oil pump question

The oil pressure in the bus gets a little low when it's hot and idling.

Looking at the oil pumps available for the isuzu 6BG1, there are a few types. Fundamentally though, most are the same except for the option of a 3 gear vs 2 gear.

Does anyone know what is the difference between a 3 and 2 gear pump when it comes to pressure and flow rates? I'm sort of hoping a 3 gear pump will increase the volume. Thanks JD

JD

A wise old mechanic once told me that everything is sweet so long as you have 10psi or thereabouts on idle. The proof in the pudding is if it comes up within the recommended range by the time you reach a gentle cruise speed RPM and stays there right up to high temperature operation on a hot day. We all know on very hot days or long hill climbs the temperature needle goes right and the oil pressure needle goes left, reversing the motion going down the other side. I see this very clearly in the little 110 Perkins in my OKA.

The old Chevrolet splash fed engines (big ends only oiled by spray going into dippers) ran perfectly well at a maximum 14psi for the pressure fed mains at cruise. In the manual for idle it says the oil pressure should be "Indicating" ie one needle width. They were certainly more reliable than the 30psi Ford V8's of the period.

I can not see how getting more flow by changing the pump will achieve much with galleries and oil ports built for the lesser pump. This assumes the pump is operating at good efficiency and not worn out or screen blocked. If it is OK the only reason for deterioration in pressure is going to be a worn engine or a loose or broken internal oil line..

Found a Cummins Forum and they said, "Cummins says the minimum hot oil pressure for a 6B series is 10 psi at idle and 30 psi at rated power."

The corollary of of Lang's needles is that I knew when the filter and oil needed changing in my 45 series Landcruiser as the pressure stayed up while going up big hills. New oil and needle would drop.

A high mileage engine with unknown or slack oil changes should get a new oil pump as a matter of course, and have you thought about rolling a new set of mains and big ends in. Interestingly after market shells for my Isuzu failed to bring the plastigauge gap up to spec so went to genuine which did the trick and cheaper too.

Ok,

so I bought the 3 cog pump. Sort of having buyers remorse!

Back to the engine; it's mostly in spec. It does drop off at idle after a run up a hill to my place. I'm sort of used to that. What got my attention was it was lower than normal when 'cruising' at about 85ks ~1900 rpm. Previously, was just on pressure spec.

The engine is a Japanese import, so I don't know the history. Sounds tight. No rattles. I've put 20,000 k'son it so far.

So, hoping the pump will lift the pressure and increase volume. Conversely, I don't want the volume to be less than a 2 gear. (which I guess is another question; is the 3 gear less volume than a 2 gear?)

Good luck with the new pump. I am sure it will be an improvement even if the reason was the old pump was worn and not the extra flow of the three gear model.

Lang

Here is the Cummins Manual. The chart at the right is pressure switch settings for engines fitted with alarms. The main chart is the actual recommended pressure. This is a shot out of the actual manual showing 5psi idle minimum not the 10psi quoted in the Cummins forum.

To my way of thinking a few things need to be checked why the motor is in bits

I assume that this has a bypass filter, is something stuck open here
Is there an internal leak
Is the pump worn

And the list could go on

A increase in volume would have to help as there is more oil to flow past worn parts , which in turn would mean that a given volume to flow the pressure would have to remain higher

However this just my brain thinking so it isnt based on any scientific thoughts just my own peanut rolling around

Maybe a oil cooler might help

You could work out the volume of oil punped pretty easily with some basic maths

Pull each pump apart and measure the size of the impeller and see what it comes up with

Would also give you a look to see if it is worn at all

Set of feeler gauges and a straight edge and measure the depth of the impellers

Might just need a sheat of glass and a bit of emery paper glued to it and a hour or so doing figure 8's to bring it into spec

Either way it will be a learning experience

Paul

The reason I rebuilt the engine in the David Brown was the 5psi at idle. Stripped it and sent the crank away and then ordered new bearings and rings etc. Now brilliant 15psi at idle and 40psi at full revs. Original old oil pumpworks perfectly well.
I too was told low psi at idle was fine but if it threw a piston through the block were they going to pay to fix it? Of course not. Strip it down and do a deep dive as to why it is as it is.

crikey! I've opened a can of worms.

The oil pressure spec is (as quoted in the manual)
approx 14.2 psi 98kpa at 600rpm
and approx 35.5 psi 245kpa at 1900 rpm

Mine is around those figures. Sometimes on the lower side. Not excessive but have noticed. More so when idling.

Lang, thanks for the images and info. This is from the 6BG1 manual. It shows the similar points.

The piston coolers open at open at 196kpa (28.4) and the overall pressure release opens at 348kpa (50psi) I reckon I'm a little bit low and coolers aren't getting the full squirt. By the specs, the pressure (aka high flow) can increase upto 50psi and remain operational.

I'm not planning on a rebuild any time soon. But would like to bring the oil pressure up a little. The pump is accessible via the sump and is a 4 bolt job.

Back to my question; does a 3 gear displacement pump (as per the diagram) give a higher flow and the 2 gear gear? By definaition, it should. But I'm not sure if the design facilitates more oil moving though the pump.

One thing seems a bit muddied in the investigation is the blurring of flow and pressure, they are far from the same thing. As the main and/or big end bearings wear out, because of the greater gaps the flow will increase and pressure drop. As Paul suggests the extra flow through the worn section may well do a little to compensate but at the same time those parts of the engine still up to clearance specifications will be starving for oil as the pressure is insufficient to push properly into the gap.

I do not know but I suggest the high flow pump has more to do with cooling than lubrication. The big blue notice in the manual above is put there to stop people panicking over lower oil pressure compared with most other engines but points out high flow is their aim not pressure.

JD I can not see where the 14.2 minimum pressure at 600RPM comes from. It seems from your manual shot that is the bypass valve opening pressure not minimum gallery pressure.

I could be talking crap but it seems to me you don't have an absolute disaster on your hands. Maybe the extra gear pump will up all the figures.

Lang

No can of worms Jon, Im just thinking out loud, I treat the forum as if Im just standing there talking to someone and we are bouncing ideas off each other trying to find the most sensible answer, takle that, prove its wrong or right lol and work from there

Lang I would think that this style of oil pump is positive displacement, what ever goes in will come out or it breaks

The only time this isnt correct is if it is badly worn and cant make pressure

Would I be correct in thinking that ?

If I am correct then would I also be correct in thinking impeller size is what determines the volume, the amount of impellers determine the pressure

I really dont know as this something I wouldn't do but I have known people that have mucked about with oil pumps and made huge differences

Some from putting high flow pumps on as suggested here and others from measuring the depth impellers and adjusting to suit

Me ? Well, why it has reasonable pressure I would keep on driving until a problem arose and then I would be in for a major rebuild, this doesnt mean my way of thinking is correct, rather I'm probably a bit rougher than most

Paul