I have a 1972 400 sbc bored out. I had a Melling M55HV oil pump. I was running 40 to 50 psi at 1800 rpm. When the rpms got to 400 I was at 85 to 90 psi. I replaced the oil pump with a M55A pump and put a lower rated spring for relief. I think it was 49 psi rated. I started the engine and I it was 50 psi at idle. Took a drive stayed at 50 psi. When rpm got to 4000 rpm pressure dropped to 25 psi. Couple days later started engine no pressure. What should I do? Do I need a high volume pump?

 

why you running 4k?