Shimming the spring isn’t expected to affect the hot oil pressure at idle speed providing the engine has normal clearances and no blockages.  The spring simply controls the pressure point at which the relief valve opens and thereby dictates the upper end oil pressure setpoint.  Shimming the spring will simply create more resistance for the relief valve and cause it to open at a higher pressure.  In your case, simply removing the pump, taking it apart, and replacing it would appear to have boosted your low end oil pressure regardless of what you did in the way of shimming the relief valve spring.  High on the list of possibilities is that the oil pump was pulling in some air at the inlet side and that issue was alleviated when the pump was reinstalled.

Too much shim can potentially limit the relief valve movement by inducing spring coil bind which then prevents the valve from fully exposing the internal hole in which it must dump oil to relieve the excess oil pressure.

The gear style pump in my ’55 with a 272 will bump 67-70 lbs oil pressure at a cold fast idle but will fall down to 20 lbs hot and idling.  It will still run 55 psi going down the road when hot and this is at 100K plus miles on the engine and using 10W-40 oil.

My roadster Y engine has its gerorotor style oil pump relief valve shimmed so that the stock spring is ~0.150” shorter which provides 90+ psi cold pressure.  Hot idle at about 1000 rpms sees the oil pressure drop to around 15 psi but immediately jumps up when the engine speed is brought up.  Going through the traps at temperature, it’s running 70+ psi pressure with 20W-50 racing oil.

The oil viscosity changes with the temperature.  Oil pressure is defined as 'resistance to flow' and cold oil simply has more resistance than hot oil due to being higher in viscosity.  When shimming the oil pump so it had 90 lbs cold, I was insuring that the highest possible oil pressure was 90 psi.  If I were to increase the viscosity of the oil being used or run the oil at a cooler temperature, then I could potentially have the oil pressure closer to 90 psi going through the traps than the aforementioned 70+ lbs.  If I had not shimmed the oil relief valve spring, then the oil pressure going through the traps was potentially going to be no more than 55 psi or whatever the stock spring tension allowed.

        The pump manufacturer will build the pump to Ford specs. They have no idea what bearing material, rotational speeds, style of lifters, or any other design parameters the engine designers are using. And, at 6000 rpm and 70 lbs the oil pump is using a subtantial amount of power. There's quite an aftermarket in heavier than stock oil pump drive shafts for almost every kind of engine.

Oil pumps are positive displacement, meaning they produce flow in Gallons Per Minute in direct ratio to your engine speed.

You're right HoLun,  if your engine can't use (or bypass) the flow produced by the pump, pressure continues to build.  As wide-open as your bypass valve can be, if it's not enough to relieve flow, pressure will continue to build until something gives.  Usually, it's the oil filter because that is the very next thing right after the oil pump, and it's the weakest component.

You're wise to look for another spring rather than shim.

Dave

Charlie.  Believe what you're looking for is at this link.

http://forums.y-blocksforever.com/FindPost22736.aspx 

Shimming the oil pump spring only raises the oil pressure if the oil pressure is already high enough to open the relief valve.  Stock relief valves open in the vicinity of 55 psi with factory fresh springs but if the spring is weak or damaged, then the relief valve can potentially be opening prematurely.  In your case, it’s quite easy to simply remove the screw in plug holding the relief valve spring and add some shims.  Be sure the shims are small enough in diameter to fit within the hollow portion of the plug.  Adding 0.100”-0.120” of shim to the spring will let you know quite quickly if you have a spring related problem or if you need to look elsewhere.

Did it using .120 shim and pressure is better.  After dropping hot idle to 600 rpm, now have 23-24 psi, and 47-48 psi underway hot.  10W30 oil.  The rotor oil pump may have a little more wear on it than I thought.

Those pressures are REAL good!  What are they cold?

Mike

That 52-psi is just fine, and you're right, ~60 would be my max.

Some things are well worth sacrificing gas mileage for.  Detonation from running too lean is tops on my list.  These carbureted engines aren't computer controlled, so our settings may stay the same, but the weather changes.  Some days your engine will run sweet.  All we can do is tune our adjustments within safe limits, then let it go.

Glad you set your oil pump up right.  Extra lubrication will pay off big dividends in the long run.

Dave