smallblock oil pressure question

[Alex Hayley]

I have a new 406 that I built and installed a few months ago. It is running well but I am troubled by the oil pressure I am seeing. The oil pump is a typical Melling M55 (standard pressure and volume). The engine runs between 170 and 185 degrees. On start up I have about 70psi at idle and 80psi when driving cold. When the engine warms up I have about 45psi to 50psi when driving. When I get on it, the pressure pops up to 60psi or more. However, I have about 20psi to 25psi at idle (950 rpm) in gear and 30psi out of gear (1100 rpm). I am running 20w50 with some Lucas Oil Stabilizer. During the build I measured the main bearing clearance with plastigauge at about 0.0030" which is just on the loose side, right? Should I be concerned with the oil pressure at idle?

[Jacfourteen]

sounds about right, most say that as long as you have 10 psi at idle and gain 10 more psi per 1000 rpm you're good. My 355 has new bearings and crank(prolly about 5000 miles), high volume oil pump, and I run 10w40 oil. It runs 60 psi at idle when cold, 40 psi when warm, and any time I'm above 1500 rpm it is at 60 psi. I ran about 70-80 psi when I used 20w50 oil before, but I thought that was a little excessive so I went thinner.

[Blazer1970]

Your oil pressure is fine, but I would also run a thinner oil (10W40). No reason to burn HP pumping that thick stuff.

[Alex Hayley]

Thanks for the advise. I would like to be up above 30 and closer to 40 on a warm idle. I did run 10w-40 but was consistently at 20 on a warm idle whether in gear or not.

When I checked the main clearances during the build with plastigauge I might have been closer to 0.0035". Not knowing at what rate oil pressure will drop as clearance increases, would a clearance of 0.0035" be excessively loose?

I would have like to check the main clearance with a micrometer and a bore gauge but lacked a bore gauge. I did have the tools to properly check the rod bearing clearance and had a consistent 0.0025" clearance.

[bradh]

That's what mine runs. I have about 500 miles on a new rebuild. I'm using 10w30 with a Melling M155 pump. It's the same as yours but with a 3/4" pickup.

[Alex Hayley]

Do you, by chance, know your main clearances?

Btw, you are getting the same oil pressures with a much thinner oil.

[BarryB]

your main clearances are a little loose. Your pressure is fine. The typical saying is 10 psi for every 1000 rpms. If it really bothers you, about the clearance, you can get .001 bearings to tighten up the clearances.
Did you have machine work done, crank turned and etc??

[bradh]

.002 on both the rods and mains. I'm running full synthetic 10w30. I have a newer 350 with a roller cam so I don't have to worry about oil for a flat tappet cam, I use the same oil my DD takes.

You could always drop the pan and throw a Z28 spring in the oil pump if it bothers you. If you are at .003 or even .0035 on the mains I would not worry. 20 psi at a warm idle, 40-50 driving and 60-70 wound up a little is fine.

[Alex Hayley]

I had all machine work (bored, honed, crank turned/polished, balanced) done except for align bored. The mains were quite straight and the crank turns easily when torqued.

Would a different spring in the pump really make a difference at a warm idle? I thought that spring only bled off excessive oil pressure which is not a problem at idle. I did change the spring, however, to a pink one if I remember correctly. I believe that is a stiffer spring. Correct me if I am wrong.

I am running a roller cam, lifters and rockers so I am not concerned about oil pressure up top.

I did read an article about mixing main bearings to get the desired clearance. I thought instead of ripping the entire engine apart I could get a set of undersize bearings and put only one shell in the main cap. Just tossing that out there. Any comments on that thought?

http://www.carcraft.com/techarticles...ance_tips.html

[llowlife]

"Usually" rod side clearances will bleed off more oil than main bearing clearances. 35 tenths is on the loose side but 10 psi is fine. As long as you have it up a little bit by 1500 or so. I am running about 10-15 psi at idle and have no intention of pulling the motor for a pump. I run a reworked melling standard pressure /volume with a "Z 28" spring from GM. I can get the number if you need it but don't have it memorized.

If you plan on measuring the bearings again find someone with the tools, plastigage is okay for a recheck but I wouldn't rely on it for 100 accuracy. How were you able to check the rods but not the mains with tools?

[BADPURPLE70CHEVY]

Originally Posted by Alex Hayley

I have a new 406 that I built and installed a few months ago. It is running well but I am troubled by the oil pressure I am seeing. The oil pump is a typical Melling M55 (standard pressure and volume). The engine runs between 170 and 185 degrees. On start up I have about 70psi at idle and 80psi when driving cold. When the engine warms up I have about 45psi to 50psi when driving. When I get on it, the pressure pops up to 60psi or more. However, I have about 20psi to 25psi at idle (950 rpm) in gear and 30psi out of gear (1100 rpm). I am running 20w50 with some Lucas Oil Stabilizer. During the build I measured the main bearing clearance with plastigauge at about 0.0030" which is just on the loose side, right? Should I be concerned with the oil pressure at idle?

Hey, This is funny because i just asked this same question about a month ago. I also have a heavily built 406 that runs the about the same pressure you describe. No reason to worry. My 406 only has 30 or so miles on it. Where you at in n.c?

