Oil Pressure at Idle

Guys, I wanted to get information on where your oil pressure falls when idling between 600 and 700 rpms and then at WOT? Mine stays between 15 and 20 when idling and during a hard run, sits at 40. Boat is a 2000 air nautique with the gt 40 in it.
I am asking because last night I was reading some threads on here about low oil pressure that literally kept me up all night. Thanks to all for their input.
TJ

Sounds like my oil pressure for years. As long as that 15-20 is once the engine is warmed up.

I believe mine is always around 40 psi. Even when idling.

Sounds normal. You don't see GT-40 engines read much over 40-45 usually. Dad's 98 with 425ish hours runs basically 45 all the time unless you come off of an extremely hard long run and it's hot outside.

My older Fords see 20-25 hot idle after a hard run.

I can't say what the PO used, but when I changed the oil in the spring I used VR1 straight 40.

Same on my GT40, 40ish, and we do a lot of high speed footin runs. Brad Penn Racing Straight 40.

So just for the record what should the oil pressure be at? I have noticed at times my gauge jumping around, guess I fungi red it was due to the boat bouncing around, but now I'm thinking it's something else. Thoughts?

Gary S wrote: I thought its about 10 lbs per 1k

I agree. That's the figure I've always known.

Above 400 RPM the oil pressure should stay at whatever your oil pump pressure relief is set at. That is why many state 40 psi all day long.
The pressure relief kicks in and by passes excess oil to keep pressure the same, when cold with hot oil it may be a little high, 10 pounds more is normal. Old Racing rules state 10 PSI per 1,000 RPM. GM for years used a Check Oil light that did not come on in their engines till you had less than 7 PSI of oil pressure.
Modern engines like the GT40 like oil pressure and thin oil. The needle bearings in your GT40 Roller lifters, assuming it does use the roller camshaft, need lubrication. The Passenger car versions of this engine used 5-20 or 5-30 oil to make sure the lifters lubricate. Going to straight 40 scares me unless PCM chose to take the Roller cams out of these engines and use standard flat tappet camshafts. I know in 2001 they advertised the engine had a roller cam and the Horsepower is still rated 310 which tells me all the 310 HP rated engines used a roller cam.
Adding a roller camshaft does give more HP.
My 95 manual states to use 40W or 15-40W in the GT 40 310 HP models.
If you have low oil pressure at idle but good pressure running above 1,000 RPM you have internal leaks. Many engines run Hundreds of thousands of miles with these internal leaks so don't stress unless it gets worse.
Internal leaks that allow low idle oil pressure are:
Oil Pumps with more than .003 clearance between the cover plate and gears.
Oil pumps with a pressure relief valve that is sticking open or partially open.
Loose Main and Rod bearings. More than .0025 clearance on rods and Mains allow a lot of oil to leak around the bearings and the oil pump has trouble keeping up at idle.
Loose Cam Bearings, often overlooked but cam bearing can leak a lot of oil. Some engines are just built loose at the factory and you have to live with low oil pressure at idle.
You may just have a bad sending unit or bad gauge and your engine is actually just fine as is.
Mine holds a steady 45 psi, I don't think it moves more than 5 PSI from idle to wide open and that is normal on a tight engine.
If your oil pressure bothers you a High Volume pump will kick the pressure back up but also waste Horse Power to pump the extra oil.
I had a Diesel powered Suburban that had 20 PSI at idle and 40 PSI at RPM and ran it for 170,000 miles with no oil related issues. After I rebuilt it because it blew a head gasket that same engine had 60 PSI all the time. There is no magic to oil pressure it is just parts and clearances.

^^^+1

The Oil pressure mentioned and leaks mentioned are facts. It is also fact that when building engines they do not all go together equally.
One will have slightly higher or lower pressure based on stack up issues in assembly. The rods may be within spec, same for the bearings cam, crank. For each part there is an accepted tolerance. If you get the crank on the low side of tolerance, the bearings on the loose side of tolerance and build the engine it will be more loose than one with the opposite components. It happens every day in production. Both are new, both are good but one will have high pressure at idle, one will be low. Some manufacturers actually started acting on this and are far more careful today. Some have the bearings color coded by size after they are manufactured and match them to the crankshaft after it is built to maintain exact oil clearance and much tighter than in the past. In 1980 crank clearance of .0025 was common with a range of + or - .0005 accepted. By 2000 some manufacturers had tightened Crankshaft clearances up to .001 with a accepted tolerance range now measured in much finer gradients. They say it has reduced NVH in the engines, Noise, Vibration and Harmonics and these are running much thinner oils.
0-20 is becoming common now. You may have noticed engines in 2000 and up see well over 200,000 miles with zero lower end issues with these modern engines in Automotive use. Marine engines though are unique, they use cold oil that does not flow and protect as well, the volume produced is far less than in Automotive and the money available for Research and Development is very small in comparison to Automotive so they used proven past history to build reliable blocks for Marine use.
I should qualify this with prior to the year 2000, I have no idea what they are doing internally with the new engines like the LS GM engines to convert them to marine use.
I had read in Water Ski Magazine that the 2001 GT40 had roller lifters if I am wrong, my mistake. You are correct if these are still Flat Tappet you certainly need oil with protection in it. The Chevron Delo 15-40 designed for modern turbo Diesel engines has all the additives needed and is cheap and available most anywhere, it will protect a flat tappet camshaft. Not an add for Delo, just a fact, other oils offer the same protection but do make sure you use quality oil with the zinc additive. My 2001 Tahoe 5.3 LS style block with 145,000 miles still has 45 psi at hot idle and runs 60 PSI when above 1,000 rpm. Tells me the by pass opens at 60 psi and my engine is a little loose somewhere inside or it would have 60 psi at idle also. At idle even 10 psi is enough, I just would not be comfortable having 10 psi at idle.
On most engines your by pass in the oil pump is starting to open at 400 RPM if the oil pump is tight. In the early 90's we had a run of oil pumps produced for GM with too much clearance between the gears and the pump cover, about .0045. These all had low oil pressure at idle. At that time we recalled these and machined them back to under .003 clearance and all was good with them at idle after the fix. Even if not fixed the engines with those pumps would have most likely have had full life service. If these were installed in cars with Oil Pressure gauges we heard customer complaints about low oil pressure at idle. If the car had an idiot light that came on at 7 psi we heard nothing as the pump was always above the 7 psi light. Hope that helps rather than confuse the issue.