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Hole in oil pressure relief valve

Tennant
Posts: 69
Joined: Sun Jan 20, 2019 7:13 pm
Location: Herne, Germany

Hole in oil pressure relief valve

Post by Tennant »

In preparation for an "in car wellness cure" I stripped an oil pump that I have to check the tolerances to see if I can just swap it out with the one now in the car. Everything measures perfect.
I looked at the infamous hole in the relief valve plunger and had thoughts how to close it, because I am sure that it does not help by hot oil pressure at the 400RPM tickover when the bearings are a bit worn.
I was reluctant to put a spot weld on the plunger, scared to distort it.
I had the idea of not closing the hole in the plunger but the larger hole in the big spring retaining nut, because what goes through the plunger can only get back to the sump through this hole. This is easily closed with a short 6mm bolt, copper washer with a nut with Locktight. This compresses the spring by a couple of mm also which is another advantage.

If there is any known cause or impediment to this procedure, speak now, or forever hold your peace!!

Thanks for any input.

Barry

silverdart
Posts: 285
Joined: Sat Feb 13, 2016 10:49 am
Location: West Midlands

Re: Hole in oil pressure relief valve

Post by silverdart »

Hi, I do not quite understand what you are trying to achieve. If you wish to increase your oil pressure, the usual way is to put a 3/16in. thick washer behind the relief spring to increase the strength of the relief valve.

Dave.

Tennant
Posts: 69
Joined: Sun Jan 20, 2019 7:13 pm
Location: Herne, Germany

Re: Hole in oil pressure relief valve

Post by Tennant »

Hi Dave,

I really only wanted to fill the hole in the valve plunger to improve the tickover pressure. It must be that some pressure leaks back to the sump through the hole, which will only be important at low rpm. Because of the very low tickover of the V8 of about 400rpm the pump needs all the help it can get when the oil is very hot. I do not necessarily want to increase the maximum oil pressure as the about 40psi is enough.
I have never seen such a "bypass" hole in an over pressure valve
I just wondered how other people have blocked the hole, what effect it has?

Thanks for your input

Barry

Chris_R
Wise Man
Wise Man
Posts: 662
Joined: Wed Feb 10, 2016 12:48 pm
Location: Twickenham

Re: Hole in oil pressure relief valve

Post by Chris_R »

10psi oil pressure at idle is quite adequate.

Ian Slade
Wise Man
Wise Man
Posts: 792
Joined: Sat Feb 13, 2016 9:54 am
Location: Sevilla Spain

Re: Hole in oil pressure relief valve

Post by Ian Slade »

By not playing with the relief valve the gauge you get a fairly good indication of the bearings wearing, change the shells at at 10psi on idle and you can avoid a full strip down and regrind, you will get at least 3 shell changes if the oil is regularly changed and if lucky 4, so about 180k to 240k miles before a regrind, the crank is made of premium steel, so with to days normal usage of these cars judging by posts on here you will get about 10 years use of a set of shells.
Owner since the 70's, Genghis is slightly to my left.

Tennant
Posts: 69
Joined: Sun Jan 20, 2019 7:13 pm
Location: Herne, Germany

Re: Hole in oil pressure relief valve

Post by Tennant »

Probably wise words Ian.

Barry

Chris_R
Wise Man
Wise Man
Posts: 662
Joined: Wed Feb 10, 2016 12:48 pm
Location: Twickenham

Re: Hole in oil pressure relief valve

Post by Chris_R »

Picking up on Ian's comment it is worth reiterating what is going on. Many people falsely believe the oil pressure is what supports the bearings. It isn't. Instantaneous pressures inside the bearings on the crankshaft can easily reach 5,000 psi and no pump can provide that kind of pressure.
What is important is the flow of oil into the bearing cavity, i.e. the gap between the journal and the bearings. The oil is pumped into the gap and flows out of the sides of the bearings and back into the sump. As the oil flows into the gap it is dragged around by the rotation of the journal and squeezed into a wedge at about the 7 o'clock position which is where the maximum pressure is exerted. It is this squeezing into the wedge that supports the journal and prevents metal to metal contact. The pressure that you see on a gauge is the resistance to that flow into the bearing gaps. A thick oil has a high resistance, a thin oil a low resistance. That's why on startup when cold you see a high pressure and once warmed up the pressure reading is lower.
Fiddling with an oil pressure relief valve to increase the gauge readings won't squeeze any more oil into the gaps in the bearings as the oil is not compressible, you can only get in as much as leaks out of the sides. All it does is create a higher back pressure on the pump and the gearing driving the pump.

bobtills
Posts: 186
Joined: Wed Sep 25, 2019 6:05 pm
Location: North Wales

Re: Hole in oil pressure relief valve

Post by bobtills »

Chris or Ian - I wonder if you would have any ideas about my oil pressure?

It's more when hot than when cold - about 34 at 3000 rpm when cold and about 38 when hot. At idle you can't see any reading on the gauge but there's no knock.

Having said there's no knock, there is a knock when pulling when cold which almost disappears when hot. I'm guessing piston slap but I'm no expert. The engine had a very expensive rebuild about 18 years and 19,000 miles ago which included a rebore but there is some oil smoke if you disconnect the pipe from the valve cover to the air filter while it's running.

The words of a wise man are always welcome :D

Chris_R
Wise Man
Wise Man
Posts: 662
Joined: Wed Feb 10, 2016 12:48 pm
Location: Twickenham

Re: Hole in oil pressure relief valve

Post by Chris_R »

I can't comment on the cold/hot difference. Having reread my previous post it might give the impression that pressure is not important which is not the case. Pressure is important to maintain a positive flow not just into the bearings but to the other parts of the engine that require lubrication. If the pressure drops too much then some parts may be starved of enough lubrication. It's a balance. The best use of an oil pressure gauge is to observe a change in pressure over a period of time (months/years), if the readings gradually drop off it is an indication of less resistance to the flow, less resistance to flow can indicate increasing wear in the bearings.
38psi at 3,000 rpm when hot is OK. Oil pressure warning lights are usually set at around 6 or 7 psi, if it flickers on idle then the pressure is hovering around that sort of figure.

Ian Slade
Wise Man
Wise Man
Posts: 792
Joined: Sat Feb 13, 2016 9:54 am
Location: Sevilla Spain

Re: Hole in oil pressure relief valve

Post by Ian Slade »

Bob you have a electric oil pressure gauge, these use a carbon pile which when under pressure changes resistance these sensors due to ageing can leak on the internal side, they were also originally fitted as a pair, changing the sensor or gauge will introduce errors, in other words what you read on the gauge is not accurate and hot oil can also affect the reading especially on an old sensor. The SP 250 uses a mechanical gauge known as a Bourdon tube which as the pressure increases straightens the tube and by mechanical linkage moves the gauge needle, once calibrated this tend to be fairly accurate throughout its life.
The V8 engine can have seriously worn bearings and give no knocks, one of my engines had a con-rod bearing pick up at 7000rpm, when stripped down the shells were stuck to the crank and the bearing was steel on steel between the shell and con-rod with only splash lubrication, there was a slight knock on idle and a drop of 10psi at idle but no knock and about a 5psi drop at 3000rpm so hot was 45psi at 3000rpm.
Owner since the 70's, Genghis is slightly to my left.

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