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V8-250 oil pressure relief valve

David S
Posts: 188
Joined: Sun Feb 21, 2016 9:23 am
Location: Forest of Dean

Re: V8-250 oil pressure relief valve

Post by David S »

I understand after talking to Russ Carpenter that the tiny hole was to stop the gauge dithering.
He also thought that higher pressure to the mains makes them last longer.

I have also put in a 3/16" spacer, I've done away with the hole in the bit in question, I also replaced the main bearing shells too and the tick over pressure is now really good and the gauge does not dither.

Fossil
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Posts: 516
Joined: Sat Feb 13, 2016 5:08 pm
Location: Helensburgh, Argyll

Re: V8-250 oil pressure relief valve

Post by Fossil »

Part of my reason for refitting the spacer in my SP oil pump was having added a remote filter head and oil cooler at the time of the rebuild. After discussing this with Russ it was felt likely that the higher pressure would ensure good flow through the whole system over a longer period of time. And as has been said it is something that he considers worthwhile for the health of the engine anyway. To me that is about the highest recommendation one can get for a Daimler V8. As I said I didn't wish to initiate another discussion about the pros and cons of altering oil pumps, but, I've always understood that in a fluid filled system of fixed resistance, an increase in pump pressure will generate an increase in flow. Is this not so? Furthermore I'm having difficulty with the suggestion that an increase in pressure could cause reduced flow? I'd welcome further detail about the mechanism of this. Meanwhile given extensive past personal experience of low oil pressure in Daimler V8 and st 6 engines, particularly at idle, I do find it more pleasant and relaxing to look at gauges registering something greater than 5 to 10 psi in hot weather and slow traffic.

Cheers

Geoff

Ian Slade
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Joined: Sat Feb 13, 2016 9:54 am
Location: Sevilla Spain

Re: V8-250 oil pressure relief valve

Post by Ian Slade »

Geoff, you are correct, any increase in pressure will cause an increase in flow, providing the restriction stays constant, the first restrictor being the filter and the plumbing to it, With the V8, SP or saloon you can have zero pressure on the gauge at idle and 50psi at 4000rpm, the pump therefore is capable of producing considerable flow in the higher rev range, one wonders whether in some cases with with a engine in good condition the pressure exceeds 50psi at the higher rpm as it seems most do not exceed 3500rpm and if they do are they looking at the gauge?
One must remember that at 500rpm and a good engine can achieve 50psi therefore an engine that is recommended only not to change gear above 6000rpm indicates it can be revved to above 6000 rpm that pump is working hard and relieving most of its pressure and flow, whilst competing I have seen the pressure rise above 50psi though to what one doesn't check, based on the small standard instrument position being vertical, all to the right is good, problems to the left.
Owner since the 70's, Genghis is slightly to my left.

tjt77
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Posts: 525
Joined: Wed Mar 23, 2016 7:01 pm

Re: V8-250 oil pressure relief valve

Post by tjt77 »

shimming the oil pressure relief valve will not alter oil pressure at idle.. the valve is shut until it reaches sufficient pressure to push the plunger against spring pressure.. opening the valve.. shimming the spring, or fitting stronger spring increases the point at which valve opens.. a 3/16 shim should add about 10lbs more at speeds above 1,800-2,000 rpm.. low oil pressure is commonly caused by main bearing wear in these engines.. you can gauge wear by how much the pressure falls at full operating temp compared with what it reads on a cold engine.. the Oil pump does not have much in the way of 'extra' capacity.. if there is wear on both main bearings and oil put pump you are likely to have low pressure at idle .. if it does not rear max pressure at 2,500 rpms there is wear somewhere.. same story if the pressure drops off significantly when hot.. anything less than 30lbs @ 2,500 rpms when hot (tested on a mechanical gauge) should be considered suspect..

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