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Hello guys need some help here. I changed my oil today to royal purple 20/50 with a napa gold filter. I'm not sure what oil it had before this change. The pressure with the old oil would build up to 80 psi at start up and drop to about 25 psi. It would build up as the rpms got higher. The new oil got to 80psi and dropped to 40psi as the car got warmer. I decided to take the car for a spin and got the rpms up to about 6500. It was fun until i noticed the oil pressure dropping to 20 at the light. I put it in nuetral raised the rpms to about 2000 and the oil pressure would build up again but drop just as fast. When i got home the pressure dropped completely to about 5-8 psi. What could of happened? What type of tool can i use to test the oil pressure directly from the block? Appreciate all comments!
 

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they have a manual guage that hooks up to block where the oil pressure sending unit is installed,you have to remove sending unit to install guage,you might be able to rent one or loan one at parts store (advance auto parts)hope this helps your oil pickup screen could have become clogged,does your eng. have alot of miles on it?
 

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Hello guys need some help here. I changed my oil today to royal purple 20/50 with a napa gold filter. I'm not sure what oil it had before this change. The pressure with the old oil would build up to 80 psi at start up and drop to about 25 psi. It would build up as the rpms got higher. The new oil got to 80psi and dropped to 40psi as the car got warmer. I decided to take the car for a spin and got the rpms up to about 6500. It was fun until i noticed the oil pressure dropping to 20 at the light. I put it in nuetral raised the rpms to about 2000 and the oil pressure would build up again but drop just as fast. When i got home the pressure dropped completely to about 5-8 psi. What could of happened? What type of tool can i use to test the oil pressure directly from the block? Appreciate all comments!
Hi,

Why the 20/50, in the first place? If the engine is in good or average mechanical condition, then 10-30W or 10-40W, would be the choice.
The O/P of the average engine should be about 10 lbs /1000 RPMs. Most healthy engines run about 35-40 at temps and cruising Rs. Idle 20 lbs, or so. There should be a steady rise in O/P as the Rs increase. Oil pressure (O/P) is related to bearing clearances and other normal built in restrictions within an engine. But for us home boys, it's basically, all about the wear in the rod and main bearings. The O/P doesn't create pressure, it pumps the oil. Again, O/P is created by the oil being pumped in and around the tiny clearances of the crank and rod journals. When the bearings wear or are prematurely scratched or scored the clearances get larger and the O/P goes down.
Now, the O/P gauge and sending unit can be giving false readings, in which case a known good mechanical gauge can be installed.
If you don't want to install the gauge in the interior, then, then install it in the engine compartment for occasional monitoring. This is what I did.

Hope this dribble helps.

What was the situation before you change the oil? Did you go "heavier" to overcome a previous low O/P situation?
 

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Robert gave us real psi numbers so he obviously already has a non-stock, 'real' gauge connected to the engine. We all know the stock gauge will never provide anyone with psi numbers. :gringreen

I don't know what was in Robert's engine before so hard telling what it might have done then. Was it oil from a prior owner? If so, it could have been 50% STP for all we know.

But the pressures you see now don't sound all that alarming to me. The only Ford spec was 35-55 psi at 2000 RPM; sounds like you meet that spec. (55 psi is what it should really be with new, stock parts.) Warm oil pressure is always lower than hot and low RPM lower than high RPM. Pressure should be directly proportional to engine RPM up to the pressure of the pump relief valve. At 6500 RPM you obviously don't have a stock '66 engine since only the K-code would turn that fast. Your oil pump also likely has a higher, non-stock 80 psi relief valve.

Every engine I own, including my Mercedes, goes to near zero oil pressure when hot and idling and have since the day they were each new. The Mustang gauge was so 'different' that it never displayed meaningful information so Ford never felt the urge to explain what it was showing or what you should expect.

Since there is no oil flow through any gauge, the pressure you read directly at the block will be the same as what you read at the end of a pipe leading to any gauge inside the car. If its an electric sender then its already reading the pressure 'at the block' since that is where its bolted on.

If there was something that actually changed it might be a piece of dirt in the pump relief valve that doesn't let it completely close.
 

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Hi,

Why the 20/50, in the first place? If the engine is in good or average mechanical condition, then 10-30W or 10-40W, would be the choice.
The O/P of the average engine should be about 10 lbs /1000 RPMs. Most healthy engines run about 35-40 at temps and cruising Rs. Idle 20 lbs, or so. There should be a steady rise in O/P as the Rs increase. Oil pressure (O/P) is related to bearing clearances and other normal built in restrictions within an engine. But for us home boys, it's basically, all about the wear in the rod and main bearings. The O/P doesn't create pressure, it pumps the oil. Again, O/P is created by the oil being pumped in and around the tiny clearances of the crank and rod journals. When the bearings wear or are prematurely scratched or scored the clearances get larger and the O/P goes down.
Now, the O/P gauge and sending unit can be giving false readings, in which case a known good mechanical gauge can be installed.
If you don't want to install the gauge in the interior, then, then install it in the engine compartment for occasional monitoring. This is what I did.

Hope this dribble helps.

