Vacuum advance. To port or not to port?

[Blew86442]

I noticed on my car that the vacuum advance for my distributor is hooked up to a ported outlet off my carb. What's everyone's thoughts on vacuum for the distributor? Full manifold or ported off the carb?

I have a 270H cam, speed demon carb, summit ditributor

 

[hurst1979olds]

I usually put all my engines on full vacuum for the advance. It gives you a better idle quality and better vacuum at idle. But thats just me. Some people like it on timed port.

 

[Bonnewagon]

Every motor is different, just look at the OEM's. I find that ported usually works best, unless I have a large cam that causes a very weak vacuum signal. Then full manifold improves the idle and boosts the vacuum nicely.

 

[Oldsmoletick]

Yup, experiment with both to see what suits your set up best.

 

[Blew86442]

Yup, experiment with both to see what suits your set up best.

I think this the most simple and sound advice! thanks guys.

 

[DoubleV]

Personally I never saw any advantage of running port vacuum. It's the same as running full manifold vacuum minus the extra advance at idle and/or cruising. Assuming proper distributor curve and vacuum advance, in what way would running ported ever be prefered?

 

[jrm81bu]

Personally I never saw any advantage of running port vacuum. It's the same as running full manifold vacuum minus the extra advance at idle and/or cruising. Assuming proper distributor curve and vacuum advance, in what way would running ported ever be prefered?

It allows more advance at cruising speed. That should help with mpg. Then as load is increased the advance goes away so it won't plug knock.

 

[DoubleV]

Personally I never saw any advantage of running port vacuum. It's the same as running full manifold vacuum minus the extra advance at idle and/or cruising. Assuming proper distributor curve and vacuum advance, in what way would running ported ever be prefered?

It allows more advance at cruising speed. That should help with mpg. Then as load is increased the advance goes away so it won't plug knock.

How does ported vacuum give you more at cruising? The way I understand it, ported vacuum is equal to manifold vacuum except at very low throttle positions in which case ported is zero. So how could ported vacuum at any time be more than manifold vacuum?

 

[Bonnewagon]

"So how could ported vacuum at any time be more than manifold vacuum?" At near WOT manifold vacuum is very low, but ported is maxed. Check this out, I love this little gem. It is a ported vacuum regulator valve offered on an '80 301 Firebird. It gives about 10" manifold vacuum to the advance right away and all the time, then any ported vacuum that exceeds that goes to the advance. It's the best of both worlds, manifold and ported and I use it on 400's too.

 

[jrm81bu]

Personally I never saw any advantage of running port vacuum. It's the same as running full manifold vacuum minus the extra advance at idle and/or cruising. Assuming proper distributor curve and vacuum advance, in what way would running ported ever be prefered?

It allows more advance at cruising speed. That should help with mpg. Then as load is increased the advance goes away so it won't plug knock.

How does ported vacuum give you more at cruising? The way I understand it, ported vacuum is equal to manifold vacuum except at very low throttle positions in which case ported is zero. So how could ported vacuum at any time be more than
manifold vacuum?

Light throttle+light load=high vacuum, which=more advance. And only under those conditions. Put a vacuum gauge on your car and you will see it. It's not about being higher or lower than full vacuum, it's more of using it like a switch for more advance. Say you have your timing as high as you can without plug knock, using your base+mechanical. Then when your cruising, your ported vacuum goes up and you get even more advance, that your engine can only tolerate at light load. As soon as you stab the pedal you lose the ported vacuum and your timing goes back to what you tuned your base+mechanical for. If you set your timing at it's fullest with the base+mechanical+vacuum(using full vaccum) there won't be any way to gain more for when your engine could actually tolerate more.