[1200]

I finished the complete rebuild on the 302 for my CJ7 last week. Sunday I decided to let my Grandson have it to put in his Bronco. I found a 401 that is in good condition after I started the rebuild on the 302, so I decided to go in that direction. We installed the engine in the Bronco and ran it for 30 minutes, keeping the RPMs over 1500 and varying. Then we changed the oil using Castrol Dino oil. Drove the Bronco for 150 miles and again changed the oil, this time we put in Castrol Synthetic. Before changing to synthetic, the oil pressure was holding at 60. After putting the synthetic oil in, the oil pressure began holding at 40. After a few miles, the oil pressure has increased to 55. I have never heard of this happening when changing to synthetic oil. The engine was hot tanked and bored 30 over and decked. All parts in the engine are new including the cam bearings.
Any suggestions on why this happened is appreciated.

 

[338]

I run synthetic in my 5.0 HO
I run a high volume pump.
My oil pressure always (well except, when Im completely vertical or upside down

) stays between 50 and 75 depending on engine temperature and rpm.

 

[GeeAea]

In reply to:

After a few miles, the oil pressure has increased to 55. I have never heard of this happening when changing to synthetic oil.

[/ QUOTE ]

..and I doubt that the apparent increase has anything to do with the type of oil. That is, I would assert that this same increase (or to some proportion) would occur with the dino as well. You haven't placed the dino oil under the same conditions ..so you don't know.

As to why it would increase at all while traveling at the same speed and at the same relative temperature (or so I gathered)? Got me swinging

Oil doesn't gain viscosity at temperature ...at least in short term use ( it does if it's shot and has already done some evaporation/volitilization) ..and not during a road test.

In reply to:

I run a high volume pump.

[/ QUOTE ]

Finally ...someone besides me that sees the sense to having one. Never can get too much of a good thing (oil flow).

 

[338]

Well a high volume pump can rob some HP at higher RPM's.
But I already make 300 HP and that is plenty for what I need my mill to do.
But since I need my engine to live well at 400-1800 rpms, under heavy load, with little to no air flow around the engine, in 100* + heat, I definitely see an advantage to a little, above average oil flow.

 

[Olin18]

This is just a swing in the dark, but maybe you were seeing oil pressure increase because the oil passages were just bleeding themselfs out. Perhaps there was some air in the system for the first few miles. And as the oil and motor warmed, and the oil flowed the air probably worked itself out. Air (unlike oil) is compressable. Does it still happen or is everything back to normal now?

 

[lindy80cj7]

Are you using a calibrated gage? Could be error there.

 

[1200]

I talked to the machine shop owner that did the machine work on the engine. For the most part he builds high performance engines for straight track people. He said that the synthetic oil causes less friction and thus less oil pressure.
As a side note, he is currently building a Mopar wedge and a hemi. I haven't seen either of those engines in many years, especially the wedge.

 

[GeeAea]

In reply to:

He said that the synthetic oil causes less friction and thus less oil pressure.

[/ QUOTE ]

That's a fine statement ...somewhat valid (conditions and restrictioins apply)....but he could have just as well said that bears $hit in the woods and it would have been just as good at explaining an INCREASE in oil pressure at operating temperature while traveling down the road (as I think you described).

 

[1200]

GA, I guess I should make it clear what happened with the oil pressure.
1. First start up after install: engine holding 60lbs idleing until warmed up, then decreased to 40lbs at idle. Changed oil before driving, still using dino oil. Pressure 60lbs while driving on highway and 40 at idle.
2. After change to synthetic oil, pressure dropped as stated and then began to climb back up as vehicle was driven for several miles. My question to the machinist was why the decrease in oil pressure. The machinist is out of the shop for a few days, so I have not had a chance to ask him why the pressure rose after driving for a few miles. which had not happened when I last spoke to him. That was my question to the engine experts on this board. Why did the oil pressure increase back to almost it's high of 60 lbs. I was concerned that something was going on inside the engine. The oil from both oil changes appeared to be perfectly clean and the magnetic drain plug had no netal on it. Today I opened both filters that we removed and they too appear not to have anything in them other than oil and filter material.
Thanks for your input.

 

[GeeAea]

Sorry

..didn't mean to ...well, alright ..I did

No two ways about it ..that's a strange occurance. The obvious thing is that you've got a 60lb pressure relief valve. That appears (again if I read correctly) to be your limit on oil pressure cold ...and at upper engine speeds (warm). Synths tend to flow better cold than dino ..but the gap is pretty thin between two oils of like weight (5w-30 synth and 5w-30 dino) unless you're in extreme cold. At moderate temps, I doubt there would be any perceived difference at any engine temperature or speed. That is, many dinos have advanced so far that unless you're looking for extended drains ..the cost effectivness of synths is questionable (I'm a synth user).
Here's a site where you can find out just how your two oil compare head to head for viscosity @ temps.

If I were forced to guess ..I'd say that there was some fitment or seating issure that was resolved with driving.

 

[1200]

Yes, you did, but I decided not to go there as I am looking for insight on what may or not be a problem with the engine. The machinist will be back early next week and I will talk with him.