timed vs manifold vacuum

[cuda66]

I'd like to add a Thank You to John Z for his excellent post.

 

[mike82]

if i go from ported to manifold vacuum, is idle speed the only thing i readjust or timing also?

 

[cuda66]

We first need to establish what you have, is this an injected ot carbureted application?
Is there any computor controls on it?
What were saying here is for naturally aspirated non-emission non-computor controlled cars only.

Gone for the weekend you guys enjoy

 

[mike82]

What were saying here is for naturally aspirated non-emission non-computor controlled cars only.

Gone for the weekend you guys enjoy[/quote]

yep that's what I thought were talkin about too.:ugh
no computers on my '78.

 

[minifridge1138]

Mike82, the answer is you might have to re-adjust your base.

If you read JohnZ's article, then you'll learn about the 3 basic parts of engine timing: Base timing, mechanical advance, vacuum advance.

And it also describes how to set all 3 so that they work correctly.

Minor summary:
Base + mechanical = total timing.
Total timing should be between 34 and 36 degrees.
Mechanical advance should be all in between 2800 and 3200 RPM.
Vacuum advance should be all in at idle (so you need a vacuum advance that is all in at 2" hg less than your idle vacuum).

So, if you get 25 degrees of mechanical advance, then your base timing should be between 9 and 11 degrees.

Since vacuum advance is not part of total timing, you shouldn't have to adjust your base timing if you switch vacuum sources.

When all three systems work properly, you can have more than 50 degrees of advance when cruising (base + mechanical due to RPM + some vacuum).

Click Here:

vettetech

And download these two articles JohnZ published in "Corvette Enthusiast" magazine:

Timing 101

Mapping Advance

They explain everything you could want to know.

Good Luck!!

 

[mike82]

thanks mini fridge