oil pressure

[dy97]

okay.. 2 races ago i begin to have oil pressure problems.. i have always had good oil pressure i didnt change anything.. just showed up to the track.. oil pressure was okay in heat.. went into the feature.. very first lap coming off turn 4 for the start as soon as i mashed the throttle it went to 10 pds.. all the way to 6 thousand.. it was a gradual fall.. as soon as i let off going into turn one it went back to 45 or so .. got back on the throttle went back to 10.. i went ahead and ran the entire race .. didnt really care what happened.. ran the whole race 6k rpm with 10 pounds on every straight away.. in nuetral.. at idle with hot oil it had around 10 after the race.. bring the rpm up to 2500 or so .. about 40 to 45 pounds.. wing it on over to 5500 in nuetral and hold it for a few seconds it would hold 60 ish pounds..got home pulled the motor (we build our own motors) pulled the pan.. i was running a stock pan with a stock pick up with a mellings HV pump.. noticed that the stock pick up was cracked.. so i assumed i had it fixed right there.. went ahead and checked some bearings they looked fine ( some how ) .. still turned easy everything felt good so we changed the pump.. i put on a 7qt pan and a new pick up for the pan .. we also changed cams while it was out.. put it back together put it in the car.. had about 70 pounds oil pressure cold.. im running 40 weight chevron oil with lucas oil stabilizer.. so we did break in on the cam.. oil pressure was fine.. all was good .. i ran it several times.. then i ran it where i test it for pops and other such issues.. as soon as i stuck it on the floor it went to 20 pds oil pressure from 50 .. and that was at 4500.. we thought perhaps from all the cam lube and other lubes from reassembly that maybe we stopped up the filter and it was bypassing.. so we changed the oil and filter as u would after break in anyway.. the filter felt pretty heavy and had some metal in it as expected.. put in new oil and filter .. ran it today.. let it warm up.. i took off in low gear this time.. it had about 60 pounds at 6K rpm.. shifted to second.. fell to 50.. shifted to third fell to 40 and continued falling until i let off then came right back to 60 before rpm even fell.. we also pulled the distributor to check for shaft slip.. we didnt see anything happening.. we changed them anyway.. same thing.. .. im not running restrictors in the block.. but iw ould think if i needed them i would have needed them all along.. still may try some anyway.. any ideas? oh yeah.. also.. it only does it on pavement.. or when the car is pulling hard.. i did a bunch of donuts and blasted around in loose dirt and it held 70 pds holding the rpm at 6500 feathering the throttle.. im stumped

thanks - daniel

[john n]

try moroso # mor22101 or mor22111 race pumps. the first one is std volume anti-cavitation pump the other is hi-volume. also use wix racing filter with a welded up oil filter bypass valve. the thing the oil filter is screwed too. any ? 210 889 7041 john.

[jwmbishop]

First would be to replace the pump - under load and increasing RPM shaft walk can cause it to dump oil at bottom of rotor and stator.

 

Most typically pressure drop with no knock under load is a stationary bearing. Rods will knock if they are spilling off pressure. Sounds like cam bearing(s). It could be your mains the oil barrier will distort more under high loads spilling off pressure - but you said they looked good. Of course I have seen bearings look good at .0035 clearance which will drop pressure under load and you did not say you mic'ed em!

 

Did you measure thrust on both crank and cam?

You had it apart - why did you not just change all bearings?

 

Did you change out lifters as well as cam (did not say if you are hyd or solid)? A bad lifter bore can dump a bunch of oil!

 

tap into the pressure port above the oil filter and compare the two pressures. that will give you an idea of how much pressure is spilling off and (kinda) where.

[floyd26f]

i had a problem like that. good oil psi at idle untill you bring it up off idle then it would drop oil psi. found the oil pump pick broke off at the screen. replaced the pick up and never did it again

[dy97]

First would be to replace the pump - under load and increasing RPM shaft walk can cause it to dump oil at bottom of rotor and stator.

 

Most typically pressure drop with no knock under load is a stationary bearing. Rods will knock if they are spilling off pressure. Sounds like cam bearing(s). It could be your mains the oil barrier will distort more under high loads spilling off pressure - but you said they looked good. Of course I have seen bearings look good at .0035 clearance which will drop pressure under load and you did not say you mic'ed em!

 

Did you measure thrust on both crank and cam?

You had it apart - why did you not just change all bearings?

 

Did you change out lifters as well as cam (did not say if you are hyd or solid)? A bad lifter bore can dump a bunch of oil!

 

tap into the pressure port above the oil filter and compare the two pressures. that will give you an idea of how much pressure is spilling off and (kinda) where.

thanks for the help.. i did replace pumps.. i did not mic em sadly.. or measure thrust.. i just assumed the pick up was the problem.. since it was screwed up .. and thats the same reason i didnt change bearings.. live and learn the hard way.. and yes i changed lifters with cam.. its a solid lifter.. no point putting used lifters on a new cam.. or new lifters on a used cam.. ive learned those the hard way too.. but i still dont understand why it would hold 70 pounds at 6500 rpm in loose dirt.. .. but not on pavement.. you would think if it was gonna lose pressure it would do it in loose dirt too..

