The Stovebolt Site Forums The Shop Area Engine Shop Oil pressure

I replaced the 235 in my '59 Apache with a 250 out of a '67 Nova. The 250 doesn't use hardly any oil but it only runs 30-35 lbs. pressure running down the highway and about 7 lbs. At idle when it is hot. I run 20-50 wt. oil in the summer and 10-40 wt. in the winter. What should the oil pressure be on these motors?
Thanks a lot,
Gene

Gene,
Are you using the stock gauge or a calibrated test oil pressure gauge? I have seen inaccurate stock gauges and would test with a proper test gauge before going too far.

Kent

Any oil pressure at all indicates an oil pump is producing more oil volume than an engine needs. If a bearing gets one drop of oil from the pump every time a drop falls back into the sump, the bearing will get lubricated properly, and the pressure will be zero. The trick would be getting exactly the right amount of oil to every place in the engine that needs lubrication.

Your cruise pressure looks about right. Anything more would just rob unnecessary horsepower from the engine to drive the oil pump. The low pressure at idle indicates you've got a bit more bearing clearance than normal- - - -replacing rod bearing, and possibly main bearings should bring the idle pressure back up if it bothers you. Unless the hydraulic lifters start rattling at idle, I'd probably just ignore the idle pressure, however. Idiot lights turn on at about 5-7 PSI, so a lot of people who don't run a pressure gauge probably don't realize how low their oil pressure actually is.
Jerry

I have seen my 250 in middle of summer run 15 psi oil pressure with 5w20 oil. The stock oil gauge for '37 only goes to 30 psi, so I have no idea how much pressure it has above idle. I would't run 20-50 in anything, 10w40 is the thickest I would run.

If like Jerry already said, if the lifters and rocker arms are quite, you have plenty of oil pressure. I prefer more volume then pressure.

Are guys running 10w40 in their 235's??

10-40 got a bad rap back in the 80's when GM announced they would not honor the engine warranty on any engine using 10W-40 oil. When I questioned some field engineers and GM training center instructors about the policy, they were pretty evasive, especially when I asked if a specific brand of 10-40 was involved. Later, the real story came out- - - -certain brands of 10-40 had polymers in them to extend the viscosity range that had a tendency to coke at high temperature and form hard carbon deposits near the heat riser and EGR passages in the cylinder heads. That restricted pushrods and wore out camshafts, particularly on Chevy 305 engines. Rather than call out a specific manufacturer, GM simply issued a ban on all 10-40 oils.

15W-40 Diesel rated oil is an excellent choice for stovebolt engines, year-round.
Jerry

One reason why you want to see some pressure on the gauge: The oil pressure gauge, or warning lamp, gives only the pressure at the point where its sender enters that part of the pressurized system — not everywhere, not an average, nor a generalized picture of the systemic pressure. Don’t trust it to guarantee bearing condition. Despite the frequent comparison to hydraulic engineering theory, this is not a “closed system” in which oil pressure is balanced and identical throughout the engine.
All engines are open-ended hydraulic systems, because the oil returns to the pan by a series of controlled leaks. The bearings farthest from the pump always have the lowest pressure because of the number of leaks between the pump and that bearing. Excess bearing clearance increases the pressure lost between the first and last bearing in a series. Depending on its condition, an engine may have acceptable gauge pressure, and still only have 5 psi pressure at one connecting rod

HRL may chime in on this but for years I've heard it said that if a GM engine had 5 pounds per 1000 rpm's it'd never quit for lack of oil....
Jerry, have you ever heard something like this?

I've heard a lot of generalized statements about oil pressure that might make sense under specific circumstances. Most people get too concerned about specific numbers. It's normal for pressure to be high on a cold start and drop somewhat as the oil heats up and clearances expand. The most common complaint I hear that's totally untrue is "the oil pump failed" when people are describing a loss of oil pressure. In nearly 60 years of fixing cars, trucks, construction equipment, boats, and airplanes I've seen exactly three true oil pump failures. One pump sheared off the drive tangs because a piece of metal got between the gears through a hole in the pickup strainer screen and locked the gears. Two other pumps got the relief valve jammed open from foreign matter and although the gears continued to pump oil, the pressure went to zero and stayed there because all the output was being bypassed right back into the crankcase. Every other "pump failure" was actually worn out rod, main, or cam bearings, allowing more oil volume to leak away than the pump was able to supply. Panic makes a good point about the last bearing in the gallery being the one at the most danger. Like the last resistor in a series electrical circuit, there's not much "Voltage" (oil pressure) left to make the lube flow. I used to tap the pressure gauge line for the race car into the end of the oil gallery above the timing cover where they once attached the line for a remote oil filter on the 1955 small block 265 engines. That port was still being drilled and tapped as late as the early 327's and blocked off with a pipe plug. It was easy to drill the passage on a 350 and tap the hole for a 1/8" pipe fitting. That port monitored the oil pressure at the last possible source of pressure, furthest away from the oil pump.
Jerry

Pontiac used an oil light to warn the driver of low oil pressure just like every other maker. Right up to the last engine built, the light was set to come on at less then 5 psi (+ -) 1.5 lbs, and this was right as the oil left the filter. The 250 used the same sensors as the V8's would have used.

I have seen two "oil pumps fail", and both were due to 20-50 oil and both still had working pumps with the drive shafts sheered off.

Joe

Thanks for all the replies. I really just wanted to know what was normal pressure for my engine. It sounds like it should make it for a while longer.
Thanks again,
Gene