[1968GTO421]

Noticing lowish oil pressure on the aftermarket gauge in my Pontiac, I've been wondering if the person who "rebuilt" the engine for the previous owners possibly used a 40 psi oil pump rather than the 60 psi.

Does anyone have have psi values for the 40 psi pump?
idle rpm -- ?? psi
1000 rpm -- ??psi
1500 rpm -- ??psi
2000 rpm --??psi
3000 rpm -- ??psi
4000 rpm -- ??psi

After reading this thread:

http://forums.maxperformanceinc.com/...d.php?t=823863

I realize several things could contribute to this.

Engine is stock '69 Grand Prix 428 with 6X4 heads, Performer intake, SMI Q-jet, log manifolds with heat riser disabled.

I've got the pulleys and water pump off now for work so can't give any readings right now from the under-dash gauge. (Have had hand surgery so will be a bit before re-assembly occurs.)

Previous owner used Mobil 1 5w30 in it with Fram filters. I currently have GTX 10w40 in it with NAPA Gold filter.

Thanks for the help!

[steve25]

A good 40 psi pump on a good gauge would output close to 50 psi with cold oil, the 60 lb pump will do close to 70 psi.
The 40 psi pumps have a smaller 5/8" pick up tube where as the 60 psi pump has a 3/4" tube which would have been the stock type In that motor.

I dought that the smaller pick up tube type of pump would have been installed during a rebuild of a 428 which would mean that more then likley your low pressure issue is due to the pump check ball being kept off its seat by some crap.

[dataway]

A 40 psi pump shouldn't effect pressures at lower rpms except when starting cold. Warm idle pressure should be very similar to a 60 psi pump since neither type of pump should be in relief at that point, so the pressure regulator spring rate should not matter.

The big difference you would see with a 40 psi pump would start around 1000-1200 rpm as the pressure fails to ever climb above about 45 psi. A 60 psi pump the pressure will keep climbing till about 3000 rpm when it maxes out at around 65 psi and stays there as rpms climb. This is all with a well warmed engine of course. Oil wt. will make a large difference when the engine is cold.

Could also be the pressure regulator spring cap loose. Something stuck in the regulator ball seat would effect pressure from idle to about 3500 rpm when the regulator ball would be in relief anyway.

Basically a 40 psi pump should not effect idle pressure of a warm engine at all compared to a 60 psi pump. Most warm engines don't have idle pressure high enough to cause the regulator to kick in.

What are you considering "lowish".

[Chris65LeMans]

Originally Posted by dataway

What are you considering "lowish".

This.

When I bought my car, it had about 8 psi of oil pressure at 700 RPM idle. After some tuning, (chasing better performance/vacuum - not oil pressure), I've bumped up the idle to 1,000. Warm oil pressure at idle is now between 15 and 20.

[Gary H]

I may be wrong but I don't even think anyone makes the 40 lb, small pick-up oil pump anymore.

[dataway]

Originally Posted by Gary H

I may be wrong but I don't even think anyone makes the 40 lb, small pick-up oil pump anymore.

Yep, I don't even think Rock Auto sells them. All you see listed now are "standard" pressure and 80 psi pumps.

Hard to imagine the original builder would go out of his way to use a 40 psi pump considering the other mods to the engine.

Mine engine had low"ish" and strange pressure behavior because I forgot to tighten the spring cap. Easy fix as it was on the stand .... not so easy in the car.

If "lowish" means 15+ to 25 at idle with 10-30, fully warm I would think you are fine, might just be built a little loose for a street engine.

[Tom Vaught]

Just for a bit of comparison info, in my old company we used to do Oil Pump Surveys.
The "Oil Pump Lady" really knew her stuff on that subject.

So we would instrument the engines with pressure taps at many locations on a modified block. We used a 100 channel data logger to collect the information. Roush would do the actual Dyno testing as our Research Cells were always booked.

