I put new rod bearings in the 327 mouse and experienced approximately a 10# loss of oil pressure. The new Clevite77 bearings gave me .002 in clearance while the GM bearings had .0015 in. clearance and thus I rationalized this as the reason for the drop in hot oil pressure from 40# at 2000 rpm to 30# +, although the cold pressure is 40#. So I wanted to check and make sure that indeed was the reason for the change, not trash in the pump relief spring or other causes. When we bought the car, there was silicone sealant oozing out on the inside and outside of the pan flange from an IDIOT's attempt to stop leaks. I had removed all of the silicone that I found when I installed a new gasket, but did indeed miss a little bit back in the pickup, but I don't believe it was the cause of the oil pressure drop.
At the end of the 600 mile break-in, there was no change in the hot 30# pressure, 15# plus at idle. I verified the pressure with another mechanical gauge.
I removed the pan on which I had obtained a perfect seal - no leaks in 600 miles, and removed the original GM oil pump. I then disassembled and compared it with two NOS Melling oil pumps, a standard M55 and a M55HV high volume pump.
The following are the results of the comparison.
A. Spring Length B. Spring Coil diameter C. Plug seat to casing out to out D. depth from casing to to back of plug E. Casing to pin F. Net spring compression length D minus E Notes

GM Pump A. 2-3/16" B. .0038 in. C. 2-19/32 in. D. 2-15/32 in. E. ¼ in F. 2-7/32 in. Note: Sq Seat
Melling Std A. 2-1/4" B .0038 in. C. 2-13/32 in. D. 2-9/32 in. E. ¼ in F. 2-1/32 in Note: Tapered Seat

Melling HV A. 2-1/4” Org . B. 0040 in.* C. 2-19/32 in. D. 2-15/32 in. E. ¼ in F. 2-7/32 in Note Sq Seat

Delima:

The GM pump has very little spring compression ~ 1/32 in. +/- on a square plug and square seat, no paint on the spring.

The Melling M55 Std pump has 1/8” longer spring and thus 7/32 in. spring compression on a tapered seat 1/8” diameter and square plug ¼” diameter, no paint on the spring.

The Melling M55 HV pump has 1/8” longer spring and thus 3/32 in. spring compression on a square seat and square plug and the *spring is painted orange.

Although there is little wear in the original GM pump, I prefer to replace it but may consider using it with the longer spring.

I don’t want to use the M55 HV pump with the 4 quart pan, although it would probably be alright, but it has only slightly more spring compression as the GM pump.

The M55 Std pump has the most spring compression, but I am worried the tapered seat exposes more of the flat plug to the oil pressure and thus would not improve the oil pressure. I c

M55 HV at top, M55 std in middle and original GM pump on the bottom of the picture.

Tapered seat for plug in the M55 Std pump v.s. square plug and seat in the other two pumps

I cannot tell any difference between the unpainted spring and the orange spring from the HV pump.

Melling has been of no help as the pumps are old models and I can find no pressure rating for either NOS pump.

Anyone have wisdom to share on these pumps? Thanks.

Ron

 

I can't help you with the springs, Ron. However:

The high volume capability of that pump is a non-issue relative to the pan capacity.

Oil pressure and only oil pressure causes flow out of the pan. Sure, that pump will suck up more oil, but that extra oil never leaves the pan. It simply recirculates in the sump. The only thing which will affect oil flow thru the engine is the pressure relief spring. If the pressure created by that HV pump isn't to your liking, it can fixed as easily as with a standard volume pump.

Jim

 

Regardless of the spring....it's going to take a different pump to change the low speed oil pressure if you want to increase it. The spring will determine the high end and certainly it will impact somewhat in the middle as to when it starts to open. You can put the big spring in a stock pump and high side will go there....but to increase low speed you have to either "outrun the leaks" with a larger pump or one in tighter condition...or close up the internal leaks with clearances. Or add resistance to flow with thicker oil. Or as mentioned, a HV pump with a lighter spring.

