oil pressure drop at temp?

Old school SBC. once warm, oil pressure drops to about 20psi at idle. Will come up with revs but constant speed it will slowly drop back to 30-35 from the 40-50 it pumps up to. What does this?

Pressure relief valve?

Pressure dropping after heating up is typical. Fully warmed up in steady state with dropping pressure sounds mechanical. But oil temp can change pretty fast if you go from high speed driving to slower speed driving - an oil temp gauge can verify this. Viscosity changes with temp so that could be happening. I experienced what you are seeing with a SBF in a 65 Mustang. Fast driving and the temp would climb while the pressure would drop. Slow down and the reverse would happen within a few minutes. I had an oil cooler and a direct oil pressure gauge with small copper tubing to the gauge so it was pretty accurate and the cooling happened pretty quickly.

Time for a bottom end rebuild? What you are describing sounds like what I have been told indicates worn out main bearings. At least before they get really bad...

For an SBC, that doesn't seem TOO out of whack to be honest, but with some follow-up questions.

How many miles on the engine? What RPM are we talking about? What viscosity are you using, and how old is the oil?

-Hans

To answer some questions: oil is unknown. Miles are also unknown. I have 10-30 jug and new filter waiting in the garage. Engine was supposedly "rebuilt recently" meaning within the last 5 years but unknown.

Finally having gauges is nice but makes me overly cautious. Also know the fuel sender is pretty close to accurate. It was sitting right under 1/8 tank when I filled it up, put 13.9 gallons into the 16 gallon tank. Yay!

20 psi at hot idle isn't concerning at all. As far as pressure at highway cruise, if it's below relief valve pressure, it's normal for the pressure to come down a little once you've been at higher rpm for a few minutes and the oil temps climb a bit.

Your oil pressure is OK. forgetaboutit.

My current SBC idles at 20, always has. Cruising can vary dependent on load, rpm, temp, 45-75. Currently has 190,000 has been the same since 135,000. I wouldn't worry too much, unless it currently has 90 weight in it.

My current SBF acts the same way but I have no gage to offer numbers from.

Pfffft..... I'd say start with an oil change and see what happens. Sounds like you probably just have some old oil and it's due for a change. An old SBC is one of the engines I'd still keep with a 3,000 mile change cycle. Particularly if it's carbureted and/or running rich, since you'll be getting fuel into the oil as it weeps past the rings.

-Hans

That's the other issue I have is the hot idle is still low and the carb needs adjusting. Still occasionally dies.

I have got to find someone to help me adjust this thing that knows what they'redoing.

On a higher mileage engine I could see spiking a 10W30 fill with a quart of 15W40 diesel truck oil. I do stuff like that, though, because life isn't complicated enough sometimes. Take it with a grain of salt.

In reply to pres589:

I've been thinking about that as well. Just for the ZDDP if nothing else.

My VW race car will do that. Cold it can hit as much as 80 psi, hot (220-240 deg.) it will drop down to less than 20 psi at idle but will jump up to 40+ psi with a quick rev of the engine. When I first had this happen on a new build I dropped the pan and checked the bearings. Everything looked good. Three years later it's still does the same thing. If I run something like 20W-50 it will read higher but cold pressures got up to 100 psi so I went back to 15W-40. I may try straight 30W or 40W on my next oil change based on advise from a knowledgeable person who has tested various oils & weights in Spec Miata engines.

Bobzilla wrote: hot idle is still low

That'll also cause lower oil pressure at hot idle. It'll probably come up a bit once the idle speed is correct.

My Opel GT has always done this, even after engine rebuild. My gage isn't marked in psi, just bars. Will go up to 3 bars when cold, drop down to less than 1 bar when warmed up at idle but at revs will go back up to about 3 bars and stay there as long as engine is above idle. Did get the idle pressure to go up to 1 bar and stay by adding Seafoam to the oil, a little less than half a bottle. Was just going to add it then flush after a few miles but since it raised oil pressure across the board and reduced oil consumptions I kept it in.
Probably is related to oil pump system though.

Sounds allright.

The rule of thumb I always heard was "10 PSI per thousand RPM is okay." I'm not saying that that's accurate, by any means.

From the time I got my 347 to the time it catastrophically kaboomed, it would make 25 PSI when cranking (cold), 40 PSI idling cold, and 20-25 PSI idling hot.

Pressure was very RPM dependent, especially at low RPM. The RPM drop of pulling out without hitting the throttle could drop the oil pressure a few PSI.

I don't like 5w-x or x-30 for old school engines myself, especially if there is any chance of wiping a cam with modern oils. I'd be running 15w-40, 20W-50, maybe 10W-40, or if I still lived down south straight 30W.

That said, while I haven't had many SBC drivers, my 4.3L stuff always idled hot in the 12-20# range no matter what oil was in there but thicker oil would improve oil pressure at speed.

In reply to oldopelguy:

A 5W-40 diesel oil is often a good choice, especially in areas where the engine might be started in colder temps. It's plenty thick when hot for what those engines need unless it was built really loose. IMO, 20W-50 is only for engines that absolutely need stupidly thick oil and won't ever be started in weather under 60 degrees.

Agree with others, 20 psi hot idle is fine for SBC.

Note that most of the diesel oils were recently neutered in the zinc/phos department by the new CK4 spec. Anything carrying a API SN/CK4 dual rating is <800ppm P. Shell are the only ones that dropped the gas ratings from most of their oils (e.g. the new Rotella T4 in 10w30 or 15w40) and came out with a single new dual rated oil so they wouldn't have to drop the phosphorus levels to <800ppm. The new low phos stuff is supposed to perform just as well with high tech additives that don't really show up on a oil analysis, but time will tell how true that is. Probably fine either way on a stock or moderate cam and springs as way back in the 50s when flat tappet lubrication issues first showed up 800ppm was found to be good enough without any help from more modern additives and motor oil was formulated as such all the way through the muscle car era. The ~1400ppm oil came later for oxidation protection in smog engines, not extreme pressure wear protection, go much higher than that and wear actually increases.

We haven't seen a ton of the CK4 stuff yet. What he have seen is ~800P/1000Zn and an uptick in Boron and Moly. Ca also seems to be lower and hte starting TBN is lower.

Rule of thumb: 10 psi per 1000 RPM when above idle. Idle can usually sustain more than 10 per

20 psi at idle is perfectly normal. Most low oil pressure sensors trip the light at 12-15psi