oil pressure expectations....

[emerygt350]

Just put a mechanical oil pressure gauge on my gt350... My gauge was doing crazy things and I just needed to know what was actually going on. I have replaced the IVR and have a new sending unit on its way (plus a nice T so I can keep the mechanical too).

So here are my numbers... Pegged to 60 when cold. 43 psi at hot idle (550 rpm). 53 psi hot at cruising speed (1800 rpm) and pegs to 60 at any rpm over 2500. After a high speed jaunt on the interstate (70, 2100 rpmish, 55 or so psi) for 6 miles the idle fell down to 38 psi (550 rpm) but after a 5 minute slow cruise at 40 on a side road the idle pressure was back at 43 psi. mobil 1 15w50.

Cooling system is running beautifully, needle never moves from between the N and O, and after a extended 'hot' run on a 93 degree day it was still sitting at 180 using a laser temp gauge on the thermostat housing.

Should these all be just pegged with a brand new oil pump on a rebuild or does this look ok?

[FB71]

oil is way too thick for the clearances. Switch to 10W-30 and reevaluate. Hot idle should be roughly 10psi. Typical pressure is about 10psi per 1000 RPM, peaking about 50-60psi.

[emerygt350]

That is kind of what I was thinking but was not sure... My old sending unit was so whacky I was scared to go lower. Now I see it was on N at 43 pounds.

[ashley roachclip]

I will second what Fb71 said .

[Ourobos]

Agreed on a 10w30 - Unless this is an engine a machine shop just built for you, then go with what they recommend for clearances.

[emerygt350]

It is an engine a shop just built for me...

[gt4494]

Rule of thumb used to be 10lbs per 1000rpm, anything more than that was acceptable.

I have pegged gauges at stone cold but as soon as the engine warms up pressures come down.

[emerygt350]

I just put in 40 weight. Just couldn't go straight to thirty. Drove 50 miles at 70 to the Dyno and the idle pressure is now at 31 psi, cruising at 2200 I get 51. Looks like I can drop to 30 weight...

[emerygt350]

Dyno guy says "what's wrong with oil pressure? Stick with the 40"

[Matt J]

Having good oil pressure isn't a problem, but if the pressure is high because it's too thick and isn't flowing properly through the engine, then that's bad. These engines were built to run on conventional 10-30 (with zinc if it's a flat tappet). If it's something that's really built up and has tighter tolerances than stock, I'd think you probably would need a more modern oil viscosity (as in thinner, not thicker), but I'd definitely defer to the engine builder on that, as they know what they put in there. Also, higher pressure will put more stress on seals and other things if they're meant to run at peak pressures of 60 pounds max when the engine is flat out.

[emerygt350]

yeah, the builders had me put 40 in it so I will check with them again for an opinion. Pressure just bleeds off at 60 anyway so it isn't like I am hitting it with more than 60 pounds.

[FB71]

don't run single weight oil in varying conditions. Single weight oil is best used in competition conditions, where the engine is brought up to temp prior to being loaded. Multi weight oils are best for casual driving, regardless of the machine shop says. The purpose of a multi weight is to allow improved oil flow, due to the lower viscosity, at lower engine temps. Oil that is too thick will not flow or lubricate properly. Here's the analogy I use with my tech students; Oil is like liquid ball bearings, just like the old Pennzoil commercial says. The weight of the oil is closely linked to size of the oil particles, or clumps of molecules. Imagine you have two sheets of plywood lying on the floor. One stays on the floor, while the other can be raised up to 2-3 inches. That 2-3 inches is relative to the 2-3 thousandths of an inch oil clearance in the bearings. So, you have two sheets of plywood that can touch, or be separated by a few inches, if something were in between them. Now, imagine a five-gallon bucket of ping pong balls. Spread the sheets, and pour the bucket of balls between the sheets. Now stand on the top sheet. What happens? You can slide the sheet back and forth with minimum friction, and while the balls will gradually drop out, they can be replenished easily and quickly by a pump recirculating the balls that drop out, at a rate just slightly faster than they drop out. That's the correct weight oil. Now if the oil is too thick; Same sheets of plywood, same conditions, but you have a net full of volley balls. Can you get the volley balls between the sheets? Actually yes... but, you need to deflate and flatten them. Now, stuff them between the sheets, and stand on the top sheet. Does it slide back and forth easily? Nope... tons of friction. But wait, there's oil between the surfaces, how can there be friction? Because the material put between the sheets has to be deformed to fit. Also, the volley balls do not drop out from between the sheets easily or quickly. However, you still have a supply of balls attempting to be pushed into the gap. Since they won't go, they just keep getting squeezed. Eventually, they just grind against each other, creating unnecessary heat and power loss. They're also harder to pick up by the pump, which increases the torque needed to turn the pump. So, by running an oil that's too thick, you're starving the engine for proper lubrication, overheating the oil, shearing it apart (which breaks it down so much more quickly), dragging power from the crank with excess resistance, and straining the oil pump drive shaft, possibly twisting or breaking it. Engine oil pressure, and oil viscosity, is definitely a case of "if some is good, more is NOT better".

