Oil Pump Pressure Relief spring

[Donald H.]

Re: Oil Pump Pressure Relief spring

Still trying to figure this oil pump stuff out.

I managed to get a gm standard pressure spring #3814903 from a Chevy dealer (picture 1), and it came in an unopened bag.


I was talking to a fellow Mid-Atlantic Chapter member and he said he had a setup to test the pump. It is a rear main bearing cap with a oil pressure gauge attached at the oil passage. Picture 2 is the setup with my pump attached and the standard pressure spring installed. Picture 3 is a jig I build to hold everything instead of hand holding it.

The drill I used is rated at 3,000 RPM at the highest speed, and since it would be equivalent to distributor RPM, at the highest speed it is 6,000 crank RPMs. Unless I'm confused!

The oil I am using for the test is Rotella T4 15W-40.

So when I spin up the pump with the drill, it goes to 60 pounds and sits there. I have been assuming that with a 45 pound pressure relief spring, that the pressure would not go above 45 pounds!

The oil is obviously cold, so with this cold higher viscosity oil will the pump push pressure above the supposed 45 pound pressure relief of the

gm standard pressure spring #3814903?

I'm still confused!!!!

Thanks,
Don

[Joseph L.]

Re: Oil Pump Pressure Relief spring

I have the correct spring in my Melling standard volume pump. At startup, I get 45+ psi. Then when warm at 2800 RPM cruising, it is 30+. I have not tried a different gauge to check this 62 year old gauge.
Your setup does not take into account oil temperature and bearing clearance. All the oil flow that is not bypassed by the spring is sent to the gauge in your setup.

Be very careful if you think about heating the oil. Measure temperature and do not go too high. Motor oils have a flash point of about 400F. I was at a site where they were testing hydraulic cylinders with heated oil. The oil was a standard flash temperature and the enclosed test chamber was heated to above the flash point. Yes, there was a fire inside the chamber until the oxygen was used up. Oil does burn.

Joe

[Donald H.]

Re: Oil Pump Pressure Relief spring

I'm going to call Melling Tech line on Monday and see what they have to say.

[Timothy B.]

Re: Oil Pump Pressure Relief spring

Don,

So what you have is a Melling oil pump body with a GM spring, correct?

I would be curious what a stock GM pump would read dead headed like that, I think approx. 45psi.

Something in the cap has to be set not exactly like the GM pump but I can't add anything to your findings, 45psi is 45psi and 60 psi is 60psi.

[Duke W.]

Re: Oil Pump Pressure Relief spring

Still trying to figure this oil pump stuff out.


The drill I used is rated at 3,000 RPM at the highest speed, and since it would be equivalent to distributor RPM, at the highest speed it is 6,000 crank RPMs. Unless I'm confused!

The oil I am using for the test is Rotella T4 15W-40.

So when I spin up the pump with the drill, it goes to 60 pounds and sits there. I have been assuming that with a 45 pound pressure relief spring, that the pressure would not go above 45 pounds!

The oil is obviously cold, so with this cold higher viscosity oil will the pump push pressure above the supposed 45 pound pressure relief of the

gm standard pressure spring #3814903?

I'm still confused!!!!

Thanks,
Don

The GM spec is 40-45 psi at 2000 engine revs, hot. The pump is driven 1:1 by the camshaft, so 1000 pump RPM is 2000 engine revs. Hot means oil temperature of say at least 150F and up to 200+.

So a 15W-40 engine oil at room temperature is going to show higher pressure, especially I you rev the pump over 1000.

You might try some 0W-20 or a very light hydraulic oil or maybe something like vegetable oil, and if it shows 40-45 at 1000 pump RPM, it's good to go and no need to rev the pump to over 1500.

Duke

[Donald H.]

Re: Oil Pump Pressure Relief spring

The GM spec is 40-45 psi at 2000 engine revs, hot. The pump is driven 1:1 by the camshaft, so 1000 pump RPM is 2000 engine revs. Hot means oil temperature of say at least 150F and up to 200+.

So a 15W-40 engine oil at room temperature is going to show higher pressure, especially I you rev the pump over 1000.

You might try some 0W-20 or a very light hydraulic oil or maybe something like vegetable oil, and if it shows 40-45 at 1000 pump RPM, it's good to go and no need to rev the pump to over 1500.

Duke

OK, this helps a lot. I switched my drill to low speed setting and that is 1,100 RPM. Close to the 1,000 pump RMP. I still get the near 60# reading with the cold 15W-40 oil. Just for grins, I switched the spring to the Melling green spring (per melling specs this is 49#). Pretty much the same as with the GM spring.

What was confusing me is that if a spring is rated to release at a certain pressure, e.g. 45#, then the pressure wouldn't go above 45# hot or cold. But the more I think about it, it makes sense that cold thick oil and higher RPM could override the pressure relief circuit and continue to build pressure.

Thanks,

Don

[Jeffrey S.]

