The Stovebolt Site Forums The Shop Area Engine Shop Oil Pressure 1955-1962 235/261

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 Year

1954: 45psi
http://chevy.oldcarmanualproject.com/chevyresto/5436.htm

1955: 30psi
http://chevy.oldcarmanualproject.com/chevyresto/55039.htm

1956: 35psi
http://chevy.oldcarmanualproject.com/chevyresto/56178.htm
double checked 56 number here: http://chevy.oldcarmanualproject.com/shop/1956sctsm/56sctsm0617.html

1957: 30psi
http://chevy.oldcarmanualproject.com/chevyresto/57042.htm

1958: 30psi
http://chevy.oldcarmanualproject.com/chevyresto/58t104.htm

1959: 30psi
http://chevy.oldcarmanualproject.com/chevyresto/59ted18.htm

I 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.jpg

Along 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?

Pump variables are confusing. Take just the pump alone. The spring pressure on the relief valve is just a part of how it works. Most are a ball bearing(some are a piston). Some sit on a seat and some move past a port. Sticky ball or piston will raise the pressure, no matter what spring is holding it in place. Add a filter system with a relief valve and it can get more complex. The pump has clearance considerations as well. Most pumps are "gear" type. So the gear mesh matters(like a pinion and ring) but most don't think about the end clearance between the gear, body and the cover. There are "restrictions" in the system as well. You may have a case where all of it adds up. Close the restriction to the valve oiler line. With the same volume of flow, it will have to go somewhere. And yes, volume matters. Close main bearings, and close rod bearings, and a bit of restriction, may lift the relief valve. That does nothing to reduce the volume the pump is moving. And the volume alone may be enough to keep the pressure up. Most engineers would worry about too little flow to moving parts. Too much is an after thought. Oil pressure that stays "high" at full temp. Should indicate a restriction some where. The "pressure" will be different in different parts of the system(I know, counter intuitive) but it is still a system based on volume, flow and pressure. Not as simple as a garden hose. Over oiling can cause issues you don't want(leaks, lots of leaks).

I know right? I mean, if you sit down with a pad and pencil and write down just the facts of the original post here and try to make sense out of how 30psi happens on a regular basis with those components installed, it's a real head scratcher!

I spend hours and hours of my time using electronics mathematics or mechanical engineering math to solve some of the problems I run into, and precision is absolutely imperative. I am always trying to understand each system in a similar way. This one has me stumped so far. To try and learn how the system works CORRECTLY then why a friends system is working differently is proving to be a challenge with those REAL numbers posted above. Mike is dedicated as much as I am to solving this problem, so we WILL prevail! Thanks Steve! Add two other things to the mix. This is a full flow system so 100% of the oil that is picked up from the pickup screen is sent directly to the spin-on filter just outside the block (then the gauge hookup) and then back in to the engine. How hard can it be, yet it IS! This is a single path for oil to flow. No bypass, no other means of establishing pressure.