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Originally Posted by JFP in PA
Actually, the spring is under compression when the shaft and seal (#4&5) are installed into the pump housing. With the spring pre compressed to its installed height, if it got hot, if anything it would tend to try and grow slightly in length, increasing the pressure it exerts against the relief valve (#26), raising the pressure at which the valve relieved. An argument could be made that heating the spring could lower its tension slightly at any installed height, potentially offsetting the slight gain from its increase in length, which would probably bring the relief pressure back to somewhere near its ambient temperature starting point.
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when the car is off, the spring and piston would be fully extended. when the car is started, oil pressure would force the piston down the bore & compress the spring. through some mechanism, heat will alter the piston's position and, in turn, the oil pressure. i started learning about this aspect of our cars when i first fired it up after installing an accusump. i opened the accusump valve & the pressure guage went sky high! made me nervous as hell. i watched as the car warmed up & the pressure slowly dropped from ~ 105psi to ~17psi at idle. started reading & found that lots of folks have experienced high/low oil pressure issues that were solved by replacing the spring/piston.
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Originally Posted by JFP in PA
Today, the focus is on delivery volume rather than high pressures, becasue high pressure actually sap horsepower due to the load on the pump.
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right. as long as the oil pressure is above the minimum required to properly actuate all of the hydraulic systems within the engine (lifters, tensioners, etc), then volume is king. it just so happens that at higher RPM's, significant pressure is often required to deliver sufficient volume. pressure without volume is useless.
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Originally Posted by JFP in PA
With the M96 design, raising the pressure by altering the spring pressure brings another issue into play as the higher pressures created by the gear section increase the load on the oil pump drive shaft (#3), which is already a weak spot in these engines. LN Engineering developed a heat treated chromoly pump hex drive that sells for about $20 after seeing several engines blow when the OEM hex drive snapped in two. If you are contemplating altering your pump’s pressure profile, you might want to look into replacing the shaft as well.
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i put one of these in when i did my 3.4L. for those considering this, be sure to lock TDC & remove the chain tensioner before you pull the oil pump.