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Discussion Starter · #1 ·
The oil pressure on my 71 usually runs just below the 35 on the oil pressure guage. Today I took it for a drive and the pressure only went to the mark between 0 and 35.
Is it likely I have an oil pressure problem or a guage problem.
If it is a pressure problem what would cause the drop? It has 69500 miles on it.
 

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In this order, check the sender / gauge, then actual oil pressure using a mechanical gauge, and finally, if you still have a problem, inspect the bottom end for main / rod bearing wear. With those low miles I would assume the bearings and oil pump are fine, and you probably have a failing sender.

Luckily they are only a few bucks, easy to replace, and will tell you immediately if you need to move onto the next step.

You can check the gauge simply by unplugging the sender wire while the engine is running. With it unplugged, the gauge will peg. Touch the lead to a ground, and the gauge should peg zero. That will mean the gauge is fine.
 

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Discussion Starter · #5 ·
Hooked up a after market mechanical guage to check oil pressure. 30 lbs at 2000 rpm and 20 to 25 at idle. I assume the oem gauge is not correct. Is that enough oil pressure until I tear it down this winter? What do you guys think.
 

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Hooked up a after market mechanical guage to check oil pressure. 30 lbs at 2000 rpm and 20 to 25 at idle. I assume the oem gauge is not correct. Is that enough oil pressure until I tear it down this winter? What do you guys think.

It's sufficient. Seems low, but it'll keep your engine lubricated so long as you don't do any seriously aggressive driving.
 

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I have 42 lbs. at 2000 rpm on the brain, but I don't recall the source at the moment. 30 seems low though. Time to investigate.
 

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I have 42 lbs. at 2000 rpm on the brain, but I don't recall the source at the moment. 30 seems low though. Time to investigate.

I seem to remember a rule of thumb being something to the tune of 10 lbs per 1000 RPMs. So long as you have at least that, with mild driving, you won't seize your engine. Obviously if you have low oil pressure and work the engine hard you'll have problems.
 

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I seem to remember a rule of thumb being something to the tune of 10 lbs per 1000 RPMs. So long as you have at least that, with mild driving, you won't seize your engine. Obviously if you have low oil pressure and work the engine hard you'll have problems.
D_B,

My engine is set-up to that rule, but the interpretation maybe different than what you're thinking or there may be another side to it that I hadn't heard. My understanding of the 10 per 1000 rule is on a high performance engine if the redline is 7000rpm you should have 70psig of oil pressure. At 1500, I've got 70psig. At 4000, it's 70. At hot idle, the pressure is 38. I'm getting this interpretation from 2 independent professional engine builders.

This could be a bypass problem. In any event, the pressure is low and I won't drive the car any more than necessary. Essentially, I agree with what you're saying at the end of your post.
 

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D_B,

My engine is set-up to that rule, but the interpretation maybe different than what you're thinking or there may be another side to it that I hadn't heard. My understanding of the 10 per 1000 rule is on a high performance engine if the redline is 7000rpm you should have 70psig of oil pressure. At 1500, I've got 70psig. At 4000, it's 70. At hot idle, the pressure is 38. I'm getting this interpretation from 2 independent professional engine builders.

This could be a bypass problem. In any event, the pressure is low and I won't drive the car any more than necessary. Essentially, I agree with what you're saying at the end of your post.
If you're running at 70 psi most of the time, then you're likely going over the pump relief which you really don't want to do. Heats and degrades the oil prematurely.
Melling stock pumps (which most are) come with a 60 psi and performance with a 70.
It may be wise to lower the oil viscosity so that you stay below 70 above idle.

Traditionally these engines would idle around 15 or 20 and run at rpm around 45.
If the op has lower pressures and worried, then go to a higher viscosity oil.
 

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If you're running at 70 psi most of the time, then you're likely going over the pump relief which you really don't want to do. Heats and degrades the oil prematurely.
Melling stock pumps (which most are) come with a 60 psi and performance with a 70.
It may be wise to lower the oil viscosity so that you stay below 70 above idle.

Traditionally these engines would idle around 15 or 20 and run at rpm around 45.
If the op has lower pressures and worried, then go to a higher viscosity oil.
I have to plead "Dumbass" and use the appeal to higher authority tactic. I hear what you're saying, but I'm not going to re-educate a guy who has been building engines for 30 years. I'm running 20W50 dino oil. The engine runs the way the clearances were set and the relief was specifically set up for high pressure. This engine was professionally built and I spoke to the builder about running a lighter weight oil at the time. I have another engine in the works now from a different builder...same thing. The builder's intention is to use 20W50 and a high pressure pump. Maybe this is old school. Other than changing to a composite gear on the distributor, I haven't seen any ill effects and the engine has been in the car for 7 years with plenty of 6000-6500rpm time. If this were a stock engine I would run 10W30 in it.

