Help interpret oil pump relief valve spec

[TallPaul]

The shop manual for my old '84 F150 300 cid I6 engine states that the oil pump "Relief Valve Spring Pressure Lbs. @ Specified Length" should be "20.6 - 22.6 @ 2.49".

I am wondering if there is a way to translate that to what oil pressure should open the relief valve (though presumably the valve opens partly to fully over a range of pressure).

Given that the pressure spec is 40-60 psi at 2000 rpm, the relief pressure must be a lot higher than the "Spring Pressure." My '95 F150 with same engine and pressure spec, maxes out cold pressure around 55 psi, but I think I have a weak relief valve spring (for example my 1990 Ford 460 V8 maxes out cold around 75 and has similar hot spec: 40-65 psi).

Hey, the other weird thing is they give the pressure spec as a single number (275 kPa), followed by a range for psi (40-60). If I recall correctly 100 kPa is about 14.5 psi, so their kPa figure is about 40 psi. Strange.

 

[Gary Allan]

I believe 100 kpa is 1 bar ..which I think is one atmosphere at sealevel ..which is 40 psi and change.

You've got to factor in the "Square inch" deal into the spring pressure at a given length. 22 lb weight rating over a .25^" poppet valve/ball will be different than if it has a 1^" relief port.

I don't have a clue on how you would integrate the free length spec to a in use pressure.

 

[Drew99GT]

1 bar is 14.7 psi

 

[Gary Allan]

Yes, I didn't edit in 14.7 and that 275kpa is 40 and change. 2.75x 14.7 - 40.23+/-

 

[mechtech]

That spring spec is for free length.
I've sometimes put a washer in the seat, to get more pressure before bleed-off, but it won't help anything if the valve is not activated - like hot oil at normal running.

 

[bulwnkl]

1 bar and 1 atm are very nearly identical, but I can't recall the exact figure for psi. I remembered something in the 15-18psi range, but I just don't recall exactly.

 

[Bret Chase]

you can figure it out if you know the area of the piston/surface acting on the spring. if the area is 1/2 sq in. the pressure required to open it would be 41.2-45.2psi.

 

[mechtech]

Bret - It depends on how much the piston has to move to uncover the bleed off port.

 

[TallPaul]

quote:

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Originally posted by mechtech:
I've sometimes put a washer in the seat, to get more pressure before bleed-off ...

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Nice idea. I understand there are a few engines where the oil pump bypass spring can be changed from the side of the block. I guess if I were in far enough to get the oil pump apart I would get a new pump (or a new spring).

 

[Winston]

Given:

k=spring rate in lb/in
L=Distance spring must compress in order open relief passage
A=area of relief piston = (pi*dia**2)/4
P=Pressure relief pressure in PSI

The formula would be:

P=k*L/A