JIM

 

Thanks Gentlemen, I am still trying to get my mind around the HV pump as I have heard it pumps more oil to the top and at high RPM but I understand it needs the pressure to move the oil. I have been using 15w40 oil, but am going to try 5W30 synthetic and would expect a further pressure drop at both idle and cruising RPM ~2500. Thus with the increased rod bearing clearance I am wanting to get the pressure back up to the 40 psi range. The original pump has almost negligible wear and I considered putting the longer orange spring in it, but then again, I have the both new pumps - STD and HV.
Ron

 

Ron,
I'm not sure you even have a problem, I would blue print the GM pump and make sure the relief spring measures to GM specs and put it back together.

 

Not quite accepting what I wrote about pressure being the only thing responsible for moving oil out of the pan places you in very good company. The CF legend, MikeM, had trouble with the concept also.

Want more?

Increasing pressure by, say, 50% does NOT increase flow by 50%. The increase in flow will be only 22.5%. This is because there is a square root relationship between pressure change and flow change.

I'll show myself out.......

 

Your clearances aren't loose at all. How high does oil pressure go if you stuff your foot in it and get some RPM? 5K or so? Does it keep climbing past 40 to 50 or so? To get more pressure at low speeds it takes a bigger pump or less leaks. The 5/30 oil is going to lower it even more. Why do you want to do that if you're worried already?

The oil bypasses back into the pan. The only issue is if it's in bypass mode ALL the time when running around at lower speeds like on the highway. That will aerate oil and cause foaming. But if it keeps climbing until it finally opens at higher RPM at 60-70 psi...it's really no issue.

Old wive's tales about pumping the pan dry. Read Melling's site...they even say it. Pans get pumped dry IF you have poor return. I run loose clearances, HV pumps and a lot of oil pressure also. The Trifecta!

But for just running around...your 30-40 psi is fine. No reason to reinstall the OEM pump at this stage. I'd either just throw in the HV pump and call it a day...or spend some time on the Melling replacement to tighten the end clearance to .001 or less on the gears and stick the "Z-28" spring or spring from the HV pump in it. One thing to ck for is to make sure once assembled the bypass piston completely uncovers the port to allow oil to leave. Some have been known to only open partially and create way too high of pressure at low speeds and beyond.

JIM

 

Don't leave Jim, my need for education will never end. I am always experimenting with things to improve them using today's technology, e.g., synthetic oil. I know these old engines used straight 30 wt oil and the thinner multigrade oils seem to affect oil pressure negatively.
tbarb, where would I find the GM specifications for the original pump? I don't have equipment to determine compression strength, only basic equipment to measure diameter and length, # of coils, and wire diameter. The original GM pump has very little preload on the relief valve plug and thus I am thinking it would begin to open sooner, thus lower oil pressure at idle and highway RPM.
I am not concerned that 30# hot at 2500 RPM is a problem as I have owned many SBC that carried 30# hot as normal pressure and driven them for 100K miles or more.
I haven't really watched the oil pressure when climbing above 2500, but have noticed it will increase a bit. Now that the engine has 600 miles on the rebuild, I'll probably turn it loose once in a while if I don't get too much noise from the passenger's side.
It is easier learning from you all rather than dropping the pan again. I think I will look into the blueprint of the GM pump and also the Z-28 spring.
Thanks,
Ron

 

I think you are putting too much thought into this. Jim explained it well for you. If you want more oil pressure switching to 5-30 is the wrong thing to do.

If you want better idle Oil Pressure install the HV pump.

If you want more pressure at speed, 2500 RPM or more use a high pressure relief spring.

You have a new pump from Melling already, bolt it in and be done. Melling tells you what spring to use for the by pass and what pressure to expect with the different colored springs. As Jim already said this only affects pressure relief and most engines do not open the by pass fully till you are over 2500 RPM.

