Auxillary oil pumps
Auxillary scavenge pumps
I have had two engines with auxillary oil pumps, that is, an extra scavenge pump to return oil from the bottom bevel housing. To make this modification, it was necessary to prevent oil from the 'top-end' descending to the crankcase by blocking the gallery to the sump just below the bottom bevel box. In this manner, only the oil delivered via the big end is scavenged by the original pump while the top end oil is scavenged by the auxillary pump. This pump is mounted to the rear of the timing cover inside the bottom bevel box with the magneto drive shaft running through it, from where it takes it's drive. On my two pumps, the gears are 'feed' size.
There are differing theories about why Veloce started using the auxillary pumps, first on the 'works' motors and then as a modification for some Mkviii's. My experience has been that there is no question about the ability of the standard pump to deliver sufficient oil to the engine and return it. My experience has also showed me that the auxillary pumps can be a bit of a liability in that it is only by being in excellent condition that oil is prevented from being pressured along the magneto pinion shaft and into the timing cover, either blowing out behind the magneto or, in the worst case, blowing oil into the magneto.
I have now removed both the auxillay pumps and have have experienced no lubrication problems. It seems to me that at best they offer no advantage and at worst, they are a liability.