When does vacuum advance kick in?

[Backally]

Got a 408 with a Holley sniper. I’m setting up the ignition, getting my start, idle(18deg) and wot(35deg) dialed in. I never had vacuum advance with a carb but can program it with the efi. Right now I have 40deg set in(still low I think but I’m increasing and seeing how it responds),but at 55 mph/ 2400rpm cruise timing is about 32deg. If I go up to 65mph cruise the timing goes up to 40. I can adjust it, but not sure where it should go to the 40. Is it dependent on rpm, MAP or what, and where would I see it advance to take advantage of it?

 

[RustyRatRod]

I wouldn't necessarily call it "kick in". That makes it sound dramatic. It ain't. lol Most any part throttle operation, you have vacuum advance. Such as cruising down the road at a set speed......or perhaps light acceleration. That's of course if it's connected to ported vacuum, which is usually best. Connected to full manifold vacuum, it's "kicked in" pretty much all the time, except for hard acceleration, when vacuum drops out anyway.

 

[Garrett Ellison]

I think AJ/formS is running around of 51 degrees total of mechanical, vacuum, and dashboard controlled timing advance at highway speed in a 367 inch 340. Sounds extreme until you see some of the smog era EGR distributors that have upwards of over 60 degrees of combined advance at cruise RPM.

 

[RustyRatRod]

I think AJ/formS is running around of 51 degrees total of mechanical, vacuum, and dashboard controlled timing advance at highway speed in a 367 inch 340. Sounds extreme until you see some of the smog era EGR distributors that have upwards of over 60 degrees of combined advance at cruise RPM.

Naw. If you read the MP engine manual it clearly says it's not uncommon to see 60* or more including vacuum advance at part throttle. So it's really not extreme at all. It's just that most people don't read the MP books at all. LOL

 

[Garrett Ellison]

True! Come to think of it, I have 51 degrees total at cruise, too (16 static, 18 mechanical, and 17 vacuum) and it runs spot on at cruise.

 

[Backally]

So as I understand you guys, me driving down the road at 55 I should be seeing the full vacuum advance I have plugged in. Thanks, I’ll start adjusting to achieve that

 

[Dubob]

With the Sniper keep in mind you can also lean out the AFR’s in that cruise range too. If you go to Holleys tech site you can find some examples that will give you an idea of what can be done. I found some and customized the idea to what I was doing.

 

[Backally]

With the Sniper keep in mind you can also lean out the AFR’s in that cruise range too. If you go to Holleys tech site you can find some examples that will give you an idea of what can be done. I found some and customized the idea to what I was doing.

Oh, I know. There's a Sniper EFI Facebook page also, lot of really good information on there also. Lots of adjustability here. I do not have the cable to hook up a laptop direct to the sniper so have to save to SD card, plug into the laptop to make changes but it's not that big a deal. Still learning (I got a long way to go) just how all this works and how it effects the engine.

 

[Mattax]

This is a repost from
Timing advance guru input needed
If you go there you can see the table the person posted.

All the same principles apply as with carburated systems.

from MTSC 1959
As rpms climb, more lead time in degrees is needed because the engine is turning faster and there's less time (seconds).
As efficiencies build, the combustion itself speeds up, reducing the rate of additional lead needed in the upper rpms.
On a distributor, this rpm related change in timing is handled by the weight and spring system.

Lean mixtures take more time for the flame to travel. So under these conditions more lead time is needed.

from MTSC 259. Ignition system analysis. 1969

This lead time for lean mixtures (vacuum advance) needs to go away when mixtures go rich again approaching wide open throttle.
If enrichment begins at 11"Hg (convert to MAP) then there should be no vac advance below 10 or 11"Hg.
If enrichment begins at 8"Hg, then there should be no vac advance below 7 or 8 Hg.

That's the basics of it.

More about mixtures relationship with load and manifold vacuum in these posts:
How lean is too lean?

Power Valve Selection: The Definitive Answer