Oil pressure dropping at higher RPM - need help

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uhcoog1

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Jan 13, 2015
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Houston, TX
Having oil pressure issues after 5200 rpm or so.
61 psi @ 3500 rpm
61 psi @ 4000 rpm
62 psi @ 4500 rpm
62 psi @ 5000 rpm
55 psi @ 5500 rpm
50 psi @ 6000 rpm
48 psi @ 6500 rpm
54 psi @ 6900 rpm

81 psi was registered on cold start up

Info:
Rebuilt 6.1 hemi
.0025 main bearing clearance
.002 rod bearing clearance
positive displacement factory 6.1 oil pump. bypass listed as 65 psi. I have seen various ratings, including= 4.72 gph at 4000 rpm (quoted by melling to me), and 2.04@ 400 rpm & 4.54@ 2000 rpm (internet forum - user quoted melling)
piston oil squirters
hemi is top down oiler
straight 30 wt conventional (tests done at 170*F oil temp)
Bosch 3500 filter

Add'l- 6.1 factory spec oil is 5W-40 synthetic


What should I do to increase my oil pressure at 5500-7000 rpm? I have both a mobil 1 and k&n filter here to try (where does the bosch rate for flow restriction vs these two?). Also have various grade oils. Was going to install one of my oil pressure sensors before and one after filter for logging pressures - however I broke one (won't get new one till after weekend). I used the sensors to find other oil restrictions (one way check valve, oil cooler).

Any help is appreciated. Thanks!
 
Wouldn't 48 psi be acceptable for this situation?

Not that it pertains to this thread but Bosch premium filters along with Purolator filters have been tearing inside the media. I would recommend a different filter for this reason.

The K&N filters are also made in Korea and various other countries and not worth the price any longer since they used to use higher efficiency media now they don't. If you have an older new stock K&N made in the USA they are good.

Mobil 1 filters are top notch as usual. I would try that since it sounds like you already have one on hand.

If you're buying any new filters I'd recommend Fram Ultra, they have the best efficiency available and very robust construction too. The full synthetic media is has excellent flower and holding capacity as well for 15,000 miles and beyond.


Also what methods did you use to obtain the oil pressure? Do you use a scan gauge?
 
I am no genius, but if you bypass opens at 65 psi, then it looks like in between 5000-5500 RPM the oil pressure goes over 65 psi then the bypass opens. Did I just embarrass myself?
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The motor is used for HPDE / road race / street driving. I shifted the last motor at 6,000-6,200 rpm, and this motor I'll pfobably shift at 6500 and maybe leave it in gear at 6500 or so when slowing for a high speed corner. The motor will see 7,000 on occasion.

I was just trying to follow the old 10 psi per 1,000 rpm rule. So looking for 60+ at 6,000+. Is 48 psi enough?

So- if the pump is riding the bypass at 4500 on up, is the pressure loss due to cavitation or a restrictive filter? Or possibly both?

A lighter weight oil (5w-20 or similar) would delay the bypass, yes? and possibly reduce the cavitation and increase the flow capability of the filter?

Dry sump is cost prohibitive for this motor. maybe one day on a more serious race car.


What filters have the highest flow and lowest pressure differential these days?
 
1). Go with a Melling pump that has an adjustable bypass spring. I would setting the bypass higher than 65 psi for sure. Mine is set at 100 psi. I don't have to worry about oil being bypassed . My cold oil pressure at idle is about the same as yours, and hot OP is in the 72 - 80 range at speed.

2). Try Mobil 1 15w-50. A very good oil when you have high heat and /or high rpm oiling issues.

3). Put in the Mobil 1 oil filter.

4). Think about getting an oil temp gauge. More information couldn't hurt.

Z.
 
With hot oil, the restriction in the filter isn't much. Search member Jim Allen's posts and you'll see a pretty fair amount of data logging, which shows what's happening in the oiling system. Very detailed info!

