V8-250 oil pressure relief valve

[Christopher Storey]

There is a fallacy , which is perpetuated by unauthorised modifications to release valve springs , that a rise in oil pressure at idle is a commendable thing . Unfortunately, it is a fallacy which can do a great deal of harm if the rise in pressure carries with it a limitation of flow. If the rise is achieved by effectively making it more difficult for oil to leave the oil pump, then the likely consequence is that the bearings , which are the important points of call downstream of the pump, are receiving a lesser quantity of oil than they should be . This has 2 very undesirable consequences : 1. less heat is carried away from the bearings by the oil and 2. the clearance between bearing and crankpin, which is maintained by the designed supply of oil, is degraded. The inevitable consequence is accelerated and increased bearing wear and , in extremis under heavy load, premature bearing failure

[silverdart]

Dave, You do not mention your crankshaft main bearings.
If the bearings and/or the journals are worn, this would reduce oil pressure.
Also did you measure big end journals for wear?

Dave.

[Ian Slade]

As I understand it, the oil pressure relief valve is set to protect the pump and the filter, the pump is a fudge set to be above the flow needed to maintain a pressure on an average worn engine, not knackered, with the relief set too high there is extra strain on the gears and shaft which can lead to sheared drives. Increasing the pressure by increasing the relief pressure setting causes a greater flow through the filter which can cause the collapse of the filter and thus starve the bearings of oil, it has been stated most engines run on bypass and only filter between 10-20% of the oil passing through the engine, certainly true when the engine was designed, whether advances in filter design have allowed a greater flow through the filter so a greater percentage of the oil is filtered is moot.
The SP pressure take off is at the pump, therefore the pressure reading is different to the saloon where it is taken from the block and will read lower especially on a worn engine, the fact that increasing the oil pressure at the pump gives a better gauge read out does not indicate there is a higher pressure at the bearings, it just means that there is a higher pressure going into the filter, not so with the saloon V8 which is read after the filter.

[Christopher Storey]

Ian : as far as I know, both the SP250 and the V8 saloon have full flow filters ; the by-pass variety tended to fade out from the mid 50s onwards because as you say, they effectively let a large part of the oil through unfiltered

[Ian Slade]

Christopher, you may well be correct, though all filters have a limit be they full flow or not, it may well be the relief bypasses to the sump rather than the engine, the relief is still there to protect the filter from collapse as well as overstressing the pump.
I have seen filters advertised for the same engine some being short and dumpy with others being the same diameter but longer, the longer one having a greater surface area of filtration therefore having a higher flow, they both can't be right, when choosing a filter you need to know the pump flow rate at the required pressure and the filters flow and pressure limit, nothing wrong wit a filter having a higher flow rate than the pump but not vice versa.

[andrewatcourtfarm]

Phil Glennerster rebuilt my SP250 engine 10,000 miles ago, I have always used Penrite Classic Light oil. Since the rebuild the oil pressure has always been 50 psi; hot, cold, tick over or high revs, so my pressure relief valve is working as it should do.

[Phillmore]

That could be a gauge fault? Hot tick over pressure will always be less than cold pressure.

[Phillmore]

Unless . . . . . . your system can genuinely deliver 50psi with hot oil and the excess cold pressure is let out by the relief valve?

[andrewatcourtfarm]

I am pretty sure the gauge is OK. It goes back to zero when I turn the engine off and it does move a few psi when I tweek the throttle.

[Ian Slade]

At 10K miles with a completely reconditioned engine including the pump being either reconditioned or new I would expect little or no drop in oil pressure, when I first rebuilt my engines one without a regrind with standard bearings the other with a 10thou undersized crank with +10 bearings neither lost oil pressure at idle (500rpm) though the bottom plate of both pumps were dressed to remove light scouring. Unfortunately the reground engine had a pickup problem on one of the big ends causing the shell to rotate in the conrod, even then the oil pressure only dropped 5 psi, I suspect it was due to taking a large right hand hairpin at the end of the Greenham Common airbase main runway onto the perimeter track during JDC sprint event before the cruise missile fiasco at over 100mph in third gear chasing a Ferrari Dino.