I just finshed building my 355,and when I floor it the oil pressure goes up to 55, then will slowly drop to under 10 psi. It would go to 0 if I didn't let off, and I as soon as I let off it will go back to 35-40. It ildes around 30-35. It has a gm high pressure pump. Everything in the motor has less than 1500 miles. Any advice would be helpfull.

 

what weight of oil are you using?

 

castol 10-30

 

draining the pan? or maybe the pickup tube tube fell off.

 

I can,t gaurantee I know the answer in YOUR case but having seen that MANY times in my shop where guys have installed a differant oil pump and had that exact symtom, Id point out that the CAUSE is the oil pump pick-up is mounted too close to the oil pan floor, whats happening is theres restricted flow access and once you rev the engine the pump can.t get enough oil and it cavatates untill the rpms drop back to the point where oil flow can keep up with demand.
the oil pump pick up must be 3/8"-1/2" from the pump pick-up to the oil pan floor, this is checked by putting a wad of clay on the pump pick-up, then temp. installing the pan,remove the pan and measure the clay thickness

now what does quite frequently happen is that the guys installing a high volume oil pump just swap out the standard pump, reinstall the stock or simular pick-up and bolt on the pan with the pick-up in the stock possition on the oil pump. the stock pick-up is mounted about 3/8" off the pan bottom,the high volume pump is normally equiped with impeller gears about .3 inches longer than stock, the high volume pump body is that much lower in the pan, resultting in the pick-up being only about 1/8" from the pan bottom. the result is that on a normal chevy oil pump pick-up this leave a space of about 1/8" x 2.5" for oil to flow into the pump. at low rpms this works but as the rpms climb the pick-up that can,t get any oil to pump cavitates as it spins and fails to pump oil, result oil pressure drops untill rpms are lowered no matter how much oil is over the pick-up. simply checking to make sure that anout 1/2" of space is under the pick-up when the pan is installed cures that problem (a simple trick is to weld a 1/2" thick nut to the oil pump Pick-up base and test fitting the pan BEFORE WELDING THE PICK-UP TO THE PUMP BODY)

 

thanks for the advice grumpyvette