[emerygt350]

Oh no, I am not using a single weight, just that the cold operation isn't really the issue here so I didn't bother stating the 10w part.

[FB71]

ok, cool. Just to be clear, my analogy applies to multi weight oils, too.

[emerygt350]

I may try a 10w30 after this oil is 'over' but I don't want to do additives for zddp and the 0w40 mobil1 has the zinc levels I like for the flat tappet... and the price is right.

[Ourobos]

Go with what the machinist says, not ANYONE on here.. Cause if there is a problem, WE won't be warrantying your engine..

[emerygt350]

hah...

[FB71]

Except I teach the techs and machinists...

[emerygt350]

I am pretty sure there is no warranty, their customers race these engines and I doubt anyone warrantees that kind of abuse.

[Ourobos]

Except I teach the techs and machinists...

Oh you set the bearing clearances up on this engine? What were they? What oil should he run?

:rollseyes:

[FB71]

Oh you set the bearing clearances up on this engine? What were they? What oil should he run?

:rollseyes:

no, smart guy, obviously not. However, given his oil pressure readings, I can tell the clearances are around stock, which is 2-3 thousandths, not the 4-5 usually used for high volume pumps or high viscosity oil. My point is that regardless of what the machine shop may or may not have told him, it was obvious the 15w-50 oil was way too thick. I've also encountered numerous machine shops around me that really have no clue what they're talking about when it comes to oil viscosity. On the opposite end of the spectrum, there are two machinists around Baltimore that I learn from, because these guys are absolutely astounding with their knowledge. So, my advice was to use caution, not to blindly follow the instructions of a shop that may not make the best recommendations.

[emerygt350]

Settle down you two

[FB71]

[emerygt350]

They had me on 15w40 rotella on break in and all they suggested was keep the zinc up and move to synthetic after 3k. It is a high volume pump they put in, new crank etc.

[Ourobos]

no, smart guy, obviously not. However, given his oil pressure readings, I can tell the clearances are around stock, which is 2-3 thousandths, not the 4-5 usually used for high volume pumps or high viscosity oil. My point is that regardless of what the machine shop may or may not have told him, it was obvious the 15w-50 oil was way too thick. I've also encountered numerous machine shops around me that really have no clue what they're talking about when it comes to oil viscosity. On the opposite end of the spectrum, there are two machinists around Baltimore that I learn from, because these guys are absolutely astounding with their knowledge. So, my advice was to use caution, not to blindly follow the instructions of a shop that may not make the best recommendations.

Then I'd be finding a new machine shop. I'd be damned if I were going to have a $10k engine machined / assembled and NOT use the machinist's recommended oil weight.. And I was an ASME certified machinist for 8 years myself if that matters.

I do agree with your synopsis on his already thick oil.. In general on a SBF, if the tolerances are in, a 10w30 is fine in all but extreme weather conditions.. But my machinist is going to need to make a suggestion as well.