Non-functioning oil pressure gauge

[Peter Carmichael]

OK. Here is the bad news. It is just possible that every R500 has dodgy oil pressure. How come the ultra-clever Stack dash doesn't show this? Well...

 

Seasoned Carmichael-watchers will have observed unaccustomed restraint being exercised at track days. With the engine barely into its stride I have been grabbing the next gear at a self-imposed 8000rpm. I have also been heard muttering about my oil pressure. At elevated rpm I have been seeing the pressure drop away, so to avoid disaster I have been avoiding those elevated rpms. Typically, once the temperatures have settled I am seeing the pressure drop away to ~53 psi at 8000rpm. At a recent Brands Hatch trackday I was seeing 48psi.

 

When the engine was first run in October 2000 it had a healthy indication of ~72psi after warmup which barely fluctuated whatever the mode of use. From the moment I started it after this year's rebuild I had lower indicated oil pressure, with warmed up oil and 4000rpm equating to 60psi. I formulated lots of theories based on what had changed:

 

-New DKE crankshaft

-Vandervell vp2 big end bearings

-QED spark eroded oil pump rotor.

-Apollo tank installation

 

It was none of these.

 

All R500s run the Stack dash, as I do. The Stack sender (a VDO unit) is larger than the standard single wire Caterham-fit sender, so it cannot be mounted at the standard location on the filter head. Caterham fit the sender at a remote site with a piece of braided pipe linking back to a banjo fitting onto a modified filter head. The tapping for the remote sender is on the pump side of the filter; the standard sensor location is on the oil gallery side of the filter. When you run a remote pressure sender you measure the pressure at a different point in the circuit. That is why the standard Caterham gauge reads 4 bar and the Stack readout typically shows 72 psi (5 bar) - the pressure difference across the filter is ~1 bar. When I rebuilt the engine this year I managed to relocate the Stack sender onto the standard filter head, sitting underneath the smaller than standard alternator. My Stack readout now shows 4 bar and lower when running.

 

The pressure on the oil gallery side of the filter is the pressure seen by the bearings. Measuring the pressure on the pump side of the filter is a pretty pointless exercise. This means that the pressure readout on the R500 is misleadingly optimistic.

 

When an engine revs to higher and higher rpm, the bearings throw out the oil faster. The oil flow demands of the bearings increase significantly with revs - the relationship is non-linear. When you measure the pressure on the oil gallery side of the filter, you get to see the bearings appetite for oil increase with revs, indicated by a pressure drop. Meanwhile on the oil pump side of the filter, more oil is flowing through the filter, causing the pressure to increase slightly - the oil pressure regulator stops the rise from being very significant. Ideally, you would want the resistance of the filter to be negligible and the two pressures to be the same. Ideally you would want to keep the bearings well supplied with ~4 bar in the oil galleries.

 

Up to standard K-series revs (7600 rpm), there is no fluctuation of the pressure. It is only at higher rpm that the effect comes into play. The effect will become more and more significant with each 100rpm above 7600rpm. R500s are running at these high revs with no monitoring of the critical oil pressure in the oil galleries.

 

It is like not having an oil pressure gauge.

[Tony Martyr]

Peter,

All that make sense and is a classic industrial plant error. You should add that, as the filter becomes blocked the delta p increases so the R500 apparent pressure goes up whereas the pressure seen at the bearing in fact goes down.

[Paul D Jones]

At work the NH3 screw compressors have before and after and this is an indication of the condition of the oil filters there is two filters so they can be changed live but most fitters choose to do the job with the machine empty of Ammonia as is not nice stuff. If you can put two sender units in remotaly then a change over switch on the dashboard the stack dash can monitor the two pressures at the flick or a switch

 

Paul the car is neary finished so hope to see you at Cadwell if the shifts allow

[Johnty Lyons]

Peter just the man I'm upgrading to 1.8 this winter and Grade A liners with Forged pistons rev limit at 7800 Pace dry sump and Laminova oil/water intercooler any recommendations on oil pump or bearing shells??

 

jj

[Peter Carmichael]

With a limit at 7800 you will be fine with standard oil pressure pump and bearing shells.

 

The first people running the Pace kit will be experimentalists and the more revs the more likely to encounter problems. Book a test session on track. Go out for a three lap session at strictly limited revs (say 6000) shut down engine while still running, coast to a halt and see where the oil level is in the tank. See how it fills up as you start the engine and idle. If it is OK at 6000 go up to 6500 and repeat.

 

Peter