My oil pressure drops over 4500 rpm...any ideas?

My 34 Plymouth has developed a new trick I didn´t know yet... I did a long and fast Autobahn run last weekend and I recognized that the oil pressure would drop to 40-45 psi whenever I reached 4500-5000 rpm for a while. Slowing down and letting the rpms drop to 3500-4000 the oil pressure would come back to 60psi.
The engine is a 11:1 327 SBC, TRW forged pistons, forged crank, Melling M55HV oil pump, Edelbrock Heads.
Do you think it is possible that the HV pump could suck the stock oil pan empty? I have 45k miles on the engine and all the bearings looked good last time I had the oil pan off, that was last winter.
What do you think?

 

tag

 

You didn't mention what viscosity grade oil you're using, but my guess would be that you're dumping over the relief valve.

 

What weight oil you running and what temps are talking?

 

If you are pumping more oil than can return to the pan try an extra quart of oil in the pan and make a run.

 

yes you are sucking all the oil from the pan. killed a Z28 doing that. larger pan/more capacity, extra 1/2 quart, or take it all apart and clean the block so the oil will drain back faster. adding the 1/2 quart is a good test.

 

Does it have a Fram oil filter? If so, get it off there immediately...!

 

I agree.

 

I run a melling high volumn pump and had to go to a 6 quart pan because it would suck the pan dry and cavitate. If I had been thinking I would have opened up the drains when I was building the engine.

 

I had 10W40 in there from day one. It was a very cool day, I think water temp was around 180 as usual, no fram filter. I use AC Delco filters.
The engine is very clean inside, no sludge at all and I keep the oil level at the full mark all the time...maybe i should try a bigger oil pan...
funny thing is this happens not immediately when I step on it, but maybe after half a minute of high rpm driving.

 

Years ago, in corners one of my cars would drop oil pressure.
I made a pressure tank from a old propane bottle, valve on bottom, truck air nipple on top. It cranked up with pressure too making motors last longer.

What you are describing is oil starvation thou. IS the motor clean and free of sludge? The drainage holes in the front, and along the cam should allow it to trickle back down. Adding to the pan size helps, but does not fix.

Pressure drops? I used to be bad to pull the relief spring, stretch it, put a carter pin back in, sometimes just put a washer behind it too, you don't know what you are getting till you got the engine fired off thou. AND I SPLIT a few oil filters, once thinking I had blown the engine. There was a WIX oil filter that could hold the high pressure. Too many scars and knots on my noggin to remember the number.

BUT then, if that 327 has the old original canister.. I got what you need.. a 5 ton truck canister about a foot plus long. (if I could find it in this mess)

 

I actually did open up the return holes in the lifter valley when I built the engine, I read this in my "How to hot rod your small block Chevy"- book, hahaha

 

Yea I would have done that if I were building your engine but somehow I managed to blow it off on mine.

 

Had the same problem on a small block many years ago, a neighbor told me to ditch the high volume pump and get the high pressure pump. Never had the losing oil pressure problem after that. Now I just wish these damn big block buicks were that easy.

 

new problem---old motor---you still have good pressure even when it drops---did you change anything when you had it down the last time?---all the suggestions are spot on for a fresh motor ( extra oil ,etc ) but your motor has plenty of run time, so what has changed , does the pressure fluctuate , what happens at 7,000 ? --replace your filter it is easy to do maybe go with the larger one ---make a test run it may be that simple--good luck

 

If oil can't return to the pan quickly enough adding more oil will just delay the point at which the pan empties, not fix the problem.

You could try cleaning up the burrs and casting flash on the block and heads, as often they are rough, or drain holes are clogged with rough casting burrs.

I've not built many motors myself, but the few I have done I took a die grinder to all the internal edges, drain holes etc and smoothed and cleaned out all the roughness,especially on the ends of head heads where the return holes are. It also removes the chance of a piece of casting coming loose and ending up in your oil system.Your hands will also thank you when you rebuild, as you get less cuts.

Things like this don't help oil return flow. Clean it out, radius and smooth the edges with the die grinder:





Or maybe your oil pickup is a little clogged with something and the pump is cavitation at high rpm, sucking vacuum bubbles into the oil?

 

one other thing to think about. The clearances are getting larger and the oil is getting "slung" out away from the rod journals and the pump can't keep up. pull the oilpan and check a couple of rod bearings, the ones towards the front.