Also keep in mind that either the oil filter (typically) or the block has a bypass built in to bypass the media at or above a certain pressure differential or PSID across the media. Most are in the 7-9lb range.

I too would tend to agree on the assertion that it's cavition, given the dip in pressure, then the increase afterwards.
 
my .02:

ditch the monograde. I'm a fan of HDEO so I would probably run 5w-40 hdeo syn. like rotella T6, mobil TDT, Mag1 all fleet, ect

or maybe something racing oriented like a poster above mentioned.

I doubt its your oil filter. Like a few have said I think it has more to do with your oil pump and bypass pressures.

A oil temp gauge will be beneficial as well. I think this is why you have 81 psi (in bypass) on cold start but when it warms up and in bypass you have less than 60psi since the hot oil can flow better through the bypass on the oil pump.
 
0.0025" for main and 0.002" for con rod bearing clearance?

To me that sounds excessive.

My days of blueprinting calls for con rod bearings (new, standard, with all the usual checks performed beforehand) to be somewhere between 0.0008 to 0.0015"; main bearings to be between 0.0007 to 0.0014" bearing-to-journal clearance (typically). Anything higher than these (from 0.0015" up to 0.002") are considered "worn" in my case.

The lowest I've ever done is a factory spec that calls for 0.0006 for main bearing clearance.
 
Got a bubby that Is a Mopar man who's got 5 old big block cars he had the same problem years ago you are pumping all the oil up stairs it has to much flow what he did for a quick fix was run a old pump . Go ask around for a low volume pump you could call Maurice petty chief Could fix you right up
 
I agree with "Quest" .0025 & .002 can't be correct clearances. That's double of what they should be. I can't believe the engine runs to high rpm. This engine should either knock, spin bearings, or explode with clearances like that.
 
I've built some street engines over the years and never used as much bearing clearance, but as I recall the racing crowd tends to open them up a bit with the thicker oils.
 
You fellas that think the bearing clearances are too big haven't built race motors then. Yes factory, OEM rebuilds for longevity use the .0008-.0015 specs but you build a 7000rpm race motor that tight and its gonna cook bearings quick! The B18C5 in my sig is at .002-.0025 and thats VERY loose for a honda engine but it holds 80psi oil pressure all day.

OP- I'd surely ditch the straight 30 weight oil, why even? Get a good syn 5W40 or 5W50 and try that but I'm almost certain your bypass is whats killing your oil pressure at high rpm, how can you expect anything over 65psi when you know the bypass is opening at that point?
 
Factory service manual states .0009-.0020 main, and .0007-.0023 rod. 0.002 main and 0.0025 Rod is pretty common practice in these motors.

I was running the straight 30 wt for break in. Was going to swap that out for 10w-40 conventional, then swap to 0w-40 synthetic. But, now I'm wondering if a 5w-20 should be tried to delay bypass engagement?

I have an oil temp gauge in place. The 170*F was accurate for this test. I've run a couple other temps as well, but had an additional restriction in the system at that point, so the data may not be apples to apples.
 
How much dwell time was at each of the engine speeds? I'm assuming this is data from an engine dynamometer. Oil pressure drops after 5000 rpm, but before that it is stable close to the regulated pressure.

Do you have any way of monitoring what the oil level in the sump is while the engine is at high rpm? What was the crankcase pressure? Maybe it's excessive blowby, and oil is not draining from the top of the engine.

Main bearing clearances could stand to be a bit tighter.

Ditch the 30-weight. You were only running 170F on 30-weight and oil pressure was dropping off. How low do you think the oil pressure will get when you get the temperature up to 260F on the race track? Use the Chrysler recommended 5w40, or maybe even a 10w40.

Use an oil filter with fully synthetic, wire-backed media. Fram Ultra is a good example of this.

As long as oil pressure is above 45 psi and doesn't drop off in the turns, the engine should be OK.

What happened to the old engine?
What are Chrysler's limits for bearing clearances on that engine?
 
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