I'm in favor of pulling the pan and checking some bearings on the OP's engine. I wouldn't drive it unless I had to move it for some reason. You know something's not right in it. If it were an everyday car that I had to keep going because I drove it to work or it had to make it to a certain date until I junked it, I'd put the heavier oil in it.
 

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well heres my 2 cents and thats about all its worth :laughing:

i have a 78 L48 350 with about 70k miles on it , i run 10w40 its a original 4 speed and stock rear end no mods to speak of and my pressure tops out at about 40 cruising but never idles less than 20-25 and those are warm numbers i run 2 gauges i was worried it seemed a bit low so i installed a mechanical under the hood at the distributer just to have a back up.
i checked and it confirmed the factory gauge ..

it idles cold at 40 and warm at 20-25
even cold and winding the gear up a bit to shift it wont get over 45..i never hot rod it ,its never seen 4k+ rpm with me..i figure ill get all i can out of this then build one made to handle it..

but honestly i think my bypass spring is weak..every time ive ever had an engine with pressure guages on it , it ran with better oil pressure than this..but that being said normally with bearing wear ive always see really sad idle pressure when warm ..and i dont think 20-25 warm idle is really bad ...my 35-45 cruise pressure does bother me and i plan on installing a high pressure spring soon to see if that helps MR.gasket sells them for $6.00 ..so for the cost of an oil pan gasket and a spring i could potentially solve this without stirring up to many gremlins ..if it doesnt i do intend to go further with it ..id like to see 60psi once and a while ....im open to any opinions on my situation as well ..it seems to be similar to yours.
 

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20-25 at idle is fine and nothing to be worried about. Unless of course it doesnt increase when you increase rpm. The 10/1000 rule of thumb is fine for both performance and passenger motors.

Keep in mind a high pressure spring ONLY effects bypass pressure. If your motor can only create 40 psi then a high pressure spring will change nothing. The spring only regulates max pressure (i. E. The bypass presure) and will not increase pressure in a motor with excessive clearances
 

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I'm not a professional mechanic, but it seems to me that everyone is focusing on the 20-25 psig at idle, and not the fact that at 2000 rpm the pressure has only gone up 5 to 10 psig to 30. That just doesn't seem right. Maybe I'm just too conservative.
 

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I'm not a professional mechanic, but it seems to me that everyone is focusing on the 20-25 psig at idle, and not the fact that at 2000 rpm the pressure has only gone up 5 to 10 psig to 30. That just doesn't seem right. Maybe I'm just too conservative.

Just curious, do you know the part number of your oil pump?

I know we're getting a bit off track from the OP's thread, but there are differences between a high volume pump and a high pressure pump.

You may not have a pressure pump. You might have a volume pump.
 

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well heres my 2 cents and thats about all its worth :laughing:

i have a 78 L48 350 with about 70k miles on it , i run 10w40 its a original 4 speed and stock rear end no mods to speak of and my pressure tops out at about 40 cruising but never idles less than 20-25 and those are warm numbers i run 2 gauges i was worried it seemed a bit low so i installed a mechanical under the hood at the distributer just to have a back up.
i checked and it confirmed the factory gauge ..

it idles cold at 40 and warm at 20-25
even cold and winding the gear up a bit to shift it wont get over 45..i never hot rod it ,its never seen 4k+ rpm with me..i figure ill get all i can out of this then build one made to handle it..

but honestly i think my bypass spring is weak..every time ive ever had an engine with pressure guages on it , it ran with better oil pressure than this..but that being said normally with bearing wear ive always see really sad idle pressure when warm ..and i dont think 20-25 warm idle is really bad ...my 35-45 cruise pressure does bother me and i plan on installing a high pressure spring soon to see if that helps MR.gasket sells them for $6.00 ..so for the cost of an oil pan gasket and a spring i could potentially solve this without stirring up to many gremlins ..if it doesnt i do intend to go further with it ..id like to see 60psi once and a while ....im open to any opinions on my situation as well ..it seems to be similar to yours.

The fact that you highest oil pressure varies from 40 to 45 depending on the rpm, shows that a poor spring is not your problem. If you had a weak spring then the pressure would top out at the same psi under all conditions at rpm. Fred Mann's engine, 70 at 1500 and also at 4000rpm indicates it's going over relief. If he had a 150psi spring, 1500 might be 73psi and 4000 might be 95 psi.

