum reading that I am getting realistic for a newly rebuilt engine?
2) What vacuum in. should I expect if all was perfect?

[Dick W.]

Re: Vacuum Testing....

Are you getting engine vacuum or ported vacuum. If you are getting ported, your readings will be low and the idle air bleed screws will not make much difference in your readings.

[Duke W.]

Re: Vacuum Testing....

The OE vacuum advance is "ported" and will not show true manifold vacuum at idle at the VAC signal line. Tee into the choke vacuum break line.

If the vacuum readings are the same, some knucklehead probably installed a very high overlap aftermarket cam.

Either way - OE or bubba cam, search the archives for information on converting ported to full time vacuum advance. Use VC-1765 and my screen name.

The idle characteristic with the OE cam should be close to 14"@900

Whenever using a vacuum gage tee it into whatever vacuum source you are using. Disconnecting a component to attach the vacuum gage changes the system configuration and you will just screw things up.

Duke

[Larry S.]

Re: Vacuum Testing....

Thanks Dick and Duke,
I guess I dont understand the difference between engine vacuum and ported vacuum.

Also, when checking the vacuum do I understand you Duke, to say, that I should leave the vacuum advance (the current one that is on the car) connected ? (Please bear with me as 'vacuum' is a gray area for me, and I will read the archives).
Thanks again.

[Stuart F.]

Re: Vacuum Testing....

Ported vacuum is usually from a port or ports at or slightly above the throttle plates, or maybe through the use of a fiitting that uses a small hole in it to restrict (or dampen) the vacuum signal. Manifold vacuum is taken from below the throttle plates and is what you want for making your adjustment of the idle screws. If you "Tee" into the choke pull off or other full time vacuum line, you can do as I have and place the tee where it is accessible yet unobtrusive with a cap on it which you can remove for future use. For show (judging), of course, you would want to replace the line with the Tee with a correct hose.

Stu Fox

[Larry S.]

Re: Vacuum Testing....

Ported vacuum is usually from a port or ports at or slightly above the throttle plates, or maybe through the use of a fiitting that uses a small hole in it to restrict (or dampen) the vacuum signal. Manifold vacuum is taken from below the throttle plates and is what you want for making your adjustment of the idle screws. If you "Tee" into the choke pull off or other full time vacuum line, you can do as I have and place the tee where it is accessible yet unobtrusive with a cap on it which you can remove for future use. For show (judging), of course, you would want to replace the line with the Tee with a correct hose.

Stu Fox

Thanks Stu. That all makes sense the way you explained it .
I will check it out accordingly.
Larry Schuyler