What is Normal Oil Pressure?

[cindy in indy]

When newly running at normal driving speed, my oil pressure on my 75 Midget is 60. After running for 20 min. or so, it drops to 40. My newly running idle is 20 but drops to 10 after driving. Is this normal? What is normal oil pressure? What point should I be concerned about the pressure? I am using 20w50. Thanks, Cindy (the girl with a recent Midget purchase and a lot of questions)

 

[Bret]

Don’t know – I’ve never gotten a definitive answer. But your numbers seem a bit low to me.

The oil pressure used to drop really low when hot and leak like a sieve. I discovered while doing some exhaust work I had to remove the manifold, that both of the side cover gaskets of the engine were all but gone. This from what I understand is a common weakness of engine’s design when it comes to leaks. Over the years the gaskets get really hot because they are located right under the intake/exhaust and down pipes (headers). In my case it’s even worse because the catalytic converter (75 on) is located just inches away from these covers. When I replaced them I opted for the rubber over the cork as I feel they will last longer.

Anyway currently my old 78B’s motor idles at about 55 or 60. But drops off after the engine has been warmed up (-10 to 15) and even more when she’s been running a long time ending up at about 30 at an idle. However under load the pressure stays a healthy 75+ PSI.

I’m pretty sure a thorough fleshing up of your motor (piston rings & gaskets) would bring your numbers up – but how much? I don’t know.

 

[Steve_S]

Those numbers sound low. First, check the true oil pressure using an external gauge. This will determine if your oil pressure gauge in the car is accurate or in need of calibration. If the pressure you are reporting is correct, try replacing the oil pressure valve. It may be worn. You may also consider inspecting the oil pump.

I don't know Midgets at all, but for reference my '65 B idles at 25, cruises at 60 and gets as high as 75-80 under heavy, high-speed load.

 

[Bugeye58]

Cindy, although your pressure numbers are on the low end, they are still acceptable. By all means, I would test the pressure with a separate gauge that is known to be accurate.
How long has it been since an oil and filter change?
A general rule of thumb for oil pressure is 10 PSI for every 1000 RPM.
The spring in the relief valve can lose it's tension over time, and allow the pressure to drop. The proper length of the spring is 1.53". Check the plunger and make sure the sealing surface is smooth and clean.
An interim fix is to shim the spring under the relief valve cap with a suitable washer. The relief valve is the long, hex looking item on the lower left side of the block, below the distributor.
Piston rings and gaskets have nothing to do with oil pressure, unless the gaskets allow all the oil to leak out, and, your engine doesn't have side covers to leak.
10 PSI at hot idle and 40 PSI at road speeds should be just fine. On the 1500 Spitfire, (same engine) the oil pressure warning light doesn't even come on until 3 - 5 PSI.
Hope this has cleared a few things up. Now I have to go out and finish assembly on a 1500 engine. /ubbthreads/images/graemlins/grin.gif
Jeff

 

[Geo Hahn]

[ QUOTE ]
...10 PSI at hot idle and 40 PSI at road speeds should be just fine...

[/ QUOTE ]

I agree. The most important thing about oil pressure is to have some. As noted, the gauge may not be all that accurate and if Midgets have an adjustable pressure relief that too can affect the reading.

So what use is that oil pressure gauge? Over time it can be a relative indicator of engine condition and in particular bearing wear. Also, of course, any sudden change in the pressure you normally see warrants immediate investigation.

 

[RobSelina]

stock numbers are 20psi idle, 60psi at higher RPM once the engine is warm. They'll be about 10 psi higher when cold.

what's the mileage of your engine?

 

[Matthew E. Herd]

Check for blockages in the crankcase breather system (I don't know if this would affect pressure or not, but some people claim it does). Also, I installed an oil cooler as pressure dropped dangerously as the car was driven. Now, after rebuilt, it runs at 70 lbs cold at idle, and once warmed, is between 30 and 50 lbs at idle (depending on how warm), with about 65 lbs pressure at speed (say 2000 to 4000 RPM). Rob's numbers seem to be consistent with my findings on a newly rebuilt engine.