Karling Racing designed this mod after flow testing a 2007-up Sportster oil pump.

Purpose: The reason for testing was due to the 07-up pump having a drain port (on top of the pump body) to drain the gearcase.
86-97 engines drain the gearcase into the crankcase sump area to be scavenged by the sump drain to the oil pump.
98-up engines drain the gearcase straight into the oil pump through the added port in the top of the pump as well as a sump drain as before.
The 98-06 oil pump also has this added drain port.
However, the 07 pump is preferred as it has bigger scavenge gerotors that carry more oil out of the engine than it's predecessor.

Several mods have been devised in an attempt to scavenge more oil from 86-90 engines.
One of which is the 5 Speed Oil Pump on 4 Speed Models (Conversion for 86-90 Evo engines)
This mod adapts the engine case to accept a newer style oil pump to an 86-90 engine.

Testing:
The pump was tested before and after the modification in Karlings oil pump test rig.

During the flow test they used a reference volume filled from the scavenge outlet.
The pump was driven at a controlled shaft speed and partially immersed in a low viscosity fluid to resemble the viscosity of hot engine oil.

Results:
The results reportedly revealed that this modification increases the return flow from the sump by 63%.

When the gearcase port was allowed to suck air with the sump port immersed in liquid, the pump needed 22 seconds to fill the reference volume. ¹⁾
With the cam chest port blocked off and the internal gerotor kidney machined to optimize sump port scavenge capacity;
The pump only needed 13.5 seconds to fill the reference volume.

This shows how much return capacity that is left on the table when a newer style oem pump with cam chest suction port is installed in XL-cases that has the cam chest drain port to the sump.
Inspired by the test results Karling therefore put some effort into the modification so that he easily can produce more modified pumps to other customers.

Karling Racing's oil pump mod for 07-up Sportster oil pumps. 2)