Here's something along the same lines from a Miata forum...
Quote:
Having recently done my first oil and filter change on my stock '97 and installing an accurate oil press. gauge, I was a bit concerned that the pressure was only about 35psi at 3000 rpm when warm.
Researching oil pressure on this forum to see what others were getting I came upon the Fram filter thread. Having just installed a Fram I read the posts with interest. I saw that a few posters mentioned higher oil pressures after changing out the Fram for a diff brand filter. My question is this. My understanding of engine lubrication systems is this. Pressure is determined by the pumps output (which is always designed to supply a surplus of oil to the main oil gallery under all running conditions), the clearances of the main bearings and mostly by the spring in the oil pump relief valve. If this is so, then how can an oil filter effect the pressure? Am I missing something or is the Miata's oil system diff. than what I described? BTW, my pressures DID indeed increase by 10-12 psi with a Purolator filter. Thanks for your input. Mike
Quote:
Mike,
Although I agree generally with the results posted above, theoretically you are right. There is supposed to be enough overpressure available from an oil pump to compensate for oil flow restriction of the filter. The oil pump is designed to discharge a fixed volume of oil on each revolution, regardless of how much pressure that requires, up to the point where the pressure relief valve in the pump begins to open and allow part of its output oil to fall back into the pan. Apparently the Miata oil pump/valve setup does not have the extra pressure capability to overcome the restriction in some commercially available oil filters, and therefore its bypass valve apparently opens, changing its output from constant volume to constant pressure.
---- Bill
Quote:
Bill If it were not for the oil pump's relief valve (which on the 1.6L engines bypasses at 50-64 psi), meaning that the pressurised oiling system were a closed loop, then the issue of more/less restrictive filters would be nearly moot as the flow rate (gpm) of oil to the engine would not be significantly affected (until something blew).
However the relief valve does exist (for very good reason) and by design reduce's the oil pump's delivery volume (by shunting flow to the pan) whenever the pump's output pressure exceeds 50-64 psi.
While in theory this would mean that the output pressure could never exceed 50-64 psi, the relief valve's flow capacity relative to the pump's output volume comes into play here. When the pump's output exceeds the valve's flow capacity the pressure will rise beyond the valve's trigger point--this occurs whent he oil is cold (which is why we can observe pressures or 90+ psi at startup), and perhaps (I suspect) at higher engines speeds when the pump's output flow is very high.
What this all means that more restrictive oil filters will cause the pressure at the pump's output (and the relief valve) to reach 50-64 psi sooner and more often than will a less restrictive filter. Therefore a more restrictive filter will cause more oil to be shunted back into the pan, and LESS oil to be pumped through to the engine.
This is not just a pressure issue, the oil pump relief valve's behaviour makes it a flow rate issue.
Here's a LSX's oil flow diagram:
So... if at idle the oil pump doesn't provide enought excess flow to reach relief a more restrictive oil filter would indeed reduce system pressure correct? FYI part #3 on the diagram is the pressure sending unit.
But I'm still confused on the oil cooler... most everything I'm reading online says oil coolers create pressure drops and to be honest I didn't pay close attention to mine when it was in because I installed a high volume pupm and my cam at the same time. All I cared was that my pressure was good on track.