i found some high quality speakers (3 1/2") worked well on either side of the center console to the right and left of the radio. they fit flush mounted and are close enough that car and road noise doesn't efect them as bad as the rear speakers.
on aircraft the oil pressure releif valve is to set the oil pressure to a fixed value and i assume it's function is the same on this engine. the pressure builds up to a certain amount overpowering the spring behind that little ball and allowing the oil to bypass! if the spring is or has been weakend by heat or what ever, your oil pressre would be low?????? :cross-eye