Oil pressure/relief valve/counselling help needed

Technical advice Q&A

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RobMcC
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Location: London
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#1 Oil pressure/relief valve/counselling help needed

Post by RobMcC » Sat Apr 16, 2011 9:06 pm

I am a bit of a worrier, and have tied myself in knots over the oil system of my 1966 S1 4.2 FHC. This is perhaps partly due to a previous oil system fault causing a lot of problems, and partly due to my slightly obsessive personality. It would be super if someone would tell me to pull myself together and ignore it, or suggest a more technical solution.

My engine was rebuilt about 3000mls ago (see above) and is now running nicely. Bottom end and pistons were all new, oil pump replaced, camshaft shells etc. Oil pressure is excellent, with 35psi at hot idle and 55-60 psi at anything above 2500rpm. I am using an XJ6 filter head and spin on filter (the one with the filter mounted vertically, facing down) which has a one piece pressure relief valve. Oil pressure is measured on the oil rail.

I am assuming that there is nothing wrong with the engine itself, and that it is just that all the clearances are all nice and tight. The oil pressure gauge has been checked against a shop gauge, and is about right.

I have replaced the oil pressure relief valve previously with a NOS replacement, and calculated its opening pressure (spring force x piston area) at 55psi. The old one opened at about 47psi. The upper limit on my oil pressure therefore is the pressure relief valve, and at anything above medium revs, some portion of the oil from the pump is dumping back to the sump, rather than circulating round the engine. I think this is not good.

In order to lower the pressure (and therefore reduce the bypass to the sump) a little, I have changed to 10w40 from 20w50. This has improved matters a little, but the gauge is still pegged at higher revs, and the pressure relief valve is open. I know this, because when reducing engine revs from say 3000 to 2000, I can hear the pressure relief valve fluttering open and closed as the pressure drops past its opening point. (Took me ages to work out what the buzzing sound from that area of the engine was.)

So finally, my questions are:

Is it not good that some of the oil from the pump at higher revs is not circulating the engine, or will the engine ?take? all it needs providing it is presented with a 55psi supply?
If not good, should I replace the relief valve with an uprated one (Rob Beere), or would this put additional strain on the standard oil pump etc.

I need to sort this out, as I have to move on to worrying about ZDDP levels in modern oils.

Many thanks,

Rob McClarence

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Heuer
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#2

Post by Heuer » Sat Apr 16, 2011 10:38 pm

Rob

If you are using the original gauge and sender be aware it is for 'guidance' only! For accuracy fit either the JEC gauge/sender or a mechanical gauge, more details here: http://etypeuk.com/forum/viewtopic.php?t=173 I would not worry about the relief valve as it is about right the way it is (mine is set at 75psi for example). The XK engine is pretty robust as long as it is not allowed to overheat or run low on oil. If you are planning to track the car oil temperature is probably more important than the by-pass pressure. You will not put any 'strain' on the oil pump.
David Jones
S1 OTS OSB; S1 FHC ODB

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christopher storey
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#3

Post by christopher storey » Sun Apr 17, 2011 2:09 pm

With all due respect, I think you are worrying needlessly, . The effect of the PRV on flow is minimal. When it opens, its opening obeys Hooke's Law since it is controlled by a helical spring, and the amount of opening is directly proportional to the excess of pressure over that necessary just to crack the PRV open . As soon as the pressure drops , the valve closes again. If there were any significant loss of flow to the rest of the engine , the measured pressure would drop well below normal , as happens in the rare event of the PRV sticking open . Incidentally, the oil pump output is enormous compared with that necessary to provide lubrication and heat transfer . If you want proof of this, try emptying the camshaft "sumps" ( those bits of the head which are not drained under the cam covers) with a suction tube and seeing how long it takes spinning the engine over on the starter to refill them - it is a matter of seconds . Just enjoy it and stop worrying!!

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