On any oil pump install it is always good to check the clearance between the pump gears and the cover plate. I have seen some brand new that had .004-.005 clearance, they also had low idle oil pressure. A little plastic gauge can quickly verify your clearance in your pump before install. If yours is loose you can quickly tighten it up by wet sanding it on a small piece of glass if you don’t have access to a machine shop at home.

If you put in the HV pump with a stronger spring and find you have 50 PSI at idle while running on 15-40W you can switch to a 5-30W if that is your choice and it will probably drop Oil Pressure by 5 psi.

The GM engineers say the engine is just fine if you have at least 7 psi at idle so I don’t follow everything they say. I like my engines to have 40 psi hot idle but that is personal opinion only.

 

I have always heard that ten pounds per thousand RPM is sufficient.

 

Thanks again gentlemen,
My confusion is the lack of information on these 40+ year old pumps which I bought back when I was racing. Melling has changed their part numbers and spring colors so it leaves me guessing.
I am proceeding with the HV pump and orange spring which came in it. The rebuild has been amazing as it used less than 1/4 pint of oil in 600 miles, even with the oil line fitting leaking a bit. Mark saved me from screwing up the cylinder walls with a hone when using moly rings - thanks again Mark.
The boss recommended that instead of synthetic oil, I use 5 quarts of Oil of Olay so I get softer skin the next time I drop the pan.

Ron

 

Ron what was the oil consumption before the rebuild/freshen up?

 

R66 mouse was using a quart of oil in 100 miles or less even with Lucas Oil Treatment. It still had 165 to 175 compression and passed the leak down test.
I did put Teflon seals on the valves as well as the O-rings, but the guides were still within GM tolerances with new intake valves. 3 angle grind on the existing seats.
Thank You again for your help.
Ron

 

Don't forget to check your clearance between the pump cover and the gears .

 

Not mentioned, the difference in HV and standard volume pumps is the width of the gears. hV is wider. With GM springs, the red/orange is the high pressure one. I’d bet Melling follows that (may even make them for GM).

 

Good.
Got the bottom end buttoned up. The blue rubber gasket is a pain. 4 passes torquing it down and the fit isn't exactly great. We'll see how it works.
Got it back together this am. I guess there is no fool like and old fool who won't leave good enough alone. I used the HV pump with the orange spring that came in it. 10W30 synthetic oil pegs the 60# gauge cold at idle. That ought to be plenty. I have a couple of 100# gauges to hook up and verify the pressure.
Ron
Update: oringinal gauge - 60#, Monit 100# gauge - 60#, SW 100# gauge - 55#. No sweat, I'm going to drive it.

 

That is normal cold start, how is your pressure with a hot engine at idle? That tells you much more. Glad you got it done.

 

I am ashamed to say I haven't driven R66 yet, yard work, planting grass, etc. Hope to drive it in the next couple of days.
I drove the 68RS to get gas for the mowers and pressure washer and it goes to 60#+ cold and 40# hot at 2000 rpm =/- with 30# idle after I put 5W30 synthetic in it. I am expecting similar results for R66 with 10W30 synthetic. I'll let you know.
Ron

 

6 hours of mowing was enough, took out R66. Cold idle 60# +, Hot idle (185 F) at 550 RPM is about 20 # after running it aggressively. Kind of surprised me it was that low. At 1500 RPM - 45 # and at 2500 RPM - 50 #. At 4500 RPM - ~55#. I don't understand when I hammer it, the oil pressure drops to about 30#, but then recovers to 55# +. I was expecting it to climb evenly with the RPM. This is with Valvoline 10W30 full synthetic oil.
With the T4 15W40 before the new bearings, the hot idle was 30# and 40# at 2500 RPM. It didn't seem to vary as much as it does now. Then after the new bearings it was 15# hot idle and 30# at 2500 RPM with the stock oil pump and T4 15W40.
I believe the larger clearance on the new bearings is responsible for a 10# or so drop and the synthetic oil also contributes to the variance.
I am satisfied.
Thanks,
Ron

 

Melling has a good video on volume vs pressure