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Oil pressure drop
#1
Am I over-reacting ?

Given the notorious unreliability of old gauges, at cold start up, the " OIL " needle sits approx halfway on the scale.  When running at full temp & 30 mph, drops to approx 1/4 position ( has lost half along the way )  I'm told this is due to oil thinning out under heat so no problem with pump etc.  Using Valvoline 20W-50 mineral oil which fits recommendations on this forum.  Recently changed + new filter,  reads O K on the dipstick.  Oil is getting to the rockers as I had weeping after replacing old cork gaskets with Felpro - now fixed.  Pressure sender replaced at the same time.

Before I get involved in fitting a temporary mechanical gauge, tee connections etc, any opinions here please.  What is " normal " pressure in P S I ?  If it drops by half when hot, is this a problem with engine lubrication ?    I'm used to 289 / 302's which went to 1/2 and stayed there.

I've done a quick search for info here but nothing that helps.  Apologies if I missed the obvious.
Many thanks.
PKJ
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#2
Easiest is a brass "T" fitting at the oil sending unit location. With 20W50 in the pan oil pressure will be higher at start up than say a 10W30 oil. If it is a "standard" engine I see no need to use 20W50 oil unless the bearing clearances are really loose (read out of stock tolerance). The factory gauges are really idiot lights with pointers. The aftermarket sending units are equally unreliable. Chuck
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#3
Typically I've experienced 70 - 75 psi at cold start and 30 - 35psi when at operating temps (195 - 200). 10W - 30 both mineral and synthetic versions

Years ago I purchased a decent quality aftermarket (dash style) oil pressure gauge and have used it as a method to check oil pressure when the factory gauges don't look quite right. I usually remove the oil pressure sender and install a fitting that works with the capillary tube. Then read and note actual pressure readings and compare them to the factory dash gauge. It's provided peace of mind a few times and once gave me warning oil pressure was no longer (less than 5 lbs) safe.
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#4
The factory oil pressure gauge in my 351 C reads 3/4 on cold start. I checked this reading against a mechanical gauge and that equals 75 psi. when warmed up my gauge is about where yours is at a little over 1/4 but still in the safe zone and that = 35-40 psi on my mechanical gauge. I am running Brad Penn 20w-50 and have a stock oil pump.hydraulic cam and stock bearing clearances. My oil pressure gauge at one point in the past read higher at the same pressures but it is 47 years old and may be getting tired. You can check the total sweep of your gauge by grounding the gauge feed wire that goes to the sending unit with the ignition key on.The gauge should peg on the H. Hope this eases your mind.

Ron
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#5
My gauge reads 90% of scale when cold and about 80% when warm.

mike

[Image: 1_11_11_13_11_50_27.png]
[+] 1 user Likes goodnigh's post
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#6
(04-09-2019, 03:21 PM)goodnigh Wrote: My gauge reads 90% of scale when cold and about 80% when warm.

mike

Excellent info - thanks to all respondents.  So a 50% drop from cold start to running temp ( 70 psi down to 35 ) should be safe providing that's what the pump is putting out.  I'll keep an eye on it but not panic in the meantime.
Well done.
PKJ
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#7
For some real numbers, my last Cleveland with a standard volume pump pushed 80psi on a cold start, 30psi at idle when hot, 60psi hot running at 2000rpms. This was with 10W40 oil.


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