Hello Richard,
The vacuum advance modules that were originally fitted to the P6B engines offered slight variations in their operationg points. Some would range from 4" Hg to 15"Hg, whilst others ranged from 4"Hg to 20"Hg. The actual degrees of advancement provided was typically 8 degrees at the distributor thus 16 degrees at the crankshaft.
The vacuum advance modules available today for the Rover V8 have been standardised, with one module being supplied as an OEM for all engines that had such modules fitted. These modules provide vacuum advance from 5"Hg to 17"Hg and will see 8 degrees at the distributor thus 16 degrees at the crankshaft of advancment.
Vacuum advance as it applies to the Rover V8 with SU carburettors is a ported system, with the vacuum supply point being before the butterfly therefore no vacuum advance is provided at idle. When cruising along a motorway with minimal throttle opening, vacuum is high so typically the module will be providing essentially its fully rated maximum advance,..ie 16 degrees, while at greater throttle openings ranging upto full throttle, vacuum will continue to diminish so the supplied advance will also decline until none is supplied.
The major difference between the Weber and the SU as I understand it is in the way the systems interface with the distributor. The Weber sits atop an Edelbrock or similar manifold which supplies vacuum directly to the distributor. At idle, there is considerable manifold vacuum and as such the vacuum advance will be receiving full suction thus 8 degrees at the distributor and 16 degrees at the crankshaft will be supplied on top of the initial dynamic advance that the engine would normally idle at. So if your engine is dialled in with 10 degrees say @ 600rpm then the vacuum advance will typically add a further 16 degrees giving a total of 26 degrees of advance at idle.
This is the only difference in how vacuum is supplied to the distributor modules between ported and manifold. In all other areas, the application of vacuum during driving in all instances is the same, although the magnitude of vacuum available from each system will be quite different.
Are you using a Lucas or Mallory distributor Richard? It is my understanding that Mallory supply vacuum modules which are adjustable so as to vary the vacuum boundary values and as a consequence the total advance provided.
All the best,
Ron.