"How much oil pressure should an engine have?" That question has been around for a lot of years, and "Opinions are like 'noses'- - - - -everybody's got one and some of them are really smelly!" Theoretically, it's possible to lubricate an engine with ZERO oil pressure- - - - -as long as the oil pump supplies exactly the amount of oil every part in the engine gets adequately lubed. If the pump supplies one drop of oil to a bearing every time the same size drop falls back into the oil pan, the bearing will get lubed adequately, and the pressure will be zero. "Pressure" builds up because the pump is producing more oil than the engine really needs. Now, that's great in theory- - - - - -but how do we know that everything from the main bearings, to the rods, to the cam, to the valve lifters, rocker arms, etc. are all getting the exact amount of lubrication they need? Oil pumps are designed to supply adequate lubrication during the "worst case" scenario an engine is likely to encounter, plus a little fudge factor to offset any margin of error there might be under less than ideal operating conditions- - - - -bearing wear, fuel contamination, restricted oil filter media, etc. So- - - - -why not just design in some massive overkill- - - -several times the pump size that's actually needed, so there's no doubt that everything gets lubed? One drawback is the power required to drive the pump- - - - -parasitic losses due to friction, internal drag, and other factors can and do reduce the power necessary to actually push the vehicle down the road! Those internal power losses also increase fuel consumption- - - - -sometimes dramatically! The simplest way to assure adequate lubrication without a lot of parasitic loss is to design a pump that has a fair amount of overkill, volume-wise, and install a spring loaded pressure relief valve that shunts excess volume back to the oil pan once the desired lubrication gets done. If I could be sure that all the lubrication that an engine needs happens at 5 pounds of oil pressure, that would be an ideal situation- - - - -but to keep drivers from obsessing about possible engine failure, most manufacturers build in several times that number. It's also good to know that the average "idiot light" switch turns that little dash light on at around 7 PSI! That's one reason a lot of vehicles switched to warning lights over gauges- - - - -several decades ago! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Interesting and timely info Jerry. I'm currently changing the oil pump on one of my long time vehicles. Problem is and soon to be was too much oil pressure, as in 100 psi cold (or more, pegs gauge) and 80 psi hot. I built the engine back in '89 thinking I wanted a long lasting street driving engine, set all the oil clearances up on the lower end of specs. Lunati balanced it and 37 years later it's still a very smooth running, happy to rev engine with all of 5,500 miles on it. Back in '89 the smart (not so much in reality) thing to do was change the pressure relief spring in the oil pump, so I did. Now I'm thinking I really don't want to wear out or shear the drive gear on the distributor shaft and wind up walking and then building another engine. This little project of changing the M72 pump out has turned into more than a day job with all the "while I'm at it details" but it's time well spent that could've been avoided by not trying to reinvent the wheel or in this case, oil pump. Too much oil pressure isn't necessarily a good thing.
We learned the same lesson racing Pontiac's, high pressure robs horsepower and wears out distributor gears driving the oil pumps. High volume insures plenty of oil where it's needed.
Curious question. In my two 327`s , one has a high VOL.pump that reads 80 psi on the test at road speed and drops to 30 psi at idle. The other has a low VOL. pump and will reads 30-35 psi at road speed and drops to 12-14 psi at idle. Both after warm up. The 235 I just had rebuilt will run today. I can check it.
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
I don't think HP has anything to do with pressure requirements. 50 years ago Cummins dropped their oil pressure from 60 to 30 on their 855 cu in engines to gain a little extra HP and fuel economy. It took some getting used to seeing oil pressure drop to 20 on a hard pull, but we got used to it. Idling at around 10 psi was common. After running a fleet with about 15 Cummins for about 60 years I only once spun a bearing on a Cummins. And that was when a turbo blew and the driver let it run until it ran out of oil. Cummins let the 855 cu in go out of the factory with 525 hp and 1850 ft lbs of torque. And lots of guys turned them up to more than that without problems. 60 to 70 years ago oil pressure was considered an indicator of engine condition. And it actually had some validity because of the lubricants we had in those days were so bad that bearings, cam bushings as well as cranks and cam journals wore in a hurry. But with today's lubricants wear is almost negligible. We tore down some Cummis with over a million miles and bearings still miked well within spec. The few miles antiques are driven, they should last forever. The most failures I have seen is where rebuilders didn't allow enough clearance, the old iron expanded when it got hot and seized and ended in catastrophic failure. I don't worry about oil pressure but I make sure everything gets enough oil.
