Once Again...... Oil Pressure

[DavidOliver]

While replacing the distributor I decided to change the oil pump rotors, obtaining new rotors from SJS. I am checking the lobe patterns between old and new. The result was oil pressure getting to 40 lbs/in2 at 1000rpm and no matter how many more revs there is no oli pressure increase. This with engine oil at start up temperature. Oil is new. I recently and previously renewed the relief valve spring and had oil pressure at start-up of 55 lbs/in2.

To open the debate on oil pump type and location on the 9XX engine, I wonder whether the engine was intended to have a scavenge sump and external pump at sump level. The actual position is hardly logical requiring considerable priming after even an oil filter change, To prime the engine after a major fettle needing to fill the draw-up pipe from the sump, attention to the nylon pipe seal under the auxiliary housing, pre-filling with oil not an option, I can only achieve by spinning the auxiliary sprocket before connecting the timing belt until pressure is registered.

Even the Rover V8 with external oil pump at sump level requires the gears to be primed with petroleum jelly and spun with an external priming shaft inserted where the distributor fits.

So how to obtain a suitable pressure for a low pressure high flow engine and to ensure adequate pressure at higher revs (6500 rpm) with warm/hot oil.
Does the special gasket between rotors and rotor housing affect pressure? Does this gasket need a fine anaerobic sealant? What tolerances are required between rotors and housing? Has anyone dared to assemble with no gasket risking catastrophic failure if the rotors bind up at higher temperatures? What holds the auxiliary shaft in position, just the spring that fits between distributor and auxiliary shaft?
Tim Engel has mentioned putting washers behind the relief spring to increase pressure relief. I do not think that pressure relief is my current problem.

Dave the cog

[Pete Boole]

I believe that SJ had a poor batch of oil pump components made some time ago (like a lot of his stuff). Are you sure that you don't have these? The oil pump clearances are given in the service notes I think, and you used to be able to buy different thicknesses of gasket but I don't think you can now.

I usually do the first fill of oil in a newly built engine via an oil gallery under pressure - that way it gets to most parts of the engine as if it had come from the oil pump itself. You're right about turning the pump via the aux shaft - it is the only way to do it without turning the crank at the same time.

The only thing keeping the aux shaft in place is the oil pump rotor - the spring under the distributor shaft just pushes the rotor lightly against the casting.

I've almost finished my oil pressure relief valve testing rig if I can be of any help.

Pete

[DavidOliver]

Thanks for the reply Pete. Rather disconcerting that you can pay so much for a rotor set and not be sure it works adequately, particularly when dealing with such a vital mechanical part. Where can I buy a guaranteed rotor set, Gary Kemp?

I think I need to again free the cam belt and spin the auxiliary shaft once I have refettled the oil pump to asses pressures obtained.
I have previously fitted a mechanical oil pressure gauge to get direct readings without electrics, this could be a good point to add a branch for pressure filling the oil ways after filter changes or other fettling. It is a pain to have to remove the cam belt to guarantee priming.

What opinion about original dry sump concept for the 9XX engine, and a Chapman FIX for the road engine by placing the oil pump in the Heavens?

Dave the cog.

[DavidOliver]

I have now fitted the Garry Kemp oil pump rotors with the help of different thickness housing gaskets to check clearance.
The gaskets are 0.15mm (standard), 0.08mm and 0.5mm. I found the 0.8 gasket to bind slightly and to dream peacefully I installed with the 1.5mm gasket therefore giving a 0.7mm clearance.
The new rotor lobe gap, between outer and inner annulus, was 0.01mm compared with 0.025mm on the original rotors.
The SJS rotors have a slightly different lobe profile and have a 0.018mm lobe gap.
All three sets of rotors measured the same depth at 12.75 mm.

The relief valve has a new spring which required a load of 12kg to compress to approx valve opening, 10mm.
I do not know what distance the piston has to travel before opening the side holes to the relief outlet before pressure is released.
This 12Kg technicaly should provide a pressure of 75p.s.i. The relief piston has a 17mm diameter.

First pressure results show a start up pressure of 60psi and at 2500rpm reaches 80psi.
With oil warmed up the pressure at tickover is over 10psi and rapidly reaches 40psi at 2000 revs.
These are first pressure observations, it all neeeds to settle down but shows the necessary pressures as compared with the Lotus Manual.
I have not revved to 6500 rpm to check the minimum manual 45psi requirement.

As a side note I see that Lotusbits provide a racing pump with an increased rotor depth, at the princely price of 750 GBP (IIRC)

As another side note, now with Brexit, importing parts to Spain has become a complete PAIN, and for the rotors implies an extra charge of 100Euros in duties and paperwork/payment procedures. Also there is no point in returning parts as there will be a double importation charge.

And as another side note, Garry Kemp advised me when priming a replaced oil pump to slip off the cam belt and spin the auxiliary pulley until oil pressure is developed.

Hope these numbers help.

Dave the cog