[Alex Hayley]

Originally Posted by llowlife

"Usually" rod side clearances will bleed off more oil than main bearing clearances.

I just happen to have the side clearance number. 0.018". Is that good? I thought it was when I built the engine.

Originally Posted by llowlife

If you plan on measuring the bearings again find someone with the tools, plastigage is okay for a recheck but I wouldn't rely on it for 100 accuracy. How were you able to check the rods but not the mains with tools?

I checked the crank journals with a micrometer and the rods with an inside micrometer with them out of the engine but torqued. I just cannot remember why I did not check the mains with the inside micrometer. There must have been a reason but it was a while ago. Regardless, a stupid oversight. Perhaps the inside micrometer did not fit properly.

Also, in all of my measurements I did not check at 90 degrees to the first measurement, nor did I check for taper.

[Alex Hayley]

Originally Posted by BADPURPLE70CHEVY

Hey, This is funny because i just asked this same question about a month ago. I also have a heavily built 406 that runs the about the same pressure you describe. No reason to worry. My 406 only has 30 or so miles on it. Where you at in n.c?

Yes. I think I saw your thread about it. From all of the responses...which have been great btw...I see a common opinion of "the pressure is fine". I just want that pressure or better WITH A LOWER VISCOSITY OIL.

[Alex Hayley]

Originally Posted by BADPURPLE70CHEVY

Hey, This is funny because i just asked this same question about a month ago. I also have a heavily built 406 that runs the about the same pressure you describe. No reason to worry. My 406 only has 30 or so miles on it. Where you at in n.c?

Btw, I am near Raleigh.

[69 longhorn]

The clearances on your mains are a little on the high side(figure about 1 tho per 1" of journal on something that you plan to rev). Still I agree, that the side clearance on the rods tend to bleed the pressure . stock min specs on side clearances are about .0025 per side(.010 per pair). I have opened them up on a perf motor, but usually run a big pump. From the oil pressure that you state, I dont think you have any big issues(it takes 10-15 psi to keep the lifters working @ idle, then 10 psi per every 1000 rpms you want to run). best of fun,crazyAl

[Alex Hayley]

Some more good advise. Thanks. Is there a max rod side clearance measurement? Is 0.018 too much or just double the minimum? Is there anyway to tighten up the rod side clearance?

I am running roller everything up top with mechanical lifters so I am not too concerned with oil up top. In fact, I have oil restrictors to reduce the oil getting to the top of the engine.

Has anyone tried mixing the main bearings as described in http://www.carcraft.com/techarticles...ance_tips.html ?

[69 longhorn]

How much is too much, I cant really say. I ran .016-.018 on a BB Olds , & it didnt cause any problems. As far as tightening it up, most likely the rods would need replaced(as a rule, the rods are narrowed to gain the clearance). You might talk with some of the chevy racers....all my chevys were street run, & didnt see the back side of 6500 too often. crazyAl

[akdmax1]

I believe the old rule of thumb is 10lbs oil pressure per 1000rpm. sounds like your just fine to me.

[Jacfourteen]

Changing the spring won't affect pressure at idle, it will just make the pump into a high pressure pump (raise the max pressure at higher rpms). If you want to up the pressure at idle you either need to change to a higher displacement (high volume) pump, or tighten clearances. If I were to drop the pan I think I would change out the pump to high volume, not just the spring. I know how you feel about the lower pressure at idle, it just doesn't feel right because all of the newer cars run steady on oil pressure.

[Blazer1970]

20 psi at idle is PLENTY of oil pressure. Quit worrying about it.

[Alex Hayley]

Thanks for all the advise. The advise has put my mind to rest somewhat. When I do pull the engine next time, which I need for some clean up work, I am going to try to tighten the clearances a bit using the mixing of main bearings. Here is another link to that:

http://www.mahleclevite.com/it_bearinginstall.asp

Thanks again!!!

[llowlife]

For all practical purposes You can only loosen the clearances on rod sides. Built racing Small blocks for over 30 years and I usually have run 015-020 with no issues. On my current 388 I have .022 on most of the side clearances and they are all new Scat crank and rods. You'll be fine. Make sure when you check your clearances that you check 90 degrees from the parting line. Bearings are slightly thinner at the parting line to account for lack of a better word "hammering". I have never used a high volume pump just a reworked standard pump. Our stock cars used gen 1 motors to over 7k on occasion. With our rules the heads would run out of air about then so n need to turn it past that at the time. You can use a BB pump too but why pay for Something you don't need. (HP loss).

I did not read the articles but I believe you are talking about using a slightly over or under bearing half with for instance a "standard bearing" this is fine and I have done it for years as long as its the same bearing mfg.

[69 longhorn]

Just to add a bit to the last post....yes , all of the bearings I have used have more clearance @ the parting line. Many yrs ago , I was told that as you rev the motor up, the rods tend to stretch & will suck the centers in. You dont want to wipe the 0il off the journals, & thats why the bearings are not perfectly round....they will "pull " round when you rev em up crazyAl