Vacuum advance connected or disconnected?

[panic]

Finally i've found the perfect ign advance for my griff 500 and not with the light pistol but trying and re-trying quite a few times,manually. Now i have it just advanced enough to have the max performance and just a bit before knocking: perfect.
Now, i've done it with the vacuum disconnected, should i normally drive the car connecting the vacuum or leave it disconnected? It might sound a dumb question but why should i drive now with the vacuum connected if i regulated the advance with it disconnected? Also, i have the impression the car revs more free without the vacuum connected, is that just an impression or it's real? Pls educate me on this iussue, thanks/Giu

[eliot]

The vacuum advance gives better driveability at low/mid throttle (when there is lots of vacuum). At wide open throttle (WOT) there is very little (or ideally no) vacuum, therefore you are relying on the weights inside the dizzy for your advance. So i would say reconnect it.

[Simpo Two]

Interesting - some time after getting my Griff back from a service I noticed that the vacuum tube was disconnected - deliberate or just forgotten?

[griffter]

It'll have been disconnected to set the ignition timing. Lots run with it disconnected, but you need to block the plenum end.

[350Matt]

Definitely connected

every time

Matt

[panic]

griffter said:

It'll have been disconnected to set the ignition timing. Lots run with it disconnected, but you need to block the plenum end.

Does it make any difference that i run on an act carbon triple plenum...to have or not to have vacuum disconnected?

[GreenV8S]

panic said:

Does it make any difference that i run on an act carbon triple plenum...to have or not to have vacuum disconnected?

No difference, it still makes sense to have it connected.

[v8 racing]

It really depends on what type of heads and cam you are running, if you are running what i would call nice manered cam and heads connect it, if you are using a fairly wild cam and big valve heads disconnect it, the reality is set your advance to 28 deg at 4000 with vac disconnected, drive it down the road with vac disconnected then connect it see which one works best for you.

[David Beer]

Always ran my standard Griff without, much smoother. New engine(ACT intake etc etc) have no choice, no tap to connect to anyway ! Did notice a .0005mpg drop in fuel economy with standard !

[Gsw500]

Disconnected for me too.

Engine rebuild at V8 Dev 2 years ago and seems to run smoother without and both ends plugged.

Gsw500

[eliot]

Consider this spark map from megasquirt for a chevy, but is very similar for rover.


Bottom axis is RPM, side Axis is effectivley vacuum.
100KPA is wide open throttle
Idle is about 40-50kpa
Cruise is around 70-80 kpa.
Overrun engine breaking - about 25 kpa.

Look at the TOP line - 100KPA - see how it starts at 10 degrees at idle (this is why you unplug the vac line when setting static timing) and by about 3k it ramps up to 36 degrees and stays there.
With you VAC line disconnected - your dizzy does exactly the same - i.e. it has a one-dimensional MAP (albeit a mechanical map) - the weights advance the timing inline with RPM and nothing else.

Now plug your VAC line back in and you now get a two-dimensional map - just like fuel injection!
At lower rpm, when the engine cruising with vacuum, the mixture needs to be lit earlier, so notice how at 2400rpm and 60KPA we are running 37' timing. This should provide better driveability and economy. When i first got my chevy it had a mechanical only dizzy, i changed it for a vac one and certainly noticed the difference.

Now of course adjusting the amount of advance for any given load site on a mechanical dizzy is what some people call a black art. You need to adjust the weights to get the desired static advance curve and adjust the vac canister to get the part-throttle areas. Clearly with electronics, adjusting it is simple.

[rev-erend]

Mine has a wild(ish) cam and it drove like a demon with it disconnected..