Vacuum Pump Vs. Oil Preasure ??

Hey Guys,

Question for you engine builders that use low tension rings and a vacuum pump.....

Having an issue that maybe somebody else has experienced.... looking for some fresh thoughts on it.

Have a Second Gen Hemi...

After a recent freshen up this winter, I am experiencing an issue where as the Vacuum increases, the oil pressure goes away. Right now...idling... as the vacuum increases to 5" and over, the oil pressure drops to 0 !

Here's the following facts...

- fresh rebuild, rings bearings ect. All hard parts are the same as before.
- Milodon swinging pickup, duel line
- KB Oil pump, 2 years old.
- Moroso 4 vein vacuum pump
- duel vacuum pickups... one in each valve cover

3 seasons ago, When I first switched the engine to low tension rings and added the vacuum pump, I had the same issue. At the time I had an older Milodon oil pump. Being told it was probably the oil pump, we replaced it with a top of the line killer KB pump.... that seemed to correct the issue.

3 years later, and a fresh rebuild, problem comes back. Now I know its easy to say its the pump... but I'm really doubting that! The kb pump worked perfectly all last season... the pump has no marks inside on the rotors. All we did was ring and bearing this engine?

Idling the engine makes 110lbs of oil pressure cold.... 90 hot. Straight 40 brad pen oil...

The Vacuum pump is setup with the small crank pulley... it pulls a total of about 14" at the stripe...

Trouble is, as the vacuum increases to 5" and above, oil pressure drops like a stone.

I have tried the following things so far with no avail...

- added more oil... there is now about 13 qts in the engine...
- replaced both suction lines on the oil pump to pickup, just in case one was internally collapsing...
- dropped down to one pickup line from the pump to pan pickup in an effort to maybe strengthen the oil pump signal...
- tore the KB pump apart and inspected it.... its perfect...

So aside from spending another $500.00 to $800.00 on a new oil pump, anybody got any ideas?? Anybody seen this before??

Thanks,

The vac does pull the inlet oil pressure away..
simple version)as you increase the vac it doesnt
let the full atmospheric pressure to push all the
oil into the pick up to the pump... when your idling
you dont need any vac.. or damn little

I was kind of hoping you would reply I read with great interest your replies here on this forum...

Trouble is, as rpm increases.. so does vacuum. This is not adjustable, only the overall amount is adjustable... using the relief valve. Bringing the engine to just over 2000 rpm, nets 5" of vacuum... which instantly pulls oil pressure to 0 from 110 lbs ? I just hate to think that my $800.00 KB oil pump is toast after 2 years! and not a mark in it...

The vacuum pump will decrease the gauge oil pressure inch for inch. But there is no way that 5 inches of vacuum is going to take away 110 psi of oil pressure.

You have some other weird problem. Since the problem is so weird it shouldn't be too hard to figure out. Standard testing should find the problem. Add a second gauge, run it without the vacuum pump, etc.

Also remember that your reading 2 different things here..
vac is measured in inches of HG(HG is mercury) and you know
the PSI... you can look up the differences.. but 2" of HG
is 1 psi... for the vac if its a belt driven you can slow
the pump and /or use the relief valve... but again.. you
dont need any vac at idle

Yep, first step is to try and isolate the issue. Try running the motor first without the pump bring employed .
From what I understand( and I have never run a vacuum pump) your oil pressure is going to come down directly related to how much vacuum your pulling. Pulling 15 vacuum should lower oil pressure roughly 15 pounds.
I am kinda surprised you seem fine with having over 100 pounds of oil pressure cold/ 90 hot.That is WAY to high in my opinion, and counter productive. With tons of 40 weight oil that can't be a positive effect on power output.

I agree, take the vacuum line off, there's no way vacuum should affect the oil pressure like that. It would literally have to suck the pan dry. Thta would fill the puke tank lickity split!

It would only come down equal IF both readings
are EQUAL... but the way these are measured they
arent equal

assume you are measuring crankcase vacuum in " of water?

Inches of HG(merc)
EDIT
This one thing that people must understand.. just
because it has a gauge on it doesnt mean that it
should read X... psi to psi is one thing... put
inch of water to psi.. thats another

With the Vacuum pump belt off... oil pressure is normal..

Yea... Ive been told that its too much oil pressure..... robs HP... now go ask how much Oil pressure a pro mod runs.... they have shut offs if it drops below 160 PSI.... I'll eat the HP to save this engine....

Some racers search for every avenue of HP savings... and I can't argue that, but at the end of the day... too much Oil pressure never hurt an any engine... but not enough has hurts lots!

If you find that the gauge was/is accurate, then there might be enough restriction or an air leak on the inlet side of the pump to cause this. I would pull all those items apart and make sure you have pan to pickup clearance, no air leaks, either above the oil level or somewhere close.

I dont build pro mod engines so I cant say what
they run for oil pressure... the next thing is..
did you build a pro mod engine... I'm guessing.. no
so in my way of thinking is.. dont look at their builds
unless yours is like it... if thats the true case and they
run 160+ oil pressure then they COULD be pulling high vac
which would reduce the oil pressure

Don't have to suck the pan dry. Just gotta keep the oil from traveling up the pick-up tube(s). I don't build pro mod motors ether, but the problem sounds pretty basic. Vacuum pump plumbed properly? Using the correct size lines?

If the oil pressure is normal with the vacuum pump belt off then you don't have any problems with your oil pump. Have you run the car down the track without the vacuum pump? If you have full oil pressure for the entire run then your pump and your plumbing is all okay.

5 inches of vacuum will reduce the gauge oil pressure by 2.5 psi. If you're seeing the oil pressure gauge drop 100 psi then you have something very odd going on. 5 inches of vacuum isn't enough vacuum to disturb the inlet side of the pump. My guess is that somehow the vacuum pump is disturbing the oil pressure gauge. I bet you have plenty of oil pressure but your gauge is wrong. Move your oil pressure line so you're picking up the pressure right at the pump and see what you have.

I agree Andy.. his numbers dont add up... but I also dont
think he needs a 100 psi.. but if he wants it.. fine

At the risk of confusing the issue I'll point out that when using a vacuum pump the oil pressure which the bearings see does not drop when the oil pressure on the gauge drops. The pressure differential across the bearings stays the same which is what is important. The pressure on the gauge side does drop, but the pressure on the other side of the bearing also drops. So the pressure across the bearing, which drives the flow of oil, stays the same.

Sounds like you're not in a situation where you're looking for every last hp, so why the vacuum pump?

Those swinging pick ups tend to not be 100% air tite, so unless you're sure the oil level keeps it submerged at ALL times, might be something to look at.

Just out of curiosity, what heads? Do they need drain backs? Is oil gathering in the puke tank? Too much oil on the top of the heads? My vacuum pump scavenges about 10-15 lbs at 7800 RPM and 14 inches. That would be from 95 to 82 lbs through the lights. The only reason I know that is because I have a racepak and left the valve covers loose one time.

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