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Old 05-14-2019, 08:14 PM
johnsonic johnsonic is offline
258 I6
Join Date: Mar 12, 2015
Location: Oregon
Posts: 255
Manifold vs. Ported - jeez, here we go

Hi All!

I've probably read every new post and most of the archives / tech articles over the last 4-5 years. I really, REALLY hate bringing this up, yet again... but just can't find an answer that makes sense to me.

1) Manifold vac for advance just makes sense to me. But since I want to use manifold vac as an accurate gauge of engine load - why unplug the vac advance when setting base timing?

2) Wouldn't we want base timing to be at the highest vacuum with timing in an advanced state through the Vacuum Advance (VA)? Then we hit the throttle, mixture gets rich, and manifold vac/timing retards to anticipate the richer load. Perfect timing all around.

3) Can't get my head around setting base timing to perfect with the VA unplugged, then throwing another 16* of advance on it when it's plugged in, and making it be way too advanced (31* or so at idle). If I floor it at every greenlight, I would be perfectly in time, but that's not the case.

4) If I set the VA to fall out completely when the powervalve opens (let's say 7.5 hg), then I'm still zero to 16* too advanced when the PV does open, because my mixture hasn't gotten richer other than the squirter shot for a tiny second.

I'm really, REALLY not trying to ask the same question that's been asked a million times before, and not trying to spark a debate. Have scoured the archives and have read every new post for years. Feel like a schmuck for asking. I just can't get my head around base timing in a manifold vac world.

If someone can shine the light - so grateful! I'm probably just being dumb but could use some knowledge here. Thank you all! Have (and continue) to learn so much from this site.
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
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