sharp oil pressure drop-off > 130C

[bbrtuning]

Hi guys, I've been using Liquimoly 10w60 in my E46 M3 for years, and I have noticed that during hard track use, the average high RPM oil pressure drops off pretty linearly until about 260F at which point it starts to drop off more suddenly. You can see this in the X-Y plot of oil pressure vs oil temp below. Because of this I usually call it quits at this temperature to be safe. Pressure recovers after a cool-down and shows little to no change in measured pressure over the life of the oil. Blackstone reports have always come back great and I haven't had any issues, but it would be nice to be able to run a bit longer without having to cool down. These S54 engines already don't make a ton of oil pressure, and are obviously notorious for oiling issues so it always has me a bit worried. I've already done what I can within reason to try and improve oil cooling. It seems to dump a bunch of heat into the oil, climbing from 80-130C in about 3 hot laps. It is a "built" engine built to slightly looser tolerances than factory, so that probably isn't helping. Typically see 55-60psi at high RPM, dropping to 40-50psi by 130C (revving to 8000+ RPM). At this temp, I'll also see as low as 9psi at idle (950 RPM). I decided to go with a higher-flow VAC oil pump to help. The pressure is now in the 70-75psi range. My question is, is it possible the sharp drop-off in pressure at that particular temp a result of the type/brand I'm using? It has been very repeatable over multiple events and oil changes. I was considering trying a different oil brand & viscosity, possibly Motul Sport 5w50 or something similar, since now I can afford a reduction in peak oil pressure. Figured I could also gain a bit of cold oil lubrication over the current 10w60 (even though it doesn't get below 50*F here). I haven't been able to find much info on non-linear change in viscosity at extreme oil temps (HTHS?) and that maybe a higher-end ester-based oil is more stable at such temperatures? I change oil after a few track events anyway so I'm more interested in high oil temp stability and reduction in friction/temps, rather than overall life.

 

[ripcord]

Not an expert, but maybe worth it to try Mobil 1 15w50, instead? Or maybe Motul 20w60 V300?

 

[CT8]

Does the car / engine have an oil cooler?

 

[Snagglefoot]

This.

 

[Snagglefoot]

Depends what you mean by "Sharp" drop off. Your loss of oil pressure seems to be following a lowering of kinematic viscosity as the oil heats up. Here is a graph that shows what to expect as the temperatures climb above 100 C (212 degrees F). The graph is in degrees F. You can see a significant loss of viscosity as the temps climb to 130 C which is 266 F.

 

[andyd]

20w50 V twin oil.A crankcase full of 20w50 dino will start at 10'F. You need a larger oil capacity and a good size oil cooler. How hot does the coolant get? Maybe a bigger radiator too.

 

[edyvw]

Did you try with Castrol Super Car? Or Redline 5W50? Most issues around that engine is related to cold performance and high rpm's when oil is cold. You do not have that issue nor I would say you are type who will spin engine to 7,000rpm minute after starting engine. So maybe W50 might work. Also, like others said, if simple brand change does not achieve anything, maybe going with some bigger hardware.

 

[Gokhan]

Oil pressure is dictated by the dynamic viscosity of the oil in the bearings, which varies with the temperature and shear rate. Shear rate (s⁻¹) = relative speed / separation (general formula) Shear rate (s⁻¹) = (π × bearing diameter) × (rpm / 60) / MOFT (formula for bearings) For the S54 engine, main bearing diameter = 60 mm. HTHS is measured at a shear rate of 10⁶ s⁻¹. At 6,000 rpm this corresponds to minimum oil-film thickness MOFT = 0.019 mm = 19 µm. The nominal main-bearing clearance for the S54 engine is 41 µm, which corresponds to zero torque (concentric bearing). MOFT decreases with torque, and it will usually be only a few µm in high-torque conditions. The point is that your bearings may experience a shear rate of 10⁷ s⁻¹ or higher, which is at least 10 times higher than the shear rate HTHS is reported at, in high-rpm, high-torque racing conditions. This is close to the full-shear region, where the only contribution to the oil viscosity comes from the base oil and additive package, with the viscosity-index improver (VII) polymer playing no role. Without going further into the details of the theory, here is the table of oils with their HTFS and HTHS values listed. I sorted them by HTFS. HTHS is measured at 10⁶ s⁻¹. HTFS applies to shear rates of 10⁷ s⁻¹ or higher. I also provided a column labeled (FS + HS)/2, which is the average of the two. Unless you want to use a SAE 40 or SAE 50 monograde, andyd's suggestion of Mobil 1 V-Twin 20W-50 will give you the highest oil pressure. It has HTFS = 5.0 and HTHS = 5.8 cP. In contrast Castrol TWS 10W-60 has HTFS = 3.0 cP and HTHS = 5.2 cP. If your oil pressure is being governed by HTFS, you will get a 67% higher oil pressure with Mobil 1 V-Twin 20W-50 than with Castrol TWS 10W-60, and your low-oil-pressure problems will go away. Liqui Moly Synthoil Race Tech GT1 10W-60 doesn't list HTHS; therefore, we can't put it into the calculator. Automated VII and HTFS calculator and table of oils sorted by HTFS Here is more information on the calculator. You can download the calculator spreadsheet as an Excel file or make a copy of it as a Google sheet, and then you can work on it. Automated calculator for the A_Harman index, VII content, and base-oil viscosity

 

[BMWTurboDzl]

I would ask Turner Motorsports. IIRC raced the E46 M3 with the S54 for a few years and I believe they ran Castrol TWS (10w60) but with an upgraded oil pump and enhanced oil cooling.

