It depends on a LOT of factors.  You can't just think of initial timing; you have to think total advance.  On a typical low performance engine you are going to shoot for like 38-42 degrees total.  On a very high performance engine it might be as little as 32.  So you need to know how much mechanical advance is coming in, and you see different specs for different engines because they often use different advance curves and amounts.  These old Ford distributors have two slots in which the advance arm rides (under the breaker plate).  You can change that to get more or less mechanical advance.  The slots are marked something like 9 and 13.  Double that number is your mechanical advance.  Most engines don't want a ton of base timing.  It can make them hard to crank over on starting and cause other issues.  I would use the highest mechanical slot, so 13 in this example, and know I have 26 degrees mechanical advance.  If my target is 38 I set base timing to 12.  Then all I need to do is see where its all in at RPM wise, and change springs if I need it in sooner or later.  I usually shoot for all in by 3,000 RPM. 

Complicating matters is that factory specs are a bunch of compromises and Fords are notorious for having too little advance and too high a curve.  If things like emissions are not a concern, and you want best power ignore the factory specs and tune as I describe above.  My neighbor used to wake up all the new Fords around here back in the day by fixing the advance curve and properly setting the lifter preload on the hydraulic cam engines.  Much better punch around town and another 500RPM at the strip.  Super smart guy because that stuff still works. 
 

EFI can be a bit of a different animal because the ECU does some or all of the timing.  On the EECIV systems we used to bump base timing 2 degrees until the car slowed down (track testing) then pull 1 degree back out until we got the best time.  The advance curve and max advance were determined by the ECU and TFI module, so without the ability to tune you couldn't really do anything about them.  The idea was we were in effect just moving the whole curve up by X degrees by changing the base timing.  A danger here was that those engines didn't use a knock sensor, so you could hurt the engine if you went too far. 

GM systems used a knock sensor, but if you got knock it pulled a ton of timing out of it.  Sometimes those sensors would "see" knock the wasn't there from a noisy timing set, etc.  The Ford systems were WAY better.

Modern EFI has complete timing control, some even doing cylinder to cylinder timing when they use individual coil packs per cylinder.  Of course there is no distributor anymore and timing is referenced off crank and cam position sensors. 
 

Sometimes...a 'pitcher' is worth a 1000 words......
THIS is where I 'go-to' trying to unnerstan Duraspark ignition curve mods...
6sal6



https://www.reincarnation-automotive.com/Duraspark_distributor_recurve_instructions_index.html

Rudi wrote:

In my case EFI and Duraspark are not relevant .
I have a Crane optical trigger distributor where I can dial in the timing for start rpm, curve and total timing. It can also do the vacuum settings.
IIRC the initial is 12, 32 total at 2900
I have adjusted and fiddled with it for years but still wonder if there is anything left on the table.
Right now the car is performing very nicely, plenty of power and good gas mileage.but still ????, I am a constant tinkerer , and don’t know when to stop
Engine is a 331, 9.1-1 cr, aluminum performer heads, mild cam tri-y’s, 670 Holley dripping pot, TT BB1🤪

Honestly, the only verifiable way to get more out of it would be a steady state chassis dyno (rolling road) and realistically the cost would not be justifiable. I'd keep tinkering, reading plugs, and listening for detonation.
 

Bearing Bob wrote:

If it ain't broke...

 
LOL, keep messing with it till it is🤪

I’d do that Dan but this distributor doesn’t have any springs in it and going to a Duraspark is not gonna happen.😁

They are rotary switches, one is for vacuum advance, three settings, the other is timing curves and limits, 10 settings.
The distributor was deleted from production when FAST bought out Crane.

RV6 wrote:

Nos681 wrote:

Try advancing timing until you just hear it ping under hard acceleration, then back it off 1-2 degrees.

If you change springs to allow earlier or later full advance, you’ll have to do it all over again.

Some camshaft manufacturers have a suggested initial timing in install instructions.

The problem with the "Just Hear It Ping" method is that some of us can't hear.

 

In fairness, some of them won't ping either.  I've seen low compression engines with the timing off by 60 degrees that never pinged.
 

RV6 wrote:

Nos681 wrote:

Try advancing timing until you just hear it ping under hard acceleration, then back it off 1-2 degrees.

If you change springs to allow earlier or later full advance, you’ll have to do it all over again.

Some camshaft manufacturers have a suggested initial timing in install instructions.

The problem with the "Just Hear It Ping" method is that some of us can't hear.

 

 
That did cross my mind.

RUDI...?  He's BB2....PLEASE!!

DUH, got my ones and twos cris crossed.😱