had the same problem with a old pro truck motor 358 the oil psi was great with out vaccum hook up vac oil psi drop check every thing called every body never found problem left vac off and raced with out vac like to know what you find still tring to figure it out

Water column (inch) is not normally used in these type applications as it is a much smaller unit of measure. 1 psi equals about 27.7 wc (inches), where as Mike posted 1 psi equals just a touch over 2" of mercury (Hg).

This is what Im thinking.... the pan is coming off it today and the pickup is getting inspected.

Something just dosent make sense here

I simply meant that the excessive oil pressure is by choice. And I don't think it has anything to do with this problem. This exact combo has bee fine and living well for a number of years. I realize that lots of people think 100lbs is excessive and detrimental to HP.... while I agree that the added "windage" from that oil pressure may rob a few HP, I believe that the added pressure decreases the odds of scuffed metal to metal should the something happen.. IE: sudden temporary loss of pressure should the pump cavitate in a wheel stand or when vacuum tries to overtake it after lifting at the big end.

Funny you should ask that.... they are stage 5 Millenium heads. And after racking my brain for any changes I made during this winters freshen up, I remembered that I installed external rear drain backs on them this winter. My thought was to double the speed at which the oil was returned to the pan.... the drain back runs from the rear of the head to each side of the oil pan. This is one of the only changes that was made to the combo. I purchased some caps and just for my own santity, Im going to block them off and try it. Id bet the farm that they have nothing to do with this problem.... but ??

Thanks. I figured, but just had to ask because of how weird the problem is. After reading some of the posts, I guess it aint that weird.

Hey Guys,

UPDATE!!

So after trying a bunch of things... and doing some brain picking... we removed the oil pan to inspect things yesterday. What was noticed (by my brother / crew chief:) ) was that in one part of the pickup swing, the pickup became pretty close to the bottom of the oil pan. Finding nothing else, we decided to go with this as a possible issue. We managed to raise the pickup slightly by elongating the holes in the side of the oil pan and grinding some off the Milodon pickup bar inside. This was needed to clear the centre tube in the oil pan for the centre ink. After all modifications, we now have about 1/4" clearance between the pickup and pan for the full swing. Back together.. fresh oil... oil pressure stable through full vacuum range now.

Best guess is that as the vacuum increased and the oil pressure increased, it was just enough to pull the swinging pickup too close to the bottom of the pan... thus killing the oil pressure.

Thanks for all the input guys... have a great season!

Glad to hear you found your issue.. at least a
possible one... you dont want or need much vac at
idle and you want to limit it to about 12"-14" hg

Sounds reasonable. It wouldn't take very much vacuum to oil can the bottom of a flat pan if it wasn't well supported. If the pan was being sucked up and blocking the inlet then you would lose oil pressure. I'd say this falls into the "weird" category. Glad you found the problem. I bet your old pump was fine also.

Glad you found it. Hope no peripheral damage.

lol... hope not ! hate to think I Pissed away $800.00 on that KB pump ! But at some point Im going to try the old Milodon and see for sure... test and tune this weekend coming... hoping for a low 8.90s or may a 8.80s ... if I have the balls to stretch the old girls legs out a little

Well oil pumps don't really go bad unless they are really trashed. Take the cover off of the Milodon pump and pull the rotors out and look at them. If they aren't all galled up then the pump is most likely just fine. You were probably fighting the issue with the pan being too close to the pickup the entire time.

On the bright side.... Im 1 step closer to having a backup motor if that old pump is good.... couple of more dollars and im good! hahahah