 

[MacManus99]

I would 100% stay away from a Mishimoto oil cooler. If doing an air/oil cooler, try a Mocal thermostatic plate with a Setrab heat exchanger. Setrab has many different heat exchanger sizes and mounting options. I was under the impression that these engines already have some kind of oil cooler system, the Z4M at least has a factory ail/oil. Makes me wonder if an additional cooler can be plumbed in to supplement the OEM. I also do not think a 20w50 hurt to try, especially with the upgraded oil pump. You obviously keep a close eye on and record data, might be a good reference point. If you experience less than ideal results, you can always switch back to the 10w60.

 

[Jimmy_Russells]

I would try Redline 10w60, HTHS is 5.8. In fact I might even try the 10w50 which has HTHS 5.0. 130 is not crazy hot but it is certainly getting up there. If that oil doesn't help you are most certainly going to have to upgrade cooling. If you can keep the temp in the 115-120 range you can probably use the 10w50 no problem.

 

[bbrtuning]

Thanks for the replies. The OE air-oil cooler has already been upgraded with a larger CSF unit. Water temps stay in the 85-90C range on track, so pretty normal. My question is specifically about why I get a rather sharp drop-off in pressure above 130C (progressive or exponential drop, as in my original post) and therefore viscosity (I assume), which differs from a more digressive curve linked in this thread and shown below. I'm curious if more motorsport-oriented oils perform better in this regard.

Originally Posted by Snagglefoot

 

[Sveina]

I have been using Lubrication Engineers Inc Monlec 20w-50 oil in a Porsche 944 turbo I used on track. It was heavily modified, but still had the original oil cooler at the time. The motor oil was at around 150 degrees C, and I still had 3,5 bar of oil pressure at idle with it. UOA was done after that, and it showed no permanent shearing so the viscosity at 100 was very much like new oil. I used Gokhans excel sheet and calculated the high temp full shear to be 4,8 cP, which is very high. High Performance lubricants 20w-50 also has a very high HTFS. Both have very low VII.

 

[Snagglefoot]

I suppose at the low viscosity ( probably 10-11 cSt), and the high rpm (7,000 rpm), the rotors of the oil pump may be by-passing oil. Why not go ahead and try the suggestion of Castrol Edge Supercar 10w60. 11 cSt puts you right between a 20 to 30 weight oil at a more normal 100 C temp.

 

[Gokhan]

Originally Posted by Snagglefoot

I suppose at the low viscosity ( probably 10-11 cSt), and the high rpm (7,000 rpm), the rotors of the oil pump may be by-passing oil. Why not go ahead and try the suggestion of Castrol Edge Supercar 10w60. 11 cSt puts you right between a 20 to 30 weight oil at a more normal 100 C temp.

Did you read my earlier post? Oil pressure has nothing to do with KV100. It is governed by HTHS and HTFS.

 

[Gokhan]

Originally Posted by Sveina

I have been using Lubrication Engineers Inc Monlec 20w-50 oil in a Porsche 944 turbo I used on track. It was heavily modified, but still had the original oil cooler at the time. The motor oil was at around 150 degrees C, and I still had 3,5 bar of oil pressure at idle with it. UOA was done after that, and it showed no permanent shearing so the viscosity at 100 was very much like new oil. I used Gokhans excel sheet and calculated the high temp full shear to be 4,8 cP, which is very high. High Performance lubricants 20w-50 also has a very high HTFS. Both have very low VII.

It looks like SAE 20W-50 is the most "stout" SAE viscosity grade among all multigrades. It is far more stout than the ultra-high-VII-content SAE 10W-60. Stout in this content means both a high HTHS and a high HTFS, also with minimal permanent viscosity loss due to permanent shear. The retired British Castrol and Afton oil blender SonofJoe also had made this point that 20W-50 is the most robust SAE viscosity grade. If your climate doesn't go more than a few degrees below freezing, SAE 20W-50 would be the ultimate multigrade to run for the thickest oil film and highest oil pressure possible.

 

[Mitch Alsup]

I can tell you that in my 1995 Ferrari F355 that the oil pressure stays at 75-80 PSI on track up to at least 300ºF. This car does have an oil cooler built into the dry sump system. In recent times, I try to stay under 275ºF. But that if I have to pit without a cool off lap, the idle pressure will be 10 PSI down from 30 PSI nominally. If I take a cool off lap, the oil pressure is above 25 PSI at 230ºF. I can also tell you that the normal street driving pressure drops by 5 PSI (from 80 to 75) after a track event (4 or 8 sessions) and does not recover until fresh oil goes in. I do not know if this is permanent shear or oil filter clogging. I suspect that the extra clearances in your built engine cause a situation where when the viscosity gets below a certain point, the oil pump cannot pump as much oil as the clearances demand; and this is why the oil pressure drops.

 

[Astro14]

I've deleted the bickering posts as they're not helping the OP.

 

[Snagglefoot]

OP, as Edyvw suggested, try Super Car 10w60. Also, OP, as I and one other member suggested, your problem might be the low Viscosity oil leaking around the rotor.

 

[Gokhan]

SAE 10W-60 is hardly a "