Technical question about timing vs vacuum on old V-8s:

I have a '72 350SL with the M117 D-Jet V-8.

I set timing yesterday to max advance which seems to work for this engine - about 5degBTDC at idle with vac.

I did not take meticulous readings, but I think obtained maximum vacuum at the above timing settings. I saw as much as 20" briefly, but I think it settled at about 18".

But, when I backed off on advance, the vacuum dropped off. I must re do this, but it seemed to me that with timing at TDC with vac at idle, I had low vacuum - maybe 10-12". If I had set it to 5degATDC, I doubt I would have seen much vac.

Does anyone know what the vacuum readings for this type of engine should be with different degrees of advance/retard?

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5