 

I had that happen in my almost OT motorhome w/ 460 Ford. I first noticed it after I changed my (amsoil) and (amsoil) filter.

I put a Motorcraft filter on, problem solved

 

Another for oil starvation.

You said it's a new thing but I would still mostly blame the oil pump. I can almost think of no reason to run a high volume oil pump. It just moves too much oil out of the oil pan.

I would also quess that *something* might have come loose and fallen into a drain hole. I've seen it enough times where someone gets a little happy with the RTV and eventually pieces of the excess falls off and plugs up a drain hole. I've also seen pieces of valve seals in the heads and pieces of fiber timing gear in the pickup which could also be causing your problem.

I'm guessing both ends of your engine may need a looking at.

 

THIS IS THE MOST LIKELY SCENARIO

If the oil isn't returning fast enough, adding 1-1/2 quarts can mask this [ simple cheap diagnosis ] If it cavitates the oil pressure guage/light flickers

But most likely there is a restriction between the pump and the journals ,or the clearances are too large and the pump can't keep up
The pump is on bypass most of the time, but with large clearances it gets too a point where the engine will dump more than the pump can keep pumping.
Put a larger micron filter in first to see if this isn't the restriction [ A cheap try it and see ]

 

If my memory serves well, the High volume pump is basically a big block pump (with five bolts holding the cover on) that has the drive shaft length modified to fit the small block, where-as the high pressure pump is a normal small block pump (in that it has four bolts holding the cover) but with longer gears/rotors.
The Melling high volume pump used to come with a note in the box that said to run an extra quart of oil if you are running high rpm's for extended periods. Honestly, I've found that the four bolt pump is all you need for a healthy motor, the only times I use the five bolt pump is on high mileage blocks that need a bit of help.

 

in his book "Power Secrets" Smokey Yunich said; "Quite frankly, I see no real value to using this(the five bolt) pump, and it does have a substantial disadvantage: at high speeds more power is required to drive this pump.If the small block Chevy has proper operating clearances and the oil pan and pickup are properly designed, the Z/28 pump will provide more than enough oil pressure and volume at any practical engine speed."

 

Drop pan.install std. vol, std. press. pump. Make sure pump is correct distance from bottom of pan. Make sure oil pan is sturdy enough not to "oil can" and reduce clearance to pickup.Weld pickup to pump.

 

I found what happened and thought I'd let you know.
So i drained the oil , cut the old ac delco filter open . The oil was still golden and fresh and the filter was clean, not clogged or anything. So i went on and pulled the pan. What i saw was my oilpump without its pickup...it had taken some time off, relaxing in the oilsump
I had pressed it in, secured with a spotweld just to be save. What seems funny to me is, the weld broke at the pickup side, not at the cast pump housing. I also checked the bearings, they all look like new after nearly 50k Miles.
The first test drive showed the same pressure readings as before, 80psi cold, 60 warm.

So finally the problem was the pump was cavitating since it could not use the whole oil reservoir without the pickup... And i am so glad i didn't destroy my bearings.

Thanks for helping!!! Chris

 

And once again talk of H/V pumps sucking the oil pan dry doesn't play out...

 

No, Mine never did. And my Engine saw a lot of hours revving 4-5000 and higher... That's what made me wonder what could have caused the pressure drop...now we know

 

I haven't read all the posts, but just replaced the bearings on my 355----all tks. to a Melling HV pump.
Cavitation of the oil at high rpm.----all the oil pumps to the top of the motor, pan is just about empty and mixes air with the oil. REAL bad for bearings, etc.
Better check the bearings, My engine guy talked me into it and he was right on.
cooger

 

Now these are the type of threads I like to read.. Good info...

 

I drill and tap the oil pump and pickup tube and use a lock nut on 10-32 screw to secure the tube to the pump. You can also make a bracket that uses one of the pump cover bolts and weld to the tube to help secure it.

 

A long time ago we ran a small block Chevy injected alcohol. It went through the lights about 10,000 rpm. I saw some marks on the bearings that looked like it mite be running out of oil. It had a windage tray in it already so I made a crank scraper out of some 1/8 aluminum and mounted it to the pan rail. It took some time to make the pattern, but it took care of the problem rite away. JC