45psi at rpm is normal for a sbc of that era. If you want more and think you might have clearance/wear issues then you can either go to heavier oil or to a higher flow oil pump.
I like using the Melling 10555 which is 25% over stock, comes with a good drive rod and is only around 60 bucks. I'm a firm believer in lots of oil and use this with a 7qt oil pan and 10w30 oil and run 60+ psi at rpm and 45 psi at idle with a 100k mile engine. I had tried 20w50 in it previously and ran over relief at spring pressure 70psi. I've also got a hefty oil slot cut for the cam gear and 2 holes shooting the timing chain.
Conversely another 350 with only 60k miles ran at 15 and 35 and switched to 20w50 and went to 20 and 45psi.
Bottom line is more wear, bigger clearances can handle heavier oil and higher flow.

You can email Melling directly to answer any engine oiling questions that pertain to pumps and pressure etc.
 

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If you're running at 70 psi most of the time, then you're likely going over the pump relief which you really don't want to do. Heats and degrades the oil prematurely.
Melling stock pumps (which most are) come with a 60 psi and performance with a 70.
It may be wise to lower the oil viscosity so that you stay below 70 above idle.

Traditionally these engines would idle around 15 or 20 and run at rpm around 45.
If the op has lower pressures and worried, then go to a higher viscosity oil.
010752,

Don't mean to hi-jack this thread.

I thought a little more about what you wrote above and what my engine is doing. I also looked up the pump part number on my bill from the engine builder and went to the Melling site based on D_B's post. The pump is a Melling M77HV high volume pump. The pump pressures vs. rpm that I stated are correct, but what I didn't think about and should have added was that at cold start-up idle (say 1200rpm), the pressure is the highest it gets and is about 75psig or so, but distinctly different than the normal hot cruise/rev pressure of 70. So this may be the maximum the engine can develop considering operating temperature, viscosity, clearances, rpm and pumped volume (not considering the bypass spring setpoint), or it may be the pressure at which the bypass opens or it may be the mechanical limit on the stock in-car gauge. I don't know which, but the fact that I get a higher reading when cold, tells me that the bypass is not opening when the oil is hot.

On a quick run-through, one would think the the oil pressure would keep building as the rpm increases. It obviously does increase to some point, but then it peaks there. It seems that the thing that keeps the pressure from continuing to rise with rpm (at pressures below the bypass setpoint) is the fact that as rpm increases, more oil is lost from the rod bearings as the centrifugal forces increase. I'm getting this from reading on the Melling site. So thinking about the rod bearing throw-off as kind of a pressure regulator, what you wrote about increasing the oil viscosity to raise the pressure at rpm makes sense since less higher visocity oil will be thrown off. Of course, it's part of the general concept of increased pressure due to flow resistence at a given volume (pumping water vs. molasses). That has the same effect as tightening up the clearances. Sorry if I'm rambling, but I never really thought or read too much about this before. I just jacked myself up one notch from dumbass.

So yeah, the OP should up the viscosity, until it's rebuild time.
 

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My pressure could very well be normal and from what ive seen on stock 350s with some miles on them from the mid late 70s now that ive done some digging, it probably is close .

My line of thinking on the spring was...most of the straight up worn out engines ive run into had some trouble holding decent pressure at idle and didnt build quick on a bit of rpm increase ...mine seems good at idle and jumps up quick with a tap of the gas but seems to stop dead around 40 give or take a few points regardless of temp ..i cant state an exact rpm/psi ..tach board is out but im thinking i should hook up a tester and check things out at 2k and 3k rpms to get some better numbers.
Most of the engines ive had that had a pressure gauge on them had been built a bit and im used to seeing the 60-70 running 30+ idling many of you have. so my first reaction was a bit of concern like the OP.

I was thinking for a $6 spring $15 gasket and a $2 chunk of clay i could check my pickup clearance...and possible clogs ..and toss in a "potentialy cheap" fix...wich may not even be a problem to be fixxed from what ive started seeing :partyon:

Anyway no danger of me hitting 5k+ rpms ..im not ready for a motor rebuild yet and ive been hit way to hard by murphy's law in the past to risk any hot rodding in what is potentially the sadest performance year the vettes had anyway...alot of potential for disaster/embarrassment there :thud: so im starting to think my pressure is ok for a while :cheers:
 

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so increasing visc of oil will "help" a bit in the pressure dept ..how thick is getting a bit too thick for an older stock motor if your trying to get the last mile out of it you can?

...at some point isnt there an issue with possible lack of oil in the first few seconds of a cold startup? or will a higher visc oil get pumped up into the works just as fast?

sorry i gotta play the idiot card on this but could use the info and im thinking im in a similar boat to the OP ..squeezing out every mile before the rebuild :rolling:
 

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...at some point isnt there an issue with possible lack of oil in the first few seconds of a cold startup? or will a higher visc oil get pumped up into the works just as fast?

With a higher visc oil the part coatings will last longer. Meaning, a thinner oil when warm runs off of the internals and out of the passages faster. A thicker oil will 'stick' around longer, giving you a slightly higher margin of protection during that initial start up as the pump starts pressurizing the system.
 
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