Strange Oil Pressure when hot

[david.g.chapman]

I have some consistent but peculiar readings from my twin cam oil pressure gauge when the engine is hot:

rpm oil pressure
1000 30
2000 40
3000 40
4000 30-35! sometimes back up to 40 again
5000 30-35! sometimes back up to 40 again

The engine is in good shape, with a recently measured oil pump (in spec) and new big end bearings. The mains are OK too.

Cold oil pressure is at 45 psi at anything above idle.

The sender unit is fitted to the engine via an adaptor block to convert the thread. I suppose the non standard gallery in that area would not affect things - the pressure should be the same everywhere.

Or am I wrong? Any other ideas? Anyone else had this?

Dave Chapman.

[M100]

While the drop off is a bit strange, and the idle pressure a bit high but I wouldn't read too much into the readings as all you probably have with the standard instruments is a rough indication. The bourdon tube gauges of that era can give slightly better accuracy than the electrical ones but don't expect anything much better than +/- 20%

To have any significance you'd need to check with a properly calibrated gauge and transducer such as the modern SPA ones for instance at 150 quid! These use industrial grade transducers accurate to around 2% of FSD

is much more accurate than

But would look crap in an Elan!

[1964 S1]

I agree with M100. Your pressure is better than mine and I've had no problems. Try tapping on the gauge while driving at constant higher rpm. A lot of times they "stick" a bit and can jump up 5 to 10 lbs. What grade oil do you use?

[alaric]

Hi. Is it standard oil? What type do people use / recommend these days?

[1964 S1]

I've used organic Castrol 20 w 50 since forever but I don't drive much in the winter months as I have no (heated) garage and cold starting can be a bit sluggish at 20F'.

[M100]

Mobil 1, I've run on 0W40 in the past but now i'm on 15W50 (Mobil 1 "motorsport")

Pressure is around 20-25 psi hot idle, 45 psi at 3000 rpm.

[Emma-Knight]

...and don't forget the oil pressure relief valve that might switch a bit slow
at very high flow rates (like "dancing" in the oil stream). If the gauge is pressurised 40 at 3000 rpm it would not have a need to move backward when rpm increases at "constant" input pressure.

Anna

[GrUmPyBoDgEr]

I agree with Anna.
The pressure regulating valve is a bit of a crude device which can be susceptable to several mechanical & hydraulic influences.
Oil temperature & subsequently the viscosity wil have their effects on the whole lubrication system and the indicated oil pressure.
At high oil temperature (There are measurable dfferences between hot & hotter) & high engine speed the oil pump has a bit of a problem keeping up with the demands that the engine places on it & therefore it is not unreasonble to see a pressure drop on the guage.
John

[poiuyt]

I use Mobil One 15W50 all of the time and get readings of 15 -25 pounds at idle and as much as 50 when hot.

I think this engine should never run on the dinosaur stuff because, unless you run it often and always get it hot, you can wind up with sludged up oil.

Two destroyed engines in cars that were used in short, stop and go driving by my wife attest to this. Since changing to the synthetic I have never had an oil related problem. My daily transport VW GTI-VR6 turned 200,000 miles a couple of weeks ago and the engine runs as good as new.

Steve B.

[lotusdelta]

steveb.,

had to laugh...see you have a vw gti vr6 and a lotus....so do i...hmmmm

anyhow as to oil i use a synthetic blend in my gti.....
but i use castrol 20-50 in my 72 lotus elan sprint..as evidenced by the above user...just feel that this is what is appropriate given that synthetic was but a dream back then ..just my two cents....
im off for a drive...its 70 today and its been a long winter......

[david.g.chapman]

Thanks for all your replies. I tend to agree that it could be instrumentation that is at fault, ot just possibly a fluttering relief valve on the pump.

Either way I am not that bothered as long as I have at least 30 psi at speed. There used to be a rule of thumb of 10 psi per 1000 rpm, but that might be for A series engines a la Morris Minor, rather than the Ford engine series of the time. Still, I will get the pressure checked with another gauge.

Incidentally, I am using Halfords 15W 50 at the moment, but would prefer Valvolene 20W 50. I have heard it is better at maintaining pressure at high revs!!

Dave Chapman