[dy97]

i had a problem like that. good oil psi at idle untill you bring it up off idle then it would drop oil psi. found the oil pump pick broke off at the screen. replaced the pick up and never did it again

yeah the pickup was cracked on mine.. not broke all the way off .. above the screen in a half moon.. i assumed it was fixed once i got it changed.. wrong

[jwmbishop]

i had a problem like that. good oil psi at idle untill you bring it up off idle then it would drop oil psi. found the oil pump pick broke off at the screen. replaced the pick up and never did it again

That would do it - as RPM increases the pump moves more oil yet oil return stays the same so the level in pan drops as more oil is in the heads etc, and you start drawing air. In the current case he is making good pressure unless there is a load on the engine. That screams main bearing (usually the thrust bearing set) to me. Of course by monitoring BOTH oil psi points you get a better idea - if both lower and upper drop pressure by the same percentage its the pump/filter/bypass - if only top drops (or drops with less percntage than the lower) its bearing clearance (or passage leaks). The actual pressures can vary with the build - the more bearing clearances, and the more lifter flow the greater the diff - but after all bearings and lifters are are fed (top port) you should have the minimum of 12 + min 5lbs per 1000 rpm extra. I typically install one guage (lower port) under the hood and the other in the dash - but you can't compare them under load unless you are on a wheel dyno.

 

As another point - lets say (solid lifter) your lower pressure is 45 and upper is 15 at idle. This means that 66% of your oil flow is washing thru the bearings. Thats part of your cooling system - you could safely increase your bearing clearance .0005 and have the bearings running cooler (no more than 75% of oil washing tho or film pressure (not to be confused with fluid pressure) starts dropping too fast to lubricate properly.)! The formula for hyd lifter is not as easy as they bleed off even more than the bearings - you need the bleed off amount to add that back to the total flow to determine how much bleed off is from the bearings alone. Conversly oil leaving the bearings is a parasitic horsepower loss - the engine has to either scrape it away or the mass of oil is being caught and spun along with the crank. You could have as much as 8-10 lbs of oil clinging to the rotating assembly!

 

Passage leaks don't usually suddenly occur - this CAN be checked visually as the passages are plugged at the ends with either expansion plugs or drilled, tapped and plugged with allen head plugs. I left this out of earlier post as it is quite unusual for them to start leaking after time - typically they will blow on first High Pressure run! and then they will dump with or without an engine load.

[Pacecar]

My guess is that there's not enough oil drainback in the lifter valley area. Are you using a screen over the main drainback holes? This is too much restriction. Did you open up the drainback holes?

[jwmbishop]

its only only loosing pressure under hard torque (pavement) not medium torque (loose dirt) - look at your block from the front with a clock face superimposed over it. The piston banks are at roughly 10 and 2. this means that torque is applied to the crank throw at 1-4 from the pass bank and 5-8 from the drivers bank. What does this translate to? simple - when you have a ratchet and extension if you push the handle of ratchet what happens at the ratchet end? you have to pull it equally the oposite direction. That same thing is happening to your crankshaft. It does not stay perfectly centered in the bearing line. It actually gets forced upward and towrds the right of the veh on a clockwise rotating engine.. As the right bank piston is loading the crank from 5-8 the crank is moving harder towrds 11 (90deg opposite the load line) - as the left bank works the load is applied towrds 4.

 

What does this mean for your oil PSI? simple - the oil pressure film is only .0015 thick (40wt at 245 degrees farenheit). If you have .0025 clearance (clearance is 1/2 the difference in size between journal and bore)- as the crank loads up and left, the lower right grows in actual clrnce. So if the clearance is .0025 static all the way around (total .005) the bearing under full load it will be .0015 on the "working" side and .0035 on the other - thats enough to hold pressure. However assume clearance of .0035 (.007 total)- less the .0015 film leaves a whopping .0055 thats almost two times the bleed off (each mult of .0015 is one times the potential flow) or a 50% reduction in pressure!

 

Dirt does not hold the same torque load as asphalt - its easily understandable that 75% less coefficient = 75% less crank load. Now with all of this said - my friend, mentor ex-boss, partner reminded me of one we saw way back when the 305 smog motors first came out. the crank broke in two just behind the #3 main journal. It did not knock and since it was "captive" by 2 bearings on one end and three on the other it ran and actually towed a trailer up Bennet Grade. With virtually no oil pressure. As torque was applied the crack widened and dumped oil from the center of the crank (Chevy uses thru the crank oiling). And the bearings looked great! Heres why - if you are starving for oil (pump problems) you will loose volume as wel as pressure (and loose that .0015 film) - if you are dumping off pressure (but still have volume) you will maintain the .0015 - and they will actualy run cooler as more oil flows through taking heat with it!

 

Don't feel too bad - I probably would have stopped at the pickup seeing damage as well. But the fact that you KNOW you had a major pressure problem and the bearings did not show overheat - well thats machinist experience not mechanic. I had a real hard time when I changed from mechanic to machinist as my work (not much diff though as I was way to detail oriented to be an effective line mechanic - and make money - I did not have a very high flag rate)! The machinist will always rebuild without regard to small parts - reliability. The mechanic will always try to make it work with as few parts as possible - affordability.