Because these engines were dyno engines vs in vehicle engines, it was very easy to swap parts for different tests. One of the tests was 5 psi oil pressure at the lowest pressure point in the system. Not for a couple of minutes but for hours of testing.

Guess what? The Pontiac Engineers probably were not too far from that mark in a pontiac engine at its lowest pressure point. (This is assuming a 40 psi pump. Pressure bypassing above 40 psi). So if, when you take the engine down for some reason other than being blown up, look at the bearings. They might look surprisingly good.
This is assuming a driver type vehicle. Like 99% of the engines Pontiac produced when the engines were made.

Tom V.

[1968GTO421]

Appreciate it, you all have been really helpful. When I get her running again, I'll make note of pressure readings at rpm's from idle to 4000 and post it here.. I would not be surprised if the previous owner's engine guy had a 40 lb pump in his shop and threw it in "for giggles" and to save $ buying another one. Who knows?

(My brother was over in Oct and replaced leaky valve cover gaskets and we saw that the tops of the heads looked "new" including pushrods, guide plates, valve springs and retainers.)

[coonhunter70]

Try a different gauge and see what it says

[steve25]

I had a 400 cid once with a 60 psi pump that got crap lodged in the check ball and when fully warmed up at 800 rpm idle with 10/40 oil would have 5 psi showing, the max it would do at 3000 rpm was 40 psi.

I drove that motor for 5 months thru a winter back n forth to work everyday ( 45 miles) with no issue until I was able to work on it in the spring.

Many times I would still rev it to 4500 rpm when accelerating and in the spring the Bearings where still good and this was using just a mineral based oil.

[Tom Vaught]

Thanks Steve for the real world example to go along with the Test Data (dyno and road) that we generated.
We ran that test on every new engine design for the "oil pump lady".

Actually she was the Lubrication System Technical Specialist and had a Doctor's degree from MIT.
A bit more than the "oil pump lady" in reality. Very smart and very POLITE when you asked questions.

Tom V.

[coonhunter70]

She sounds like the perfect women,

Originally Posted by Tom Vaught

Thanks Steve for the real world example to go along with the Test Data (dyno and road) that we generated.
We ran that test on every new engine design for the "oil pump lady".

Actually she was the Lubrication System Technical Specialist and had a Doctor's degree from MIT.
A bit more than the "oil pump lady" in reality. Very smart and very POLITE when you asked questions.

Tom V.

[Tom Vaught]

She was. Kung Fu Master Black Belt too. You were ALWAYS polite around her, LOL!
On her lunch hour she was always going thru her Kung Fu "Forms" like David Carradine
https://www.youtube.com/watch?v=5z1ziP2lY3A

We are talking a 40+ years old Engineer here.


Tom V.

[1968GTO421]

Thanks to Tom for advice and for telling us of the "oilpump/kung-fu lady

Steve, appreciate hearing about your 400 with the low oil pressure. Sounds like I can get through to next spring before pulling the engine. It has no bad sounds through the stethoscope so I think I may be OK.

Coonhunter: the underdash oil gauge has no name on it so I like your idea of another gauge to verify things--perhaps an Autometer.

[dataway]

Three ways to look at it ... if it's an oil pump issue you are probably fine till spring, if it's a bearing clearance issue ... but stable (like loosely assembled), you are good. If it's a bearing going south issue ... you are already screwed and spring probably won't make much of a difference

I could be wrong ... but usually a bearing issue great enough to cause a drop in oil pressure will make itself known in other ways, like noise.

[steve25]

Well do the old cut the oil filter open / search for crap deal and if your ok then you can sleep better or at least drive the car without waiting for the other shoe to drop.

[Tom Vaught]

Originally Posted by 1968GTO421

Thanks to Tom for advice and for telling us of the "oilpump/kung-fu lady

I have mentioned her several times over the years. She was a key player in getting a Pontiac Oil Pump design that would allow 8500 rpm passes at 60 psi using a Luhn modified Pontiac Oil Pump.
Mike Leech RIP.

If you do the design right, the "problems" never appear.

Tom V.