[knighthawk]

how can you tell if a distributor uses ported or manifold vacuum ?

[onovakind67]

Originally Posted by knighthawk

how can you tell if a distributor uses ported or manifold vacuum ?

The distributor doesn't really care where the vacuum comes from.
Ported vacuum removes the vacuum advance at idle which raises the exhaust temperatures and reduces smog levels, and also keeps the idle from hunting around if your VA can is mismatched to your engine.
Hooking your properly selected VA can to manifold vacuum will smooth out the idle and allow you to reduce the throttle opening at idle. This will have several benefits, including smoother idle, less chance of run-on and less rpm drop when going into gear with an automatic tranny.

[knighthawk]

so basically, any engine that your not concerned with the pollution stuff, should be manifold vacuum ??

[onovakind67]

In order to successfully run manifold vacuum, the VA can must be matched to the engine. A common mistake is to run a VA can from a stock motor on a motor that makes less idle vacuum. You don't want to be in the middle of your curve while you are idling because the idle will be unsteady. You want to select a VA can that is all in at your idle vacuum. You can test the VA can with a mighty-vac to see how it operates and if it's going to be suitable for your engine. You can buy VA cans that operate at lower vacuum, and you can also buy limiting devices that will tailor the curve even finer.
Lars Grimsrud has some excellent info on VA systems:

http://65corvette.nonethewiser.net/t...um_advance.pdf

You can get VA limiting kits from:

http://www.kendrick-auto.com/ignition.htm

Here's a nice write-up from a former GM engineer on VA systems:

http://www.camaros.net/forums/showthread.php?t=17599

[knighthawk]

read your suggested link,,, WOW, very informative, thank you. After all these years, you'd think I'd know that

[curtis73]

I use the suck on the can method

If you suck on the nipple of the advance can and the advance moves, its designed for ported. If you have to suck so hard that your eyes go numb, its designed for manifold.

[onovakind67]

Originally Posted by curtis73

I use the suck on the can method

If you suck on the nipple of the advance can and the advance moves, its designed for ported. If you have to suck so hard that your eyes go numb, its designed for manifold.

What's the thinking behind this?