123 Distributor vacuum advance set up and curve, Advise on seting up the vacuum advance curve on 123 distributor Options

[Clydeskeeper]

I have searched the forum and watched too many YouTube videos. I am either missing the answer or just don't quite understand.
I am replacing my distributor with a Bluetooth 123. I am a little confused on setting up the vacuum advance curve and maybe even the mechanical curve. On a basic level, I understand that it needs to go from a static of 10 degree at idle up to 27 or even a little more at 3,500 rpm's. Of the videos I have watched, seems like most are not even using the vacuum advance part and just using the mechanical to achieve the above curve. Others go into details I just don't understand including using the vacuum to retard the timing at certain rpm's.
Is there a simple answer and old guy like me can understand? I grew up with cars that actually had mechanical distributors with points and carburetors for fuel delivery. So these concepts are not completely foreign to me.

Just for reference, the engine in question is a modified 2 liter that is now 2549cc with dual 48 Webers.

I await your thoughts, should be interesting at always.

Robert!

[wonkipop]

I have searched the forum and watched too many YouTube videos. I am either missing the answer or just don't quite understand.
I am replacing my distributor with a Bluetooth 123. I am a little confused on setting up the vacuum advance curve and maybe even the mechanical curve. On a basic level, I understand that it needs to go from a static of 10 degree at idle up to 27 or even a little more at 3,500 rpm's. Of the videos I have watched, seems like most are not even using the vacuum advance part and just using the mechanical to achieve the above curve. Others go into details I just don't understand including using the vacuum to retard the timing at certain rpm's.
Is there a simple answer and old guy like me can understand? I grew up with cars that actually had mechanical distributors with points and carburetors for fuel delivery. So these concepts are not completely foreign to me.

Just for reference, the engine in question is a modified 2 liter that is now 2549cc with dual 48 Webers.

I await your thoughts, should be interesting at always.

Robert!

whats vacuum advance do beyond mechanical advance you ask?

vac advance was something all air cooled vws had back in the 60s and 70s.
the world over. (Not sure date it came in VWs).
its pretty simple. kicks in at steady revs (ie cruise). advances the timing even further than the mech (centrifigal) advance took it. did this for a couple of reasons.
fuel economy. and as emergygt350 notes also made for cooler running/lower head temos. the vac line for vac advance in a vw at least came off a port above the throttle body so it only ever operated at steady state cruise. as soon as you gassed it at cruise it came straight off so your engine responded to the throttle. and your timing fell back to advance position of the mechanical part of the distributor. you had power.
its not an emissions device - rather it was a fuel economy device in the main part and enabled greater efficiency and smooth/cooler running at varying cruise speed.
side effect was greater emissions of NOX.

it started to be discontinued due to emissions standards in the usa.
for instance even though the distributor had an advance can function it was disconnected in the 74 L jet cars in california and disconnected across the board in the 75 L Jet cars.

as to retard. dual can distributors with a retard function were strictly limited to the usa market back in the 1970s. did just what it says. retarded the engine at stand still idle.
again for emissions. except that made your engine idle hotter. the port was below the throttle plate. as soon as you gassed it to take off from standstill the retard snapped off as the engine vacuum dropped. otherwise your engine would just cough and hesitate at the retarded ignition position.

as emergy says you don't really need the retard set up. that was there purely for emissions. it reduced NOX at idle.

i dont know anything much about 123 ignition systems. others will. but there will be an advance curve that is ideal for most situations. all you lose with the old vac advance function was efficiency at cruise.