There are a LOT of misconceptions bandied about vacuum advance, which stems from the fact it is generally so poorly understood, even by those who should probably know better.

It might be considered "low performance" by some from the standpoint that drag racing application distributors don't even include it. Performance engines aren't driven on the street, these engines are intended to operate exclusively at wide open throttle, consequently the vacuum advance mechanism itself is simply another potential failure point. Since it isn't ever going to be operational, it isn't needed, therefore it isn't used. See the difference?

But vacuum advance has nothing to do with inhibiting or retarding performance as such, because there is no affect at all if it is working correctly i.e. no diaphragm leaks, advance plate sticking etc. Another source of confusion is the terminology itself. "Total Timing" numbers often discussed, does not include vacuum advance. Sounds strange, but it's true.

For street driving, part throttle operation, and cruising on flat ground especially, there is no practical way to achieve the optimum necessary ignition timing advance with mechanical centrifugal weights and springs alone. That's why vacuum advance was invented in the first place. The proper "curve" is not possible.

Estimates vary, though there is somewhere around a 3 to 4 hwy mpg economy loss without it. Engines have better all around driveability and run significantly cooler. I found my Y likes about 52° BTDC cruising. Now this isn't an exact figure because it is measured in the garage, in neutral, with no load on the engine. Vacuum advance is inherently a load operated device. So far though, I've been unable to find anyone willing to sit in the engine bay with a timing light to check it out on the highway at speed.

The Shop Manual shows somewhere around 45° BTDC - total + vacuum advance using a distributor machine if everything is added up - keep in mind OEM numbers are always listed in distributor degrees - not crankshaft degrees - and most Ys had an initial timing of only 3° or 6°.

Short answer: "Yes you want vacuum advance". Nod your head yes.

Leave it alone.



"FREE SAMPLE"

So far though, I've been unable to find anyone willing to sit in the engine bay with a timing light to check it out on the highway at speed.

... hmmph ...

Can we stop at DAIRY QUEEN afterwards  ...

____________________________

While you can't check timing easily while driving, you can map the mechanical curve according to rpm and the vacuum advance according to vacuum and use that while taking note of engine rpm and vacuum while driving in order to figure it out.  It's probably not all that important, unless you just want to know.


Lawrenceville, GA

Now for the question, with a  holley dual bowl replacement carb would you use ported carb vacuum or manifold vacuum and why ?

I understand the concept of bringing advance in at cruse during high vac low load cruse but from which port ??

Use which ever you like best or works best with the engine.  At cruise, the advance behavior should be the same with those.  The difference is at or near idle.


Lawrenceville, GA

As Charlie mentions, you can try it both ways.  I find that the engine will have a cleaner idle using the staged or ported vacuum port at the carburetor assuming you are using an unmodified distributor.  Unless you set up the distributor accordingly, you could end up with a rough idle with the vacuum hooked up to a direct vacuum source either at the carburetor or the intake manifold.  Here’s the link to a past thread on the subject.
 
http://forums.y-blocksforever.com/FindPost44648.aspx

Lorena, Texas (South of Waco)

Something around 50 sounds right for me for light highway cruising.
I tried 55 but it was quite high for stock Y.

Even Hollow Heads dragster got cruising area in the spark map. It may never use it but wont hurt a bit.


Hyvinkää, FI

Ted, thanks for the explanation.  I am still confused about a few things.

You stated the following.   "For street driving, part throttle operation, and cruising on flat ground especially, there is no practical way to achieve the optimum necessary ignition timing advance with mechanical centrifugal weights and springs alone."

If this is so, why did Ford use all centrifugal advance distributors in their performance engines?   HiPo 390, 406, 427 and 289 engines did not have vacuum advance and they were street operated.  I have driven many cars with these engines and they seemed optimal to me.  However, I can see how that would have a negative effect on gas mileage.

Assuming I can get all my centrifugal advance in by 2,500 rpm, would that make my TBird accelerate faster?

Would timing my engine at 50 degrees total advance (centrifugal plus vacuum) with the vacuum line connected be better than 36 degrees with the line disconnected?

It may have something to do with the intent behind those performance engines.  I don't know any of the history but for instance, if it is a street / strip intent then simply leaving out the vacuum advance would be a benefit for the strip portion of that.  It could also have something to do with warranty. In a drag racing scenario, the vacuum advance can result in over advanced timing in some circumstances, which might cause engine damage.  The vacuum advance would still be a benefit for the street portion but since it is a production vehicle with warranty considerations, the manufacturer has to make decisions that reduce warranty claims.
Were some distributors for those vehicles dual points?.  Were there dual points distributors with vacuum advance?  There may simply not be much room left for that in that case.


Lawrenceville, GA