I received new information from Federal Mogul today. They own Sealed Power who makes our OEM oil pumps. To catch everyone up, this started when my friend Mike told me he has 45 psi constant oil pressure on his 1958 261 engine. This did not make sense to me because in my limited experience, I have never seen it. Keep in mind, we all take for granted that you can have oil pressure go up beyond the OEM spec, then drop as the engine warms, etc. This goes without saying, however for it to remain at 45 psi is 50% more pressure than GM specifies. So that everyone is on the same page, I did a little research by year at Keith Hardy's amazing resource:
Oil Pressure by Year1954: 45psihttp://chevy.oldcarmanualproject.com/chevyresto/5436.htm1955: 30psihttp://chevy.oldcarmanualproject.com/chevyresto/55039.htm 1956: 35psihttp://chevy.oldcarmanualproject.com/chevyresto/56178.htm double checked 56 number here:
http://chevy.oldcarmanualproject.com/shop/1956sctsm/56sctsm0617.html1
957: 30psihttp://chevy.oldcarmanualproject.com/chevyresto/57042.htm1958: 30psihttp://chevy.oldcarmanualproject.com/chevyresto/58t104.htm1959: 30psihttp://chevy.oldcarmanualproject.com/chevyresto/59ted18.htmI couldn't find 1960-62 in his library. With the exception of a hiccup in 1956 (no idea why) it has been 30 psi since. So, here is a link to the Sealed Power Spec Sheet for the OEM pumps for the 235 engines. The 224-4141 is the one that RockAuto.com lists as correct for these later year 235/261's.
You cannot take RockAuto as a technical resource however. Gotta be careful because I have caught them wrong in their database several times., but in this case that is the correct number for 55-62.http://devestechnet.com/Images/Projects/SealedPowerOilPump.jpgAlong with the chart above, the tech rep also included this statement (which I think we all pretty much know)...
"Deve, oil pressure specification for the pump are not the same as engine specifications, because you have to take in consideration the condition of the engine and how much oil clearance you have in the main bearing, rod bearing & camshaft bearings, also other things can cause lower oil pressure in the engine, like sloppy lifter bores, leaking lifters, leaking oil gallery plugs, oil pump pick up screen, type of oil, engine temperature and others. Attached are the oil pump specifications for all three of the 235 Chevrolet oil pumps."
In addition to this information, I called them back and asked them to tell me exactly what the pressure relief valve is set to on that specific part number of oil pump.
They said Pressure Relief Valve is set to between 35 and 40psi.It leaves questions I know, thus the reason I wanted to discuss it with people who might have clues. For example, if Operating Speed you get 41, but the relief valve is set to lets say 38 (middle) then the relief valve must not follow the actual reading on the gauge exactly. In actual practice, every 55-62 I have ever seen the number is 30-32 psi or so. Yes, that number varies depending on condition of the engine, ambient temperature, etc, but that number is the norm as well as it is the manufacturers specification.
Anyway, thought I would share with you what I found out. No idea why real life numbers vary by almost 10 psi, but they just do. Was hoping for an explanation, but nobody seems to know. This is an engineering phenomena and probably not something anyone would know.
If GM's specs say 30psi and the pump manufacturer says 41, apparently when you put the two together, you get less pressure to the 30psi spec. I do not know why or why my friends engine is being different! Thoughts?
