Sorry... but can someone please explain the advantage of Manifold Vacuum vs Ported Vacuum as far as vacuum advance goes?

I understand the difference but I do not understand why my '73 even with the new top end still seems to prefer the ported vacuum with the Qjet
The Holley SA 670 was running on manifold vacuum and it was very nice (I did get some detonation while ******* it)

I switched back to the QJ and ran it off ported vacuum and the thing screams..no comparison to the Holley...no detonation (both were set up 12*/36* and 52*)
So...why does my 350 NOT like the manifold vacuum?

The vacuum before the top end rebuild was about 20"
After the cam and heads and headers I have about 16" to 17" vacuum

Confusing.... I doubt a mild cam and smaller cc heads required more than 670 CFM

Any suggestions?
Thanks
Bob

 

I bet the jetting on the Holley is too lean. I'm also betting the secondaries on the Holley are coming in too slow. The timing thing has me confused too. You should have excellent throttle response with manifold vacuum.

 

use the venturi vacuum port.

 

Thanks Big G
I tried all 3 springs on the Holley (Secondary)
I did not re-jet the Holley as I dont have any parts for a Holley But the damn QJ just whipped the crap out of the Holley.... Maybe I should spend time learining about Holleys now.... Thank you



Sorry Turttle ...are you saying Venturi vacuum is the same as ported vacuum (Above the throttle plate?)
Thanks

 

There are three kinds of vacuum referenced here so I'd like to clear the air.....


manifold vacuum is clear....whatever the vacuum level in the intake is.

ported is a vacuum signal that is taken from just above the throttle plates. there are A LOT of folks who think this connection somehow is venturi vacuum. I can tell you that at least on holleys, this IS NOT venturi vacuum and it does not differ from manifold vacuum except when the throttle is closed. I bet a q-jet is the same.

venturi vacuum is just that...taken from the narrow of the venturi and as such would increase as RPMs climb.....the proof in the pudding is, there can't be much venturi vacuum (in a holley at least) at low throttle as the secondaries would slam wide open as the manifold vacuum is high.....if the vacuum was so incredibly high, we wouldn't need boosters in the venturis for crying out loud.


so in a nutshell;

1. venturi is a very weak signal except at high RPMs

2. ported is like manifold except when the throttle is closed (at least on a holley).

3. manifold is self explanatory.


A LOT of people make claims about the different connections' vacuum without ever checking with a vacuum guage while driving (under load where vacuum is different than in your driveway in neutral). I've seen many websites that have pages on this that are F.O.S.. I'd also be very surprised if a q-jet is different than a holley. So since it's my favorite rant.......



PORTED ADVANCE ON A HOLLEY IS NOT SOME MAGICAL VACUUM CONNECTION THAT IS BOOSTED HIGHER THAN MANIFOLD VACUUM!!!!! THE CONNECTION IS RIGHT ABOVE THE BUTTERFLIES!!!!!

 

My original owner 70 came from the factory with a QJet 350/300 and the vacuum to the vacuum advance was controlled by the TCS solenoid. One of the first things I did to my new car was bypass the solenoid on the intake and run 100% full vacuum to the vacuum advance unit. City gas milage went up. Another advantage with full time vacuum is the car will run cooler at idle and slow traffic because the timing is advanced by the vacuum going to the vacuum advance unit on the distributor. What got my attention one day after taking delivery of my 70 and it sitting and idling in park was the rpm's picking up and no one in the car. The TCS is also controlled by engine temp, when the engine temp reached a temp detected by a separate temperature sensor, not the one that goes to the console guage, the solenoid would permit full time vacuum to reduce the increased temp. Have by passed the TCS now for over 36 years, all it took was a new longer piece of vacuum hose from the carbs full time vacuum to the vacuum advance unit.

 

Thanks
70ZZ3
I will try the full vac with the QJ
Bob