Hot Rods Oil pressure/ viscosity

Everything being equal , how much ( percentage) would oil pressure rise changing from 10/30 to 15/40 ?

 

SAE 30 viscosity can be from 9.3 to <12.5 centistokes (cSt) at 100C
SAE 40 viscosity can be from 12.5 to <16.3 cSt at 100C

So the answer depends on where in the range both oils fall. Different brands will have a different typical viscosity, but will fall somewhere in those ranges (at least when new). So the answer to your question is probably anywhere from ~35 - 70%.

 

Wow , 50% average ? That's from 40#-60#/?

 

Or from 10# to 15#. What's the pressure relief valve opening at?

 

I haven't seen anywhere near that much change in oil pressure when changing oil weight.I don't think that pressure in an oiling system, is proportional to viscosity.

And there are other variables too...temperature, engine clearances, etc.

If you want to know for sure, change the oil and see what happens.

 

The relief in the filter is the biggest culprit. Open relief means unfiltered oil.

 

You're probably right about that Jim. Well, there's probably a constant, I just don't know what it is.

 

Just for a bit of color. I used to be the lead test engineer at a Powersports manufacturer. We had a situation in the field were the engines were blowing lines off the oil cooler during cold (-20 deg. F) starts. At the time, the engine and transmission shared a common case and we used 20W-50 oil, primarily to maintain good lubricity between the gears. So, I instrumented the engines with thermocouples, pressure transducers, flow meters, etc to find out exactly what was going on. On cold starts, we would see system pressure spike to well over 600 psi (the failure pressure of the line fittings), despite the pressure relief valve on the oil pump being set at 85 psi. The viscosity of the oil was such at those temps that the pressure relief valve couldn't flow enough oil to keep the system pressure below 85 psi. It would often take a few minutes (***uming the oil cooler line held) for the oil to warm up enough to have the system pressure drop below 100 psi. Switching to a 0W-40 oil limited the oil spikes on cold starts to less than 500 psi. The reduction in viscosity allowed better flow across the pressure relief valve and the corresponding drop in system pressure.

Most people think pumps produce pressure. This is FALSE. Pumps produce flow, restrictions to that flow down stream of the pump create pressure.

 

I switched from 10W30 to 10W40 on my 350 crate. It increased pressure at the gauge sender somewhat.
After sender failed, the replacement sender increased gauge pressure some more.

 

My dad got a gig hauling in Idaho? I think it was with his Bottom Dumps, he ran Dello 400 in is ***mins. Starting in cold weather was almost impossible, said he’d trade that big Cam in for a small cam with a compression release, and his Oil PSI pegged the gauge.
The locals set him straight right quick with some sort of block/pan heater. Only thing that was an inconvenience was parking it where he could plug it in. He wound up getting a suitcase generator and fired it up a few hours before he was going to start the truck.

 

I think he saying he had a faulty sender

 

I got that ....

 

Read again my last words:

Since different things happen at different RPM and with different oil temperature, I really doubt you're going to get a solid answer, like "7.3%"

Try it, see what happens.

 

With all due respect , you're final line is a suggestion lacking information . I don't wish to change the oil twice .

 

Yes, it's true that I don't know the magic number.

Do you have a range of acceptable pressure change, that would make you happy with the results of the oil change? What do you expect from it?

 

Maybe, tell the crowd what engine you have, milage on it and current oil pressures at different RPMs. It may lead to something, maybe not.

The viscosity % has been given, but surely a fresh engine with known bearing clearances, etc, would behave differently than one with 300K miles on it, right?

 

I have done some experimenting with 5w30, 10w40, 20w50...and seen some change, but never enough to make me think it was going to bring a worn engine back to life.

 

Hey Squirrel,
Even with different oils,,,,,,did the change stay at a constant ,,,after hot and running for a while ?
Also,,,,,did they settle back close to normal after a couple of thousand miles ?

Tommy

 

I don't remember the details, and didn't write stuff down, unfortunately. I do recall it wasn't worth bothering with.

 

FWIW!
Straight eight Buick hydraulic lifters are gravity fed. Increased valve spring pressure was causing them to "collapse" as the engine warmed up. Changed from 15/40 to 20/50 and that helped somewhat. I guess the stronger springs caused the 15/40 to squeeze out of the lifter. The 20/50, not so much.

Ben

 

I'm saying the change in oil viscosity helped some. Later the change in oil sender helped some. Still a little concerned as pressure is about 20 +/- at idle.

 

Pontiac's factor low oil lights came on at 3 to 5 psi, just how much pressure do you need?
Ask around and see how many motors running thick oil have had distributor gears or oil pump drives go out.

 

Had a 64 Malibu with a worn out 194 L6. 20-50 kept the oil light from going on at idle. When I sold it I clearly told buyer the engine was badly worn and what oil to run for the time being. I got a call the next day about oil light. Clearly he wasn't listening. He had changed the oil.

 

The way I understand it, and I may not, it is largely the bearing clearances that dictate what viscosity oil is optimum. And most people want a multi-viscosity oil that is suitable for use year round.

If the OEM specified say, 10w-30, it might sound crazy, but that's what you want to use. The exception of course if the mains and conn. rods (and piston rings) are worn to excessive clearance.

But if the engine is in good shape, or it was overhauled to factory specs then a higher viscosity than called for isn't what you want, especially in cold weather.

Motor oil flow is also critical to cooling internal engine parts that the antifreeze/coolant jackets can't reach. Again I might be wrong but the way I take it, the lightest viscosity that still achieves proper oil pressure within the zone is what you want. High oil pressure as such, for high oil pressure sake, is not necessarily a good thing.

 

I've used 10w-30, 15w-40, 20w-50 and now trying out 5w-50 now. Pretty much stays the same no matter what viscosity I use. 20 lbs. at idle hot and driving 60 lbs. This is on a BBf stroker with a high volume pump. I guess if I needed more at idle I could shim the relief spring in the oil pump but at 20 lbs. it's fine.

 

Thanks everyone , never mind , not important.

 

I'm confused, was there an original question that I'm not seeing?

 

Hi Lloyd,
Yeah,,,,but it got kind of muddled up a little bit .
And besides,,,,,after it went to C versus F in the next post,,,,I was out .
I know I’m not an engineer,,,,,,and I’m okay with that,,,,,,,but I usually just go back to first hand experience,,,,,,rather than the book answer .
About the only engineer I listen closely to is Jim,,,,,,mainly because he just talks plain sense,,,which I can relate to .
It’s just over my head,,,,,,I’m a plain old dumb hillbilly,,,,,,and I’m reasonably happy with that ,,,,,LoL .

Tommy