Those of you who are setup up with manifold vacuum
on your vacuum advance. Wondering if you have any
adverse affects with poor idle quality, pinging on light load etc. I've always used ported vacuum but thinking about eperimenting with full vacuum mainly because I've installed a vacuum switch for lockup on my 700 to ported and don't really like having to tee off the distributor and vacuum switch. I know Dave Ray is adamant about using maifold vacuum. Also do you still set base timing with vaccum to the distributor plugged and just reset the base idle.
I currently run 14 initial with 22 mechanical and
about 12 vacuum. My car is weekend street driven. Thank for any input.

Under light load, ported and manifold vacuum are exactly the same. WOT tey are the same, at or near 0. The only difference is at idle, where ported is 0, and manifold has some vacuum.

In my experience, I've had better luck with manifold vacuum, as it helped improve idle quality in my engines. They idled much smoother, and ate less plugs. You still set the mechancal timing with vac advance disconnected. After setting the mech timing, when you plug the dist into manifold vacuum, the idle will rise. Reset the idle speed at the carb.

Some people that have larger cams and automatics seem to prefer ported vacuum, to lessen the RPM drop when going from park to drive. If you have manifold vacuum and you set the idle speed in gear, when you shift into park it may idle too fast. (Or "crash" into gear from park) This is because when you shift to park and the rpms rise, the vacuum goes up, adding more timing, and causes even more rpm rise. The cure for this is ported vacuum....

Try both ways... it's easy, and really only requires an idle speed adjustment at the carb smile.gif

Chris

I was resistant to using manifold vacuum for a long time. Finally I tried it and wouldn't go back. The idle improved a bunch. I don't have VA on the bracket car but on the driver, I set inital at 16 plus 10º VA for 26º at closed throttle idle (36 total).

This is from a post by Fried_Guy several months ago.

You're probably thinking, "Sure there is no manifold vacuum at WOT but aren't I supposed to use ported vacuum for the vacuum advance." Hold onto your hat, THEY ARE THE SAME THING! Except ported is shut off at idle. There are a lot of misconceptions when it comes to the ported vacuum source. After hearing 20 different theories I decided to hook up two vacuum gauges, one to manifold and one to ported, then drive my car and watch it. I found out they are the same, except the ported is shut off when the throttle is closed. Even then I had a hard time convincing guys so I hooked up a couple MAP sensors and a throttle position sensor to a data logger and recorded them while driving then dumped it into a spreadsheet and made a chart. As you can see, there is a direct relationship between throttle position and vacuum. When the throttle is closed vacuum is high, when the throttle is open vacuum is low, and ported vacuum is the same as manifold except when the throttle is closed. So which one do you want to hook it to? I prefer manifold vacuum. This pulls in more timing at idle which is good since there is virtually no load. Your motor will idle smoother and cooler with the extra timing. One night I was at the drags and my car was running hot in the staging lanes, I swapped the vacuum advance from ported to manifold then it would idle all night at 175°. Believe it or not the purpose of ported vacuum is to raise the temperature at idle, to lower NOx emissions. If you're like most hotrodders that is of no concern to you. If you have a big cam with a choppy idle then a vacuum advance hooked to manifold vacuum can really help. It will idle smoother and requires less throttle to maintain speed. Often a big cam requires you to open the throttle so far that the curb idle adjustment needles won't work. Hooking the vacuum advance to manifold vacuum will allow you to close the throttle some which may be enough for the idle mixture screws to work. Someone told me he noticed less dynamic braking with the vacuum advance hooked to manifold. I didn't notice it on my car but it makes sense. If the motor is running more efficiently with the added advance it will make a less effective brake. So which should you use? Try both and see which you like best.http://cochise.uia.net/pkelley2/vacuum_chart.jpg

Notice how both ported and manifold vacuum track together once the throttle is opened.

Fried_Guy don't look all that fried to me....

that's always been one of my favorite handles on this board.

Tom

Good info for sure!

Nice link Pat...Actually I'm aware that once the port is uncovered its the same...Seems though that its possible some driving conditions where the port could be partially covered which would equate to diffent timing. Don't know that just speculating. Would have bee cool if the graph showed RPM's or TPS voltage. I'd have a better idea.. I did hook up to manifold vacuum last night and as you've stated the idle was noticable improved in the garage. If it cleans up my plugs I'd be very happy. Thanks guys.
Jack

Edited, because I didn't pay attention to previous posts. Sorry.

Just thought I'd post back with some results.
I did the swap and did some cruising last nite.
My idle did smooth out quite a bit..Almost sounds stock..This morning I pulled the plugs and all
were MUCH cleaner than they have always been. In fact I may have to rejet a little richer. Looks like I'll be staying with manifold vacuum for a while. FWIW I failed to mention one part of Fried GUY's info is inaccurate IMO Believe it or not the purpose of ported vacuum is to raise the temperature at idle, to lower NOx emissions Actually NOX occurs with higher temperatures. EGR valve redirects inert exhaust gases to LOWER NOx. You need ported vacuum for EGR so the valve doesn't open at idle.

SO on a Holley 750 DP (4779) which would manifold vacuum be?

MaTt