Oil pressure loss SB Chevy
Hello, I am new to this board. I've been lurking for a while now, but now I ran into a problem and decided to ask this board for help. I tried the search function and searched the Knowledge Base, but did not find any clues.
I just finished building a 383 cid small block Chevy. The engine has a '72 350 cid block (.030" stress bored), an Eagle cast crank, 6" Eagle I-beam rods, KB pistons etc. Everything is practically new except for the block.
I started the engine for the first time on Saturday, and it seemed to run fine. I have a new Melling M55HV oil pump (with the heavier spring), and the oil pressure was 60 psi at idle when cold and 40-45 psi at idle when warm, rising to about 60-65 when driving. I drove it around for a while and everything seemed fine. The oil in the engine is a mixture of two mineral oils, some 20W-50 and some 10W-40. I thought I'd break the engine in and then change to synthetic.
Today I took the car out, having driven only a couple of miles on the weekend b/c the hood was still off the car. I went to a test drive with a friend, and while warming the car up the oil pressure was a constant 60-65 psi. When the engine had warmed up, I took it to around 4500 rpm on 2nd gear, and when I changed to 3rd I noticed the oil pressure gauge falling rapidly. I let off the gas pedal, and the oil pressure stopped falling at 20 psi and stayed there. I idled back to the garage (1/2 mile), the gauge at 20 psi all the time and shut the engine off.
We opened the hood and checked for visible leaks. None were to be seen. I checked the fittings on the oil pressure gauge, an Auto Meter mechanical unit, and they were fine. I started the engine again, and now the pressure rose slowly to 40 psi at idle and stayed there. I tried 2500 rpm and the gauge stayed at 40 psi and then fell slightly. The engine idled and ran fine, with no extra mechanical sounds, but the pressure was down.
I have an extra gauge, so I decided to try that. I put the other gauge on, with a different piece of copper tube, and started the engine. Same results: slowly to 40 psi at idle and a slight drop when at 2500 rpm. Again the engine seemed fine, with no extra mechanical sounds.
I took the car in, drained the oil: it seemed fine. No metal shavings were visible, even after pouring the oil through a paint filter. The oil had a strange, bronze-ish color but otherwise it seemed fine.
I'm at a loss. What could cause the oil pressure to drop all of a sudden like that?
I'm sorry for the long post, but thank you if you made it this far. Thank you even more for any ideas.