Posted From: 220.127.116.11
|Posted on Friday, 26 September, 2014 - 07:13 am: |
Oil pumps suck oil from the sump bowl and pump oil around the brgs.
Because the suction pressure can only ever be a maximum of 14.7 psi, a proper filter can not be used, only a mesh strainer which is not that fine, the strainer will catch the big bits enough big bits will block the strainer by action of the vacuum and cut oil to the engine brgs. Once the engine is stopped the debris falls away and pressure is restored. If the strainer is too close to the bottom of the sump bowl the oil will not be able to move fast enough to the strainer. Oil doesn't compress and neither does it stretch. This causes the pump to cavitate. Again once the engine is stopped the oil has time to fill the strainer and the pressure will return.
Either way it's sump off. To check for strainer to bottom of bowl clearance either use plasticene or a rule.
The pick up pipe must be vacuum tight. Usually a gasket or 'O' ring is used.
The RR V8 uses a gear pump. One gear is driven from the crank and that gear drives the second gear. The gear are in a case which is just a small amount wider than the gear. The case dia is a small amount larger than the dia of the gears.
As the gears turn the oil is sucked up and goes round the OUTSIDE of the gears it doesn't go between the gears. So the mesh backlash has no effect on the pumping action.
When gear pumps wear it is the width that wear not the diameter. Unless the gear brgs are worn allowing the teeth to touch the sides.
Take the pump apart and check the sides and brgs for wear and scoring on the gears sides and the case sides. Under no circumstances alter the clearance to less than the specs because it will not make the pump work better and the pump could seize and wreck the pump drive gears which will cost lots to repair.
One would think that oil pumps due to lots of oil wouldn't not wear, unfortunately they do wear, however one would think that wear will seriously impair the operation of the pump providing the pump is not to bad it will work fine. So if the pump is in tolerance a new pump is not needed. However if the whole engine is being rebuilt then one might as well fit.a new one. Some oil pumps are only £30 new and some are mega more.
To test pumps clamp in a vice gently with the strainer upwards and horizontal and fill the strainer and pick up pipe with oil turn the pump until oil comes out of the outlet port and stop. Top up the strainer. The oil must not run through the pump a good pump with hold the oil back for hours and over night maybe an egg cup full. A lot of pumps I have tested are still more or less full overnight.
Place the strainer in oil and turn the pump by hand the pump should pump within one turn and suck, if primed oil should issue from the out port immediately. Turn the pump slowly and it should pump at 10 rpm and even slower. Block off the outlet port and the pump.should immediately lock solid and if allowed to readily squirt oil a good few feet.
The pump drive gears on a RR V8 are either bronze on the pump and steel on the crank or Visa versa. Never steel steel or bronze bronze.
The back lash is 0.012" and measured with a dial test indicator. A good clue is with no oil a bit of lash then with heavy oil a lot less. Or use a feeler gauge. However because of the service history of the drive be extremely fussy, and maybe go and acquire a dti, used one will do Verdict angle stylus one is ideal. A verdict can be used on brake rotors etc as well.
If the pump is fitted to the engine and the engine turned upside down and then oil is pored into the strainer and the engine turned one can watch the oil squeezing out of the crank bearings. Or the pump can be turned on its own for the same effect. This will show that the system is diffinately good. The starter motor should be fast enough to put the oil light out.
Pressure relief valves. These are sprung loaded ball or spool which dumps oil usually out of the pump direct which splashes out and ends up back in the sump bowl.
The spring controls the relief pressure. The average pub mechanic thinks that packing the spring to raise the specified relief pressure is a good performance mod. This overloads the pump drive.And uses engine power.
Some engines when revved immediately reach relief pressure. MINIs do that. RRs don't they eventually blow at 42 psi. RR didn't like oil pressure gauges because typically at idle the pressure is about 5psi and when revved the pressure slowly picks up. This is normal because the flow rate is high. Turbo charger brgs like high oil flow for cooling. This fact is the undoing of many after market turbo installations. The oil system is high pressure low or lower volume, when tapped for the turbo feed the oil pressure drops so the diyer fits a restrictor instead of fitting either a bigger pump or a second pump. Austin Turbo A series engines had bigger pumps. Still had turbo trouble though.
High flow rates aid cooling of the brgs. Which is another reason for grinding race cranks loose and fitting a bigger volume pump. The pressure only needs to move the oil fast higher pressures cause problems with pump drives and oil filters.
(Message approved by david_gore)