Re: Oil Pump Pressure Relief spring

I don't know if this is relevant to this discussion but in 2013 when I had my '69 engine rebuilt (with much help and advise from Duke), the builder installed a Melling M55NV and simply switched the Melling spring with the original stock spring from the old pump. Pressure is always about 45# cold and goes down to around #30 when hot at speed and somewhat lower at idle.
Jeff

[Joe L.]

Re: Oil Pump Pressure Relief spring

All------


A standard pressure GM spring in a GM oil pump is going to produce the oil pressure that GM specifies for that engine assuming that the engine is stock and not excessively worn. Period. Of course, the pressure will be within whatever tolerance range the GM specs call for. I'm sure the specs don't call for the pump to produce, for example, a pressure of 45.00 PSI.

A standard pressure GM spring in a non-GM oil pump may or may not produce the GM specified oil pressure for that engine. There's no way I know of to determine if the non-GM oil pump is in every way identical to the GM pump. Some might say, "the GM oil pump is made by Melling". That may be true in some cases. However, that does not necessarily mean that Melling makes the oil pumps sold under their own brand the same as they make the ones they supply to GM. GM may have certain specs that Melling must meet and Melling may have reasons they do not use these specs for Melling-branded pumps. As a matter of fact, oil pumps that Melling makes for the automakers including GM may not even be manufactured in the same factories as those they make under their own brand for the aftermarket.

We do know that the standard pressure spring that GM offers is, based on appearance, not the same as the equivalent Melling spring. So, that's at least one component of the pumps that is not the same. There may be others as well.

The oil pumps of old were definitely manufactured internally by GM. I believe they were manufactured at the Bay Cities, Michigan engine components plant. I believe that plant is still operating but they very likely don't make any SERVICE oil pumps for old applications and probably have not made any for a VERY long time.

[Page C.]

Re: Oil Pump Pressure Relief spring

Thought I would add this to the oil pump pressure thread. Started my 327/365 for the final time prior to putting the body back on the rolling chassis today. This engine had been run before and the cam break-in was done months ago. This engine has a GM oil pump with the GM#3848911 high pressure spring. Today the temperature was about 60 degrees. As was the coolant and Rotella 15w/40 oil. On start-up the engine was running at 1100 RPMS and the oil pressure was 80 psi. The oil pressure dropped to 66 psi when the engine reached 160 degrees and dropped to 56-58 psi when the engine temperature reached 180 degrees. All of these temperatures were at 1100 RPMS. After about 20 minutes, the oil pressure was steady 56 psi. This looks like about a 24 psi drop in oil pressure due to the oil getting up to operating temperature. The results might be a little less on the 327/250-300 engines.

[Dan D.]

Re: Oil Pump Pressure Relief spring

Page, that kind of pressure can blow the seals out of the FI distributor. You need to change that spring.

Lots of good info on this thread. I have been following it closely, as I have a similar setup.

-Dan-

[Joe L.]

Re: Oil Pump Pressure Relief spring

Page, that kind of pressure can blow the seals out of the FI distributor. You need to change that spring.

Lots of good info on this thread. I have been following it closely, as I have a similar setup.

-Dan-

Dan------


A GM pump with the 3848911 spring is exactly what the L-76 and L-84 engines were originally equipped with. So, whatever pressure it produces now is what it produced way back then. If it was OK then, it should be OK now.

[Dan D.]

Re: Oil Pump Pressure Relief spring

Joe, the OP said the engine was rebuilt with a high pressure pump. It did not come from the factory that way.

I would like John DeGregory to chime in on this, if he would. He is the FI expert, and I would take his word as the gospel.

-Dan-

[Joe L.]

Re: Oil Pump Pressure Relief spring

Joe, the OP said the engine was rebuilt with a high pressure pump. It did not come from the factory that way.

I would like John DeGregory to chime in on this, if he would. He is the FI expert, and I would take his word as the gospel.

-Dan-

Dan------


A GM "high pressure pump" and a "standard pressure pump" are exactly the same except for the pressure relief spring. All 1964-65 L-76 and L-84 were originally equipped with a "high pressure pump". If anyone tells you anything else, they are wrong.

A "high volume pump" is something different. No PRODUCTION small block was ever originally equipped with a "high volume pump" and I do not recommend them for any STREET small block.

[Dan D.]

Re: Oil Pump Pressure Relief spring

Yea Joe, the OP did say "High Pressure Relief Spring". But that is not the way the factory configured these engines. So the result is the same - too much pressure.

p.s. Did not intend to offend you in any way.

-Dan-

[Joe L.]

Re: Oil Pump Pressure Relief spring

Yea Joe, the OP did say "High Pressure Relief Spring". But that is not the way the factory configured these engines.

-Dan-

Dan------


If by "high pressure" you mean these engines were not originally equipped with a pump using a GM #3848911 relief spring, wanna bet? Be advised that I'm not a betting person. If I do bet, I only bet on sure things.