there is a small plunger that is the pressure relief valve. it is inside the oil pump body. if you put a washer under the base of the spring, it pushes against the plunger more, so it takes more oil pressure to move it.. so oil pressure in engine is higher.
basically, increaseing thickness of the retainer 15135 in this pic
http://opc.mr2oc.com/online_parts_ca...oil%20pump.GIF
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