Dry sump question

[Jack Gifford]

Assuming you meant to say "a pair of -12 scavenge lines", your sizes look okay to me (excepting some very small or large engine with unusual oil flow rates).

In general, it's best to avoid the pressure pump "pulling", if possible. Gravity feed is best, to assure liquid oil always at the pump (during startup, etc.). Although my reason for using dry sump was unusual (avoid pumping any air while front of tractor danced nervously in the air!), I had the tank (funnel-shaped bottom) above the pump and close to it. [This photo was right after doing the dry sump setup. The S-shaped hose wanted to trap some air in its high spot, so I raised the tank a couple inches and changed to a very short all-downhill hose]. It served me well for a number of seasons after that.

[fordboy628]

Assuming you meant to say "a pair of -12 scavenge lines", your sizes look okay to me (excepting some very small or large engine with unusual oil flow rates).

In general, it's best to avoid the pressure pump "pulling", if possible. Gravity feed is best, to assure liquid oil always at the pump (during startup, etc.). Although my reason for using dry sump was unusual (avoid pumping any air while front of tractor danced nervously in the air!), I had the tank (funnel-shaped bottom) above the pump and close to it. [This photo was right after doing the dry sump setup. The S-shaped hose wanted to trap some air in its high spot, so I raised the tank a couple inches and changed to a very short all-downhill hose]. It served me well for a number of seasons after that.

X2.

Your dry sump tank should always gravity feed the pump inlet.    If the pump has to "suck" the oil, your pump pressure output will be reduced by the amount of suction.    Remember, a pump is only a "differential pressure device".    Ie: with a pump pressure differential of 60 psi, with gravity flow of 1 psi output = 61 psi.    With a suction of -5 psi, output falls to 55 psi.    Line size has a HUGE impact on oil flow when the oil is cold.    Pressure pump inlet size should be -12 minimum, maybe -16 if the run is long.
 
Fordboy

[Rex Schimmer]

99% of all dry sump pumps are gear type and although gear pumps have pretty good ability to suck, they should not be put into a situation that requires them suck, they should always have a positive pressure head on their inlet (talking about the pressure section). That means the oil level in the tank  needs to be above the pump inlet, the higher the better, the hose from the tank to the pump should be large, -16 at least , even if you have to use a reducer at the end of the hose to fit the pump inlet port, and short, and direct. The oil tank needs to be well vented. Although Fordboy is correct that a pump is a differential device the out put pressure of the pump is (should be) controlled by the pressure relief valve that is connected to the pump outlet, if the situation occurred that he describes, pump out let pressure drops because of a reduction of the inlet pressure, then your pump is undersized. You should have, under all conditions oil flow from the pressure control relief valve to insure you maintain your target oil pressure which is an indication of proper oil flow to the engine. A note regarding Fordboys comment about the reduction of the inlet pressure by 5 psi, which would make the pump inlet pressure 9.7 psi absolute, at this inlet pressure and if the pump is turning at 2-4000 rpm you will most likely cavitate the pump, which means there will be air entrapped as "bubbles" in the outlet flow and air is not a good lubricant.

I also need to make a comment regarding filtration, if you check the NASCAR set ups you will see that they use a large and  fine (usually below 10 micron) filter in the return line from the scavenger pumps to the reservoir, they also use the small screen filters that Bob Drury  recommended but these only catch rock, birds and small children so any trash that may come out of the engine will then be sent to the reservoir and then to the high pressure pump. This contamination can, will, cause the high pressure pump to degrade and eventually not be able to provide sufficient flow to maintain system pressure. Having  a filter on the pressure section out let is not a bad idea as it can (will) catch the gear teeth from the pump when if fails because it ingested some large chunk of metal that was not filtered out of the scavenge return flow. Buddy has been using a large 6 micron filter on the scavenge return line  on the Flatcad with great success.

Rex