View Single Post
Old 05-15-2019, 09:00 PM
johnsonic johnsonic is offline
258 I6
Join Date: Mar 12, 2015
Location: Oregon
Posts: 267
OK, maybe I'm being REALLY dense, and probably missed something here:

1) We set ideal base timing at 650 RPM or whatever with vacuum plugged for the highest vacuum or RPM.
2) We also adjust springs, weights, and stops to make sure that we're 34*-36* advanced at 3000 RPM, with the vacuum advance unplugged.
3) We now plug in the VA, and we're 16* over ideal timing from 650 RPM to 3000 RPM in a no-load state.
4) I'm going down the highway at 2000 rpm with not much load. I'm still 16* over perfect timing.

In this scenario, I have ideal mechanical, no-load timing all the way from 650 RPM to 3000 RPM without vacuum advance. With vacuum advance I have 16* over ideal timing with no load from 650 to 3000 RPM. If I completely FLOOR it at any point, I will be back at perfect no-load timing, but I obviously have a load and a rich mixture at this point.

If the reason to use manifold vacuum is that it retards timing when the mixture is rich because it lights off more quickly and advances it when lean, why don't we set base timing and total advance with the vac canister hooked up so we are sure we have perfect timing in a no-load state?

All the vac advance is doing is retarding timing when there is a load on the engine and the mixture is richer. So wouldn't we want timing set with vac advance in place starting from a no-load condition from idle to 3000 RPM? Then at any point along that curve timing gets retarded when there's a load.

I can just set all of this up and test it out, but really want to know the theory behind it, because it feels like I'm missing something fundamental.

Babywag, thanks for your input! I do want to go tbi at some point when I can afford it, but want to understand the fundamentals. From what I understand there are still timing adjustments driven from the TPS or MAP sensors and understanding this stuff will help.

Sorry for flogging that dead horse!
1984 GW
Comp 260H
Magnum Roller Rockers
Wiseco -21cc Forged Pistons
Performer Intake
Holley SA 670
MSD 8523
Dakota Digital custom cluster
Serehill headlamp harness
NWMP aux tank

1987 GW deceased
...but the parts live on
Reply With Quote