Re: Oil Pressure Problems

Posted by Tim Cole On 2012/7/5 19:22:32

Dear Jim:

Glad your car is running okay.

Depending on the axle ratio the speed per 1000 RPM is as follows: Standard 3.54:1 23.8 mph, 3.28:1 25 mph 3.07:1 27.5 mph.

So with the standard ratio 3000 rpm translates to 71.4 mph.

So the minimum required oil pressure to run at that speed is 30 psi. Usually Packard speedometers read fast so running the car 65-70 would probably be alright, although given Packard liked higher oil pressures I would want to take it easy.

This rule of thumb comes from my SAE reference and is pretty consistent with a lot of vehicles. Let's look at my modern car:

Idle: 11 psi
3200 rpm: 46-57 psi.

The car will probably rev up to 6500 rpm and so I'm sure it satisfies the rule of thumb, 4000 rpm translates to around 100 mph so I'll bet governed top speed is 125 which is 5000 rpm and 50 psi. Pretty darn close. Typically maximum oil pressure is tied to 10 psi per maximum rpm.

Let's suppose you are at Indianapolis and running 6000 rpm on the straightaways. If the oil pressure is 60 psi and you start pushing the engine higher for sustained periods it will probably blow up.

So I think the 10 psi per 1000 rpm rule of thumb for oil pressure is good.