A high volume oil pump does exactly what the name implies: it delivers a higher volume of oil to lubricate the engine. It has taller gears with a larger surface area so it can pump a higher volume of oil. The one shown above is a Schumann high volume pump for a small block Chevy.
The horsepower required to turn the oil pump depends on the surface area of the pump, the speed of the engine (and pump), the viscosity of the motor oil (thicker oil takes more power to pump) and the relief valve pressure setting on the pump (the higher the relief pressure, the greater the resistance that has to be overcome before the relief valve opens to dump oil back into the crankcase).
The taller gears in a typical high volume oil pump are capable of delivering 15 to 30 percent more oil volume than a standard pump. The question is, do you really need the extra oil flow?
Back in the 1970s, I worked part time as an engine mechanic in a VW dealership. Those were the days of air cooled Beetle engines. Replacing the anemic stock pump with a thicker high volume pump was a definite improvement for those engines because those old Beetle air-cooled engine relied as much on oil cooling as air cooling. Increasing the oil flow to the bearings and oil cooler helped prolong the life of the engine.
Increased oil flow in a present day water-cooled engine can also help cool and prolong the life of the main and rod bearings, and increase splash lubrication to the undersides of the pistons. But on most engines, the stock pump provides sufficient volume for these purposes. So for normal driving or even moderate performance use, a stock pump can usually provide adequate oil flow.
Some technicians will install a high volume oil pump to compensate for sloppy or worn bearing clearances. This trick works okay but a better fix is to replace the bearings or rebuild the engine with tighter bearing clearances so it can maintain normal oil pressure.
Most if not all NASCAR race teams today run very low oil pressure (around 5 PSI per 1000 RPM) and use stock volume oil pumps to minimize parasitic horsepower losses to the oil pump. But they are also running thin 0W-20 synthetic racing oil and very tight bearing clearances (.001 inches or less). This saves them maybe 30 to 40 horsepower, which in a NASCAR engine might make the difference between winning or losing a race. On a low mileage street engine with normal bearing clearances, you can save some horsepower (maybe 5 to 20 hp) by using a stock oil pump and low viscosity synthetic oil (5W-20).
Melling (a leading supplier of aftermarket oil pumps in the US) makes standard and high volume oil pumps. Like other oil pump manufacturers, they tout the advantages of high volume oil pumps and the improved flow numbers for many of their stock pumps over the original equipment oil pumps. Yet they do NOT recommend installing a high volume oil pump unless an engine really requires one. Such an engine would be one built with looser bearing clearances and running say 20W-50 or straight 40 or 50 racing oil, and/or equipped with an external oil cooler (which does require increased oil volume) or piston oil cooling jets. Otherwise there is not a lot to be gained by installing a high volume oil pump unless your engine is a high revving performance engine that could benefit from increased oil flow at high RPM.
If your engine isn't built to rev beyond 5,500 RPM, therefore, you probably do not need a high volume oil pump. On the other hand, it you have an engine that can rev beyond 6,000 RPM or higher, a high performance high volume oil pump would probably be a good upgrade and provide added lubrication insurance. A high volume oil pan would also be recommended so the pump doesn't suck the pan dry at sustained high speed. Some high volume pumps are capable of flowing up to 12 gallons per minute!
Here's another little known fact: Most double spur gear crankcase mounted stock oil pumps flat line above 5,000 RPM due to cavitation. Though the engine and pump continue to rev beyond 5000 RPM, the pump's output does not increase because the gears are moving faster than the oil can move through the pump (often due to restrictions in the pump inlet port or inlet tube).
Some racing pumps have larger and better designed inlet ports or dual inlet ports so they can perform well up to 8,000 RPM and beyond.
The clearances inside the pump between the gears and between the gears, housing and cover have the most influence on oil pump performance. The tighter the internal clearances, the more efficient the pump and the more oil it can move. A worn pump or one with sloppy clearances will not work as efficiently resulting in low oil pressure and less oil flow.
Restrictions in the oil pump inlet tube can also reduce flow. The screen mesh that covers the pickup inlet can become clogged with varnish and debris, restricting flow to the pump. Many performance pickup tubes have a larger diameter tube to the oil pump and use a more open honeycomb screen design or a drilled cover to reduce restrictions.
Something else to know: a camshaft driven twin gear oil pump inside a crankcase (like that in a small block or big block Chevy V8) turns at half the speed of a front mounted oil pump driven directly by the crankshaft (like that in a Chevy LS engine). The flow rate of a front mounted pump is therefore theoretically 2X that of a crankcase mounted pump, but the actual flow rate can vary depending on the thickness of the gears.