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oil pressure

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TWF
Posts: 126
Joined: Wed Mar 09, 2016 7:10 pm
Location: Liverpool

oil pressure

Post by TWF »

Hi, I have the oil pump from my V8 250 engine stripped at the moment and whilst everything looks good I have decided to renew the relief valve and spring, is there a way that I can put a bit more tension on the spring to increase the oil pressure and as I don't have the means to bench test the pump what would be the ideal way of doing it? Regards ,Terry .

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

Re: oil pressure

Post by silverdart »

Russ Carpenter told me to put a 3/16in. thick washer behind the spring and I now get 60psi at about 1500 rpm onwards when hot.

Job done.

Dave.

Christopher Storey
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Location: Cheshire

Re: oil pressure

Post by Christopher Storey »

The raising of oil pressure is not necessarily a good thing. In the first place, it absorbs a measurable amount of power, but more seriously it may have deleterious effects on the flow rate through the system, and this is just as important as pressure , particularly in this engine where the main bearing areas are small . Also, altering pressure from the design level can have unpredictable consequences such as increasing the likelhood of cavitation at the pump

fredeuce
Posts: 76
Joined: Mon Feb 29, 2016 10:56 am
Location: South Australia

Re: oil pressure

Post by fredeuce »

There were posts about packing up the relief valve spring on the old forum. As I recall it the thickness of the packing mentioned was 3/8".

http://archive.dloc.co.uk/forum/topic.a ... l,pressure

Chris_R
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Re: oil pressure

Post by Chris_R »

I agree with Christopher Storey. As a general rule of thumb about 10psi per 1,000 rpm is sufficient. Oil pressure is a false measure. What you are in fact measuring is the resistance to the oil flow in the system. That resistance in mainly due to the gap in the crankshaft journals and to a certain extent the other orifices where oil flows out. The most important thing is the amount of oil that is flowing in the system. By increasing the relief valve you won't get any more oil flowing through the system, oil is not compressible and you can only get a certain amount through a gap at any one point in time. Equally important is the viscosity of the oil. You should use an oil of the viscosity as originally recommended by Daimler and not more.

tjt77
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Re: oil pressure

Post by tjt77 »

It makes sense to shim the relief valve when undergoing an engine rebuild.. factory setting is too low for the gearing of the saloons at higher cruising speeds.... as is posted above 'volume' of flow is the more critical issue.. and the oil pumps on these engine do not have extra capacity and consequently .003" of wear on crankshaft bearings will give a low oil pressure reading (sub 35lbs @2200rpms) on a a hot engine factory setting of 40- 45lbs is too low pressure to run at continuous high speed as these engines do put high loads on the bearings.... its also true to state that excess oil pressure saps power.. hardly noticeable however, but not much more than you'd use by adding a high output alternator.. .what DOES suffer if the oil pressure is set too high (above 60 lbs) is that the bronze drive gear for oil pump/distributor can wear prematurely and you run the risk of oil leaks in places such as the feed pipe to valve train.... the common reason for low oil pressure on the daimler V8 is worn main bearings.. setting the pressure a little higher helps prolong bearing life as it reduces the possibility of metal to metal contact..however, shimming the relief valve wont help if the engine is already worn..in which case pressure will read higher initially, but will still drop off when the engine reaches operating temp. The current aftermarket oil pressure relief valve springs tend to blow off at just over 45lbs.. a 3/16 - 1/4" spacer should add another 4-6 lbs before the relief valve opens .. 50-55lbs is ideal. also.. do NOT use thicker oil than recommended.. 20w/50 is as thick as one needs to go.. I personally use 15w/40 ...with the P relief valve shimmed approx 1/4", engine maintains 50lbs at speeds above 2,500 rpms @ full operating temp..

Phillmore
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Re: oil pressure

Post by Phillmore »

I wouldn't have thought packing the relief valve would have any effect once the oil got up to temperature (only on cold start up which is what it's for). Once oil pressure has dropped below 40-45 blow off point the relief circuit will be inoperative surely?
Andy

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

Chris_R
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Re: oil pressure

Post by Chris_R »

The oil pressure delivered by the pump does not support the bearings in the crankshaft. Instantaneous pressures in the bearing can reach 6,000psi and your oil pump cannot deliver that. This is why the oil flow is important.
What happens is that during running the crankshaft actually rotates slightly off the centre line, at about the 7 o'clock position. When the crankshaft and bearings are assembled there will be a clearance between the journals and the bearings, typically about 0.0010". While the movement off centre is very slight it nevertheless does happen within that gap.
As oil is fed into the bearing under pressure from the pump it is dragged around in the bearing gap by friction and due to the slight offset running position it forms a wedge under very high pressure and it is this wedge that supports the bearing. The flow of oil is critical to the continuous formation of that wedge. This is also why the viscosity of the oil is important. You might think that a thicker oil will help but in fact the reverse can happen. It does not flow so freely and because it is thicker it has more internal friction as it gets dragged around in the bearing gap which generates more heat and conversely as oil thins with heat it may instantaneously be thinner in the bearing than a thinner oil!

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