Using oil pressure to determine viscosity?

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cfromc

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First off, I don't want to offend anyone who uses oil pressure to determine viscosity requirements. I just want to clarify the method and get opinions of whether or not it is the "right" way to choose a viscosity. Should oil pressure be a primary factor in choosing viscosity-is it that simple? To clarify, I'm talking about "normal" passenger vehicles, not raced. Driven in normal conditions: highway, city, hot, cold, unloaded, loaded, long-trip, short-trip, bumper-to-bumper, and 80 mph on the highway.

The main vehicle I am concerned about is a 2001 Suburban with a 5.3L engine. I currently get 60-70+ PSI at cold start and usually 50-65 PSI at idle when hot. Sometimes 45-50 PSI at idle when it is very hot. At cruising RPM I get 55-65 PSI and on acceleration 60-75 or so. The PSI seems to be higher than is needed so can I safely go "thinner" or will that sacrifice durabilty/wear rates? I'm currently using XL 5W-30 with ccs of 58.2@40, 10.5@100 and 4341@-30.
 
To a first order, if you watch the pressure very carefully you can.

The first thing to figure out is where does the engine reach full pressure when compared to the RPM with the engine nice and hot (like 1 hour of city traffic in 103dF heat). So, in one of my cars, I get to peak oil pressure at 2700 RPMs with the oil at 220dF. Everything above RPM range operates at peak oil pressure (in non-race track operation). This indicates that the pop-off valve is opening early in the RPM band, and than a thinner oil could be used*.

I suspect that you have neighter the oil temperature guage nor the tachometer in your suburban. Without these you don't really have a means to carefully monitor the situation.

Taking the above example, one step thinner (5W-30) oil would move the RPMs at which the pop-off valve operates from 2700 to 3500 RPMs, and a 5W-20 might move this point to 4200 RPMs. The car in question is a Ferrari F355B, and operates at 85-90 PSI oil pressures and has a 8750 RedLine. So reaching peak oil pressures at 4200 RPMs still has full oil pressure for over 1/2 of the RPM band, and since 90 PSI is way above the old rule of thumb {you want 10 PSI for every 1000 RPMs of operation} then there is no excess wear occuring due to lack of lubrication flow.

The best way to conduct this experiment is to use a tachometer and oil temperature guage in concert with the oil pressure guage and keep a record of the relationship beween these 3 measurements.

  • I don't because of the race track use on this vehicle
 
Well, I have a tach and an oil pressure gauge, no oil temp. But after 60 minutes I would have to imagine the oil has reached "operating temperature" of ~100C. After extended driving I still have a lot of PSI and, while I haven't checked this yet, I'd say the maximum psi is reached at a fairly low RPM. I'm not sure I'm ready to jump to 100% 5W-20 but I was thinking of mixing half 5W-20 and half 5W-30 of the same brand. The viscosity numbers I came up were: 61@40C, 10.0@100C and <5500@-30C. These equate to a little thicker at start-up (less noise hopefully) and slightly thinner at 100C (slightly more MPG-maybe). I will likely do a UOA afterwards and if everything goes good, try a 67% mix of 5W-20.
 
A couple of questions before I add a couple of comments to Mitch's excellent dissertation. What are the oil pressures and conditions listed in the service manual for this vehicle? Does it have an oil cooler and where is the oil pressure sending gauge? How sure are you of the accuracy of your oil pressure gauge?
 
I used pressure to increase my viscosity. Spec for my engine is 40-60 psi at 2000 rpm hot. I was getting about 36 with 5w30. I went to 10w40 and am getting around 42-44. It probably would be fine with 10w30 though as it was not that far off.

I would not be inclined to move to 5w20 to decrease pressure though. For one thing, your engine may not be back specified by GM for 5w20. Second, you will get a lot lower HT/HS (high temp/high sheer) viscosity with 5w20, which is viscosity where it counts, in the bearings. You could go with Redline Oil as their 5w20 has an HT/HS similar or better than most 10w30s.

I had a motorhome that ran 75 psi cold and about 60 to 65 psi hot, with 40 psi idle. It was fine. Max pressure would be achieved shortly off idle. Fine running engine. I put 35000 miles of its total 87000 when I sold it and it ran just fine. The 10w30 didn't hurt a bit with those pressures.
 
