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Engine starts up and oil pressure is 25psi. Rev engine up to 3500rpm and goes to 55psi. Make a run down the track and it will be 20psi. or lower, let off the throttle and it will come back. The engine is a ls1 chevy. Main clearance is .0028 on all but but #5 that is .0029 It has a Katech oil pump and a c5 oil pan. I started the engine on mobil 1 5w30 then went to 20w50 valvoline, gained 3 psi. I have had this engine out of the car 3 times. It is NOT the pick-up "o" ring. I am hoping that Chris will chime in. Thanks fellas for any suggestions. JC
 

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I just went thru the same thing on my Big Block Chevy. If you are saying that the oil pressure drops when you lightly accelerate and comes back up when you let off then you could have a bad relief valve in the pump. Mine was either that or a collapsed filter (filter relief was blocked off)

I dropped the pan, changed the filter and the oil punp and the oil pressure returned to what it was when I had the motor built. Made 8 passes since and it stays where it should.

Good luck with it and keep us posted.
 

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if you have a high volume pump it could be running out of oil to pump. Have you tried putting an extra 1/2 or whole quart? I've had to do that before...
 

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Could be a few different things. Sucking air into pickup/passages, trouble flowing around sharp corners if you havent ported the passages running from the pan to the block, pumping all the oil to the top end/not draining fast enough.
 

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Fram filter if you have it on there could be an issue. I just had a similar problem but reversed on an 06 GTO with a Katech pump Had TOO much pressure Put the stock pump in and worked fine. I would go back to the stock pump.
 
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