A friend just picked up a 1958 XK-150. It runs well, but after getting up to highway speeds for a while, oil pressure drops to around 28-30 lbs. I believe he said the book says 40# is normal. I think he is using 10-30 multi-grade oil. Is this pressure safe ? He hasn't had time to do much checking yet. I guess he should check on the accuracy of the gauge. Any other thoughts ?
James

 

james, first off, welcome to the Forums. There is lots of good information to obtain from here. But, we do ask a small favor from you. Please stop by the New Member Section and tell us a little bit about yourself. We pride ourselves in being a better car group than others out there and as part of this, we like to get to know a little bit about each member and in return you get to learn a little bit about us. So, grab a cup of joe, sit back, do some more reading and enjoy.

As for your question, this is not a simple yes/no answer. The big question is when the motor was last rebuilt and when it was rebuilt, what were things dimensioned to. What I mean by this is that a lot of older motors had much bigger gaps between things (say the pistons and the cylinders or the gaps in the main bearings) that cars of today have dropped to a minimum. Newer cars today need a thinner oil to get into those harder to reach spots and the thinner oil needs less space to develop the proper oil wedge (the pressure the oil creates to keep from getting metal on metal contact). Where, the older motors need a thicker oil to fill the larger gaps and the thicker oil to be able to develop an oil wedge that will survive in the big gap. Along those same lines, if you have bigger gaps, the oil is going to see less restriction (think of trying to pass the same amount of water through a garden hose and a fire hose). This will result in the oil pump outputting a lower pressure. A thinner oil will also not need as much force to move through the same size opening (think of how much work it would take to pump molasses vice water through a given size hose).

If your friend has a recent rebuild on the motor where things where built with modern day specs, then that pressure should be fine. If they rebuilt the motor with 1958 gaps in the motor, he will be having issues in the near future. Granted, like you mentioned, the gauge may be reading off too. So, this is where attaching a gauge that has been calibrated to get a much better idea of what the real pressure is will be critical too.

 

Jaguars with XK engines should be about 40lbs at 2000rpm. Using 20w/50 oil. 10w/30 is too thin for summer driving. All my XKs run 60lbs cold start to 40lbs running. Might be a little higher after a rebuild, but usually settles in at the factory 40lbs at 2000rpm.

 

On the last Jaguar I had with an XK engine, I used 15W40, as 20/50 had tended to disappear, although you can still get it if you look around. The XK engine commenced production in 1948, so the period of production covers the whole post-war period of engine oil development.

I certainly think that 10W30 is the wrong oil to use in an XK engine for the reasons outlined, although it will probably be OK, but give the owner a heart attack whenever he looks at the oil gauge !

 

Thanks everyone, it turned out that someone assembled the pressure regulator wrong or left out a part. Pressure appears fine now.
James

 

When I rebuilt my first 3.4L I had 60lbs on start up and would keep lowering the warmer it got. It would reach 30lbs and I would kill he ignition. Would try again and it repeated. Ordered new oil pump and same thing happened. Changed oil a couple of times and remove oil filter assembly until I found a piece of wood jamming oil pressure relief valve. Wood chip came from wooden handle used to install pistons. Use a plastic push from then on.

 

20-50 or similar, pick your favorite brand, noting that zddp should eithr be in the oil already (such as certain Valvoline, Joe Gibbs Classic etc oils) or added, as a protection to your flat tappet cam.
yes, about 60 or so at start, 40 lb at 3000 hot is normal, dropping to 20 at idle.
things do get caught in the oil pump, but first check the oil filter housing, filter, pressure relief valve, these should all be clean and free. As has been said, a separate pressure guage can be used to check accuracy of your guage, and of course the oil line to the pressure guage must not be blocked. Most other additives are snake oil, probably do no harm in mft recommeded amounts, but early on Jaguar put out a service bulletin advising non-use of additives..of course oils were a bit diffferent in the early 50s (they did have zddp in them at that time..now std ones do not).