Quote:

I used pressure to increase my viscosity.

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Probably the reason your UOAs are always terrible.
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The service manual lists: Minimum-Hot 6psi at 1,000RPM, 18psi at 2,000RPM and 24psi at 4,000 RPM with 5W-30 oil. The oil pressure gauge appears to be at the rear of the intake manifold and I do not believe there is an engine oil cooler. I have never done an oil pressure test so I can't be 100% sure of the accuracy, however, it is a real sender, not some computer generated "warm-fuzzy" readout. I've heard these engines do normally produce a lot of psi. My co-worker has the same engine in a 2003 but his has a little lower psi at 30-40 hot and up to 60 when cold-still well above the spec. I've heard many owners say they get 45-60 hot at idle. I doubt if the sender is 100% accurate but I believe it to be relatively close. Even a 20% error still puts me at over 40psi hot idle.
 
The oil I had in mind was Redline. The mix would result in HTHS of about 3.55 (3.3+3.8/2)
 
I believe from Cfromc's description of oil pressure Psi & engine Rpm's, that he is referring to the standard dashboard OEM gauges found in the GM family of 5.3L powered SUV's and Pickups.

"Where is the pressure sender?" posed by 427Z06 is a very good question.

My 2001 Sierra with the 5.3L displays a very steady 53 psi at 1650 rpm and 60mph with 5W-30 (Winter Months) and after 1/2 hour of drive time.

Cfromc, correct me if I'm misinterpreting your question, but you don't have a specific oil pressure problem, it's just that you are asking about the effects of viscosity on oil pressure in your application?
 
cfromc,
I am inclined to say go for it.I have the same 5.3 engine in a silverado with the same pressures,and i will be putting in 5w20 on the next oil change.will see if i like the outcome.
gary
 
Blue99: correct about the gauge, also not necessarily an oil pressure "problem" although it does seem a little high. The service manual shows the oil pressure sender at the rear of the intake but I must admit I've never looked. The GM's I've worked on in the past were usually at the rear of the block.

Lt2000: that's the way I'm headed also. I've never towed with the vehicle and don't plan on it in the near future. It isn't raced and almost never "floored" and rarely sees over 3,000rpm. Occasionally it gets loaded up with 7-8 people and some cargo but that's about it. If I were planning to tow I would likely switch to a 5w-30 like RL or Amsoil HDD. Right now it is a people-mover/cargo-mover.
 
Quote:

The service manual lists: Minimum-Hot 6psi at 1,000RPM, 18psi at 2,000RPM and 24psi at 4,000 RPM with 5W-30 oil. The oil pressure gauge appears to be at the rear of the intake manifold and I do not believe there is an engine oil cooler. I have never done an oil pressure test so I can't be 100% sure of the accuracy, however, it is a real sender, not some computer generated "warm-fuzzy" readout. I've heard these engines do normally produce a lot of psi. My co-worker has the same engine in a 2003 but his has a little lower psi at 30-40 hot and up to 60 when cold-still well above the spec. I've heard many owners say they get 45-60 hot at idle. I doubt if the sender is 100% accurate but I believe it to be relatively close. Even a 20% error still puts me at over 40psi hot idle.

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OK. Given the above, looks like the abnormally high pressures aren't part of the design.

Now remember, oil pressure just insures the oil gets to where it's suppose to be. Oil Film Thickness is determined by viscosity under shearing conditions among other variables. So if you don't plan on running it hard or towing a lot, you'll probably be OK. I'd do an UOA just to insure something isn't wearing at an increased rate. 5w20 won't blow your engine, but it may cause increased wear if the minimum film thickness threshold is reached.

Keep in mind, many 5w30s sheared to a 5w20 in the past.

Keep us posted.
 
Talk about delayed gratification...I plan on running 10K OCIs and I probably won't have my first UOA from the switch-over back to me for about a year. Of course, I will be able to tell almost immediately if it starts easier, is quieter and gets better MPG. At least that I can report back on within a couple months.
 
cfromc, GM has gone on record as not wanting any 20Wt. oil used in any of their vechiles not specificly recomending it. To date only those GM vechiles with Honda V6's are calling for a 20Wt. oil!! We only have a few UOA of GM vechiles on 20Wt. oils. If you are trying to save money with thin oil I would instead look for a more fuel effecient vechile!A synthetic 0W30 or 5W30 are about as thin as I would go. Last I checked GM alloyed 0W30's to be used in all of their engines. The "0W" is what is going to save you fuel dureing the first 20minutes of cold start operation. Most 0W30's shear dureing usage.

