[kenneth77]

hello,is anybody here member on forum http://www.nastyz28.com/forum/?

i have3 an argument with one friend. i tell him that my 66 389 engine needs manifold vacuum to distributor when idling.meaning that i have example 10 degrees initial,pluss i add 20 degrees of vacuum advance.that ends up with a total of 30 degrees when idling.
he dissagrees about the manifold vacuum,he means it should have ported vacuum,and only idle on initial timing when idling.

if someone is a member and and can come with replys,i will be thankfull.
thread on the timing issue is below.the first question in the thread is from my friend

http://www.nastyz28.com/forum/showthread.php?t=153064

[Cliff R]

It doesn't require 20 degrees of additional timing at idle, that's just the way they set them up on that engine in that model year.

Most Pontiac engines in later years used ported vacuum for the spark advance, adding the additional timing right off idle and cruising, but not on decelleration or at idle.

The factory also employed other methods to operate the spark advance in later model years with ever tightening emission standards. The vacuum could have been routed through temp switches, transmission switches, and a variety of other devices to bring it on and off under various driving conditions. Most of this was strictly for emission reasons, and these days it's quite rare to see any of that "odd ball" stuff still in place, being used and working correctly.

In any case, the timing is always set with the engine fully warmed up, vacuum advance disconnected, and the engine idling low enough to not employ the mechanical advance. Then the vacuum unit is hooked back up, idle speed and mixture set, etc, before driving the car.

The ignition points, if used, should be set before setting the timing. Air conditioning, head lamps, or any other accessories should be turned off, unless otherwise noted in the vehicles shop Service Manual. Hope this helps some?.....Cliff

[kenneth77]

i know it doesent require 20degress of vacuum advance.it all depends on your engine set up.but the bottomline is that my engine and other engines should have full manifold vacuum connected to distibutor when idling.other/newer engines have ported vacuum or other stuff,due to emmisions

[Cliff R]

So what's the arguement about, sounds like you already know what you need to know about the subject?

Therefore, why ask for advice or opinions?

Here's a more direct answer:

No, your engine does NOT require manifold vacuum to the advance, most other engines don't either.

Yes, it was set up that way from the factory, and yes it will work fine hooked up that way with a completely "stock" application.

This topic causes more problems that one can even imagine with tuning, as folks continue to install goofy distributor weight/spring kits, larger than stock camshafts, lowering/raising the static compression ratio, aftermarket intake manifolds, cylinder heads, and dozens and dozens of other components that directly effect the vacuum the engine makes at various engine speed, load, rpm, etc.

In any and all cases, tune the carburetor and distributor in all areas exactly for the application. If manifold vacuum to the advance works, then use it. There should never be an arguement with this topic, but so many folks really don't understand how distributor and carburetor tuning should be applied to these engines, we often end up in lengthy discussions that in most cases only continue to confuse those who are trying to learn about the topic.....FWIW....Cliff

[OINK 1]

I like going to manifold vacuum, you can set initial timing at 2. Makes it easy to start.
Once it starts, Ya give it a little gas to get the vacuum up.
Then the vacuum pulls the advance to like 16 degrees @ 1100 r.p.m..Nothing on the mechanical advance for you set up the springs/weights to come on at like 1600r.p.m. Then you put a restricter in the vacuum line ( a tube with .015 hole through it, 1" long). The advantage is when you stomp the gas it takes like 1 1/2 seconds for the vacuum advance to back off, by then the rpm's are up and the mechanical takes over. The vacuum advance stays out of the picture cause the manifold vacuum is low. It's kinda like having a twin timing curve. It's a cool trick,( my own Brag Brag trick) and you can run higher compression on todays unleaded gas.
My typical sea level to 1500' elevation motors are set up at 10.75 to 1, run on pump 91, iron heads, no problems. 1500 to 5000' I'll go to 11 to 1. When you go to the ported advance on the carb( above the throttle plates), the vacuum comes on + the mechanical
too, this causes pinging.

Pontiac" vacuum advance" Gregg