What was the situation before you change the oil? Did you go "heavier" to overcome a previous low O/P situation?
I will have the same set up in mine. I love it.
 

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Discussion Starter #6 (Edited)
Hello Butch88,
I don't know the actual miles. The previous owner had the motor built by Martin Motion. He's an engine builder here in Houston TX. I don't know his reputation in the performance world. I'm new to the performance scene. I can tell you that the engine is a 302 bored 30 over, TFS twisted wedge aluminum heads, Victor jr intake, Roller rocker arms, F-303 cam, Forged pistons, Milodon 8 quart oil pan, Hooker long tube headers
 

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Discussion Starter #7
Royal Purple 20/50

Hi,

Why the 20/50, in the first place? If the engine is in good or average mechanical condition, then 10-30W or 10-40W, would be the choice.
The O/P of the average engine should be about 10 lbs /1000 RPMs. Most healthy engines run about 35-40 at temps and cruising Rs. Idle 20 lbs, or so. There should be a steady rise in O/P as the Rs increase. Oil pressure (O/P) is related to bearing clearances and other normal built in restrictions within an engine. But for us home boys, it's basically, all about the wear in the rod and main bearings. The O/P doesn't create pressure, it pumps the oil. Again, O/P is created by the oil being pumped in and around the tiny clearances of the crank and rod journals. When the bearings wear or are prematurely scratched or scored the clearances get larger and the O/P goes down.
Now, the O/P gauge and sending unit can be giving false readings, in which case a known good mechanical gauge can be installed.
If you don't want to install the gauge in the interior, then, then install it in the engine compartment for occasional monitoring. This is what I did.

Hope this dribble helps.

What was the situation before you change the oil? Did you go "heavier" to overcome a previous low O/P situation?


I was told to use 20/50 in hotter weather by the guy who sold me the car. I bought the car from a local mustang dealer here in Houston TX. He was given a list of mods that were done to the car. The guage i have is a mechanical guage. It was installed at the bottom of the dash. It was time for an oil change. Thanks for your information.
 

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Discussion Starter #8
Robert gave us real psi numbers so he obviously already has a non-stock, 'real' gauge connected to the engine. We all know the stock gauge will never provide anyone with psi numbers. :gringreen

I don't know what was in Robert's engine before so hard telling what it might have done then. Was it oil from a prior owner? If so, it could have been 50% STP for all we know.

But the pressures you see now don't sound all that alarming to me. The only Ford spec was 35-55 psi at 2000 RPM; sounds like you meet that spec. (55 psi is what it should really be with new, stock parts.) Warm oil pressure is always lower than hot and low RPM lower than high RPM. Pressure should be directly proportional to engine RPM up to the pressure of the pump relief valve. At 6500 RPM you obviously don't have a stock '66 engine since only the K-code would turn that fast. Your oil pump also likely has a higher, non-stock 80 psi relief valve.

Every engine I own, including my Mercedes, goes to near zero oil pressure when hot and idling and have since the day they were each new. The Mustang gauge was so 'different' that it never displayed meaningful information so Ford never felt the urge to explain what it was showing or what you should expect.

Since there is no oil flow through any gauge, the pressure you read directly at the block will be the same as what you read at the end of a pipe leading to any gauge inside the car. If its an electric sender then its already reading the pressure 'at the block' since that is where its bolted on.

If there was something that actually changed it might be a piece of dirt in the pump relief valve that doesn't let it completely close.

Thank you for the info. I turned the car on today and the pressure built up until the needle could not go no more. That was the first time it went that high at start up. As the engine got up to 180 the oil pressure dropped to 20psi. It did'nt get any lower.
 

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Thank you for the info. I turned the car on today and the pressure built up until the needle could not go no more. That was the first time it went that high at start up. As the engine got up to 180 the oil pressure dropped to 20psi. It did'nt get any lower.
Hi,
OK, 20 PSI, is not bad at idle. It's what's going on at speed. Obviously, there should be a steady increase as has been mentioned. I didn't notice, you live in the "great state" of TX, so, I understand the reasoning for running the 20/50W.
Happy Trails!
 

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In Houston 20/50 is probably fine any time of the year. What you see has nothing to do with the oil weight you are using. A lighter oil would only make the 'problem' you report worse.

As I thought your engine is not quite stock. :) Performance bearing clearances are often larger than stock which means more oil will flow through them. That is the main reason you might want a high volume oil pump. Higher flow through the bearings is mostly to keep them cool and has little to do with lubrication. But any time more oil flows the pressure at low RPM will fall more quickly so what you report doesn't surprise me at all. The fact that the pressure comes up quickly with RPM says to me that everything is working like it should.

The actual lubrication pressure inside an engine bearing has nothing to do with the gauge pressure. The gauge pressure only indicates that you are delivering a full volume of oil to the bearings. Once the oil is in the bearing the actual pressure inside as the bearings turn is thousands of times higher than what any oil pump can produce. Its that immense internal pressure, and not the pressure on your gauge, that keeps the metal parts from touching each other as they turn. It called Hydrodynamic Lubrication. The only thing super high oil pressure does is waste HP because the oil pump is much more difficult to turn.

Lubrication
 
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