PSI recomendations would be listed in the service manual. As long as you stay within the minimums for the given RPM range and temp. you should be fine. The real problem you might runinto is what volume of oil the pump will be able to pump and how much hemorage you might get at the rod bearings. I do not see this as an issue but with out any hard data I am just giveing you my best guess.
 
Most of GM's OHV engines with out displacement on demand can live a long life as long as idle pressure stays above 6-8 psi and they show positive oil pressure increase with RPM to about 30-35psi. This is based on a lot of observations on the old chevy small blocks. I have no idea how their newer displacement on demand and roller rocker set ups will do. I would imagine that everything other then the displacement on demand system would behave much like the old 5.7 small block since general design and materials are so simalar.
 
I'd love to have a vehicle that can seat 7-8 people, is fuel efficient, and will last for 200K+ miles. There just aren't very many. Oh, no mini-vans please. Already tried that and hated it. I understand that the vehicle is not known for fuel economy but I love everything else about it. If there are some reasonable ways to improve MPG's then I'm all for it. So far I've mostly done things that needed to be done anyway so I don't think I've spent more money than the money I've saved in gas. So far new O2s, fluids, filters, increased tire psi. Recently I had the PCM professionally tuned which is normally $250-500 but I know somebody and paid a fraction. It is still too early to tell what affect that will have. Anyway, if I can protect the engine and gain .1-.2 MPG than I consider that a win-win. Also, I tried the 0W-30 and my engine was noisy on start-up (piston-slap) and it was really, really bad in the winter. the 5W-30 doesn't make nearly as much noise. I will be performing a top-engine cleaner in hope of getting rid of the noise altogether but that chances are probably 50% that there will still be some noise. I'll also be using Redline at a mix to make 5W-25 so I think the HTHS and film strength/thickness will provide protection.
 
Quote:

Also, I tried the 0W-30 and my engine was noisy on start-up (piston-slap) and it was really, really bad in the winter. the 5W-30 doesn't make nearly as much noise.

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my 98 S-10 ZR2 has a 4.3L v6. Different engine, for sure, but Ive noted a few things that may be useful.

Since I bought it new, the oil pressure was always ~55 cold, ~25 hot idle, and ~55 any touch of the throttle, from fast idle through higher RPMs. Always the same, regardless of if Im using 5w-30 or 5w-40, summer or winter, etc. And Ive watched it carefully, and know it has not deviated in the 54k Ive used it so far.

However, I put GC 0w-30 in it last OC, and now I always get a cold start of 60+ psi, but hot idle and hot operating RPM, as well as the constant PSI regardless of RPM, stay the same. FWIW.

Now, lots like to correlate RPM to psi, etc. My MB diesel does it differently, and granted the miles on that car, its engineering and longevity, I tend to trust the daimler engineers... The car has a gauge that goes 0-3 bar. A bar is approximately an atmosphere, so 3 bar is approximately 45 psi (gage). The way they do it is, when the oil is cold, itll be pegged at 3bar. When the oil is hot and at idle, it sits at 1.5 bar. When you tap the throttle, the needle pegs back to 3bar. Supposedly, unless your oil pressure at hot idle goes below 1, or your operating pressure doesnt go all the way to 3, immediately, when the throttle is touched even slightly... then youre fine and the engine is operating properly with sufficient pressure. The gauge in this car is directly plumbed into the pressurized portion of the oil system, BTW, there is no sender... don't screw your oil line into your gauge, but start the car and you have a mess!
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Id venture to guess that the GM gauges our trucks have are similar in quality, so though our engines are apples to oranges, perhaps that sheds some sort of insight..

JMH
 
I was wondering which 0w-30 you tried. Castrol Syntec 0w-30 (GC as its called here) sold at Auto Zone is a thick 30 weight. Are there Auto zone near you? I remember reading here somewhere that GM engines like a thick 30 weight. I would be curious how it would work for you.
 
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