I have a 1970 Cutlass with a rebuilt 350. After start up oil pressure on the factory gauge is in the middle. After driving approx 10 miles oil pressure drops off...at a stop light pressure is at the lowest line. When revving pressure fluctuates up and down but does not go above a quarter. When driving presure seems to rise then drop off again...any hints?

 

Even a healthy engine generates less oil pressure at idle than at 2,000 or 3,000 rpm. What is the numerical value? A fresh/healthy engine will do 40 psi easy, my 455 developes 50 psi at 2,000 rpm and at idle I think it holds 35 psi all day long. An older engine with worn bearing and bushing surfaces might hold 20 psi at idle, any less than that and I would be concerned. If you engine is low then it probably just needs to be rebuilt, then there is always the chance that the gauge is off some too.

 

This seems to be a somewhat common complaint with oldsmobile engines, I 've been seeing lots of these complaints. Here's the deal, you may not really have an issue here. The problem may be just in the olds design of where they measure oil pressure on the engine. By reading oil pressure up front, above the timing cover on the driver's side, you are reading pressure at the worst possible place, the end of the line off the driver's side lifter galley. This is the worst place to read oil pressure. You want to tap into the main line feed, just out of the oil pump so to speak. We have found one easy way to do this is to tap right into the oil filter housing, there is a perfect flat circle area on the left hand side above the oil filter that you can easily drill and tap with a 1/8 inch pipe tap, this is the oil galley from the pump to the filter. if you take your readings at that point, you wil have a better idea of what is going on inside the engine. If you don't get good oil pressure reading there, you have a serious problem. With the 2 pressure gauges hooked up side by side , the gauge in the original location continued to show pressure drop offs with rpm, yet the gauge hooked up to the oil filter housing showed us a steady 70 psi under full load at over 5000 rpm. Now I was happy.
Reading pressure at the front factory location is in my opinion a bad choice. the oil has been through who knows how many 90 degree turns, through all the lifters, out the pushrods, and is a resticted passage to begin with. I want to know what the oil pressure is getting to the crank, bottom line. I don't care what it is at the lifters, if there is oil there, you are fine.
We fought this on a 455 olds on the dyno just yesterday, had great pressure cold, even great pressure hot, but as the engine got up into over 4000 rpm range under load, the oil pressure would simply begin to fall off, like to the tune of 30 to 40 psi, as soon as you let off the throttle, it came back up. We disected the filter, no debri, pulled the pan and changed the oil pump, no debri in the pan, and no change.
I really wanted to get to the bottom of this because we have sorked for years to solve oil problems on the 455 Buick engines, and I figured if we could make those engines live at high rpm, we should be able to make an olds engine live.
the olds engine uses a way better and bigger oil pump than the Buick engine, so I beleive it should not be that hard to maintain good pressure in one of these.
Hope this information helps some people out.


Jim Burek P.A.E ENTERPRISES www.paeenterprises.com

 

Jim, excellent post, there really seems to be some validity in that. I can't wait to try it on the next 455 I install, should be soon. Thanks.

 

Jim,
Great post!

I'm certainly no expert, but wouldn't you be reading the output plus back pressure (possibly) giving an artificially inflated reading? It seems like "downstream" will give a better idea of how much pressure is actually pumping where you need it.

Again, I'm no expert...just thinking out loud (kinda).

C.J.

 

C.J., nope, not a problem. You just want to be tapped into main line pressure. This was the easiest location we found without havving to do a bunch of weird plumbing and drilling. Jim Burek