Vacuum Advance V8
- Thread starter quattro
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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.
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.
ghce wrote,..
I need to add a caveat,...the ported vacuum system as it applies to the Rover V8 fitted with SU carburettors means that only partial vacuum is available to the vacuum advance module which is designed to function correctly with such a limitation.
Full manifold vacuum as it applies to an engine which is fitted with a Weber carburettor (the vacuum take off is not on the carburettor as it is with the SU) means that for a given throttle opening there will be substantially more vacuum available, and the Lucas vacuum module will not provide the correct degree of advance for such an engine loading. This quite often means that the engine will have excessive ignition timing. This of course can lead to all sorts of problems, such as a hard to trace engine misifre, surging at cruise speeds and ultimately costly engine damage.
Ron.
I need to add a caveat,...the ported vacuum system as it applies to the Rover V8 fitted with SU carburettors means that only partial vacuum is available to the vacuum advance module which is designed to function correctly with such a limitation.
Full manifold vacuum as it applies to an engine which is fitted with a Weber carburettor (the vacuum take off is not on the carburettor as it is with the SU) means that for a given throttle opening there will be substantially more vacuum available, and the Lucas vacuum module will not provide the correct degree of advance for such an engine loading. This quite often means that the engine will have excessive ignition timing. This of course can lead to all sorts of problems, such as a hard to trace engine misifre, surging at cruise speeds and ultimately costly engine damage.
Ron.
Thanks Ron
The edelbrock carb (right hand vacuum port) produces no vacuum at idle.
I am going to rig up my vacuum gauge and do some tests to see what sort of vacuum is produced at various revs, and throttle openings - not easy but I am going to try. I was just wondering what vacuum the SU produces.
I have heard that the edelbrock is not suitable for use with the Rover dizzy (Rover contactless Electronic distributor from an SD1) and was just trying to find out why.
I am not happy with the idea of removing the vacumm and advancing the igniton so was just trying to find out some more.
The edelbrock carb (right hand vacuum port) produces no vacuum at idle.
I am going to rig up my vacuum gauge and do some tests to see what sort of vacuum is produced at various revs, and throttle openings - not easy but I am going to try. I was just wondering what vacuum the SU produces.
I have heard that the edelbrock is not suitable for use with the Rover dizzy (Rover contactless Electronic distributor from an SD1) and was just trying to find out why.
I am not happy with the idea of removing the vacumm and advancing the igniton so was just trying to find out some more.
Hello Richard,
If your Edelbrock carburettor has an integral vacuum port and exposes the distributor vacuum advance module to no vacuum at idle, then that means that it too is ported vacuum. When you undertake the vacuum measurement tests, will you be doing so on the engine free revving or whilst driving under varying load conditions? My reason for asking is on a free revving engine, the results may well be quite different to those that might normally be measured out on the road. Using a rolling road for such testing is ideal as all manner of loads and speeds can be tested for.
With the SD1 distributor that your engine is fitted with, did RPI regraph it so that the rate of mechanical advance and maximum advance provided would be correct for your 4.0 litre engine? Normally, a 3.5 litre engine will require a maximum of 36 degrees of mechanical advance whilst a 3.9 / 4.0 litre engine (which are identical in capacity) requires a maximum of 34 degrees of mechanical advance. Ideally if the engine can receive full mechanical advance by the peak torque rpm, then best performance will be obtained. Of course on top of this you will also have the vacuum advance although the amount provided is directly dependent on engine load and throttle position.
When under hard acceleration and at WOT, vacuum advance is essentially zero, so there will be no increase over the built in mechanical advance during such occassions. Light throttle use is when vacuum advance will be provide the greatest lift in overall ignition timing and as such when driving on a motorway at 60 or 70mph, ideally the fully available 16 degrees of vacuum advance (from the OEM Lucas module) should be delivered.
Ron.
If your Edelbrock carburettor has an integral vacuum port and exposes the distributor vacuum advance module to no vacuum at idle, then that means that it too is ported vacuum. When you undertake the vacuum measurement tests, will you be doing so on the engine free revving or whilst driving under varying load conditions? My reason for asking is on a free revving engine, the results may well be quite different to those that might normally be measured out on the road. Using a rolling road for such testing is ideal as all manner of loads and speeds can be tested for.
With the SD1 distributor that your engine is fitted with, did RPI regraph it so that the rate of mechanical advance and maximum advance provided would be correct for your 4.0 litre engine? Normally, a 3.5 litre engine will require a maximum of 36 degrees of mechanical advance whilst a 3.9 / 4.0 litre engine (which are identical in capacity) requires a maximum of 34 degrees of mechanical advance. Ideally if the engine can receive full mechanical advance by the peak torque rpm, then best performance will be obtained. Of course on top of this you will also have the vacuum advance although the amount provided is directly dependent on engine load and throttle position.
When under hard acceleration and at WOT, vacuum advance is essentially zero, so there will be no increase over the built in mechanical advance during such occassions. Light throttle use is when vacuum advance will be provide the greatest lift in overall ignition timing and as such when driving on a motorway at 60 or 70mph, ideally the fully available 16 degrees of vacuum advance (from the OEM Lucas module) should be delivered.
Ron.