oil pressure pt. 33

[keelue]

proud owner of the ugliest k5 in arizona! i've owned plenty of muscle cars, primarily, ahem...mopars, but always had a soft spot for classic chevy trucks and the greatest small block ever built. i've had a '77 blazer for almost a year, my first true off road toy.

replaced or rebuilt most of the essentials and just had a 350 put in. which leads me to my question...

what should the oil pressure be in a freshly rebuilt mid 70's 350?

the block in it was shot, though still running. i had a cheap but fairly reputable shop replace it. they said they replaced it with a four bolt from a '77 suburban, oil pump to manifold. is there an easy way to indentify the block? they put a pretty big cam in it which i wasn't thrilled about but it seems like a solid build. no leaks, rattles, or squeeks and the power is defintely there.

i get 30 lbs of pressure cold. at warm idle i hover around 20. there's about 250 miles since the rebuild and i haven't changed the oil. i was expecting more like 30 - 40 lbs of pressure though the shop says it's cool. what do you bowtie guys think?

thanks!

 

[4X4HIGH]

I've been building engines professionally for 22+ years and every SBC i've built had 50-60psi at idle cold or hot. My current engine in my rig stays right around 65 lbs all the time and that is with a stock pump.

 

[82355]

Seems low for a fresh engine. That being said, you don't need more than 10 psi per 1,000 RPM.

Martin

 

[TerryD]

Probably loose bearing clearance. Alot of people don't take the time to check it. 20-30lbs oil pressure in a SBC is fine. Oil is moving, and that's good. If you're not turning it over 5000-5500 RPM you won't have any problem at all. If you were sustaining 6000RPM+ for long periods of time, I'd probably look into it, but not for just between the shifts. My 7000RPM 350 only has about 12-15PSI at an idle, but I get about 50-55 when its wound up. The oil pump in that motor came factory on the block in 1976 and the only thing we've done was pull it apart and check to make sure the clearances in it were still within spec.

There could be a few other things that they missed that's keeping the pressure lower, like cam bearing lube hole placement or the plug in the back of the block near the rear main bearing, but again, I wouldn't concern myself with it.

 

[keelue]

yeah the 10 lbs per 1000 rpm rule. thing is i get 20 at 1k, 2k, 3k, after that it starts climbing but never gets much above 25psi. strange...thanks for the help though guys.

been trying to baby the enging during break in but had to go get the tires wet last friday

http://www.youtube.com/watch?v=3tQRWtnC3xo

 

[djtejas]

I've been building engines professionally for 22+ years and every SBC i've built had 50-60psi at idle cold or hot. My current engine in my rig stays right around 65 lbs all the time and that is with a stock pump.

i've built 3 motors and have always had about 40 psi cold and around 35 hot at idle...run around 55-60 at higher rpms...also with stock pumps

 

[chulisohombre]

im with the other guys on this one.my gm crate motor ('84 block) still puts 35-40 psi cold at start and about 25 to 30 warm.new rebuild id like to see a lil more than what you are getin.if you arent running it hard,it shouldnt have a problem.but at high rpm the top end is gonna have issues with not enough oil as everyone else has said.is your guage accurate is the next thing to maybe look at.and also is it a mechanical or electrical sending unit guage.may be something wrong there also.you can pull the valve covers and make sure there is enough oil coming up to the rockers.at least you will hve a better idea if there is low pressure all the way up top from there.every time the rocker moves it should have a pretty good squirt up through the lifter at idle to keep the top end happy.just a couple things to think about or try.

 

[doonjumper]

Bad gauge

 

[TerryD]

but at high rpm the top end is gonna have issues with not enough oil as everyone else has said.

The top end of a SBC or BBC always gets enough oil, rather the opposite is true. High volume oil pumps will flood the top end, starving the bottom end and not keeping enough oil in the sump to allow it to cool. If you are running a mechanical lifter cam, you can even put restrictors in the oil passageways that feed them to help maintain more stable pressure going to the bottom end. What the higher oil pressure is for is keeping the rod bearings and main bearings from slamming into the bearing journals when the higher piston speed when turning them hard.

You have all that mass moving in one direction and it suddenly stops at the top and bottom of the cylinder. Low pressure is rumored to be over powered by the movement of the piston/rod and squish the oil out, allowing the bearing to make physical contact with the journal.

I've never had any problems with our engines hurting bearings, but we've never built and engine that sustains over 6000RPM for more than the time it takes a transmission to shift. Swapping a high pressure spring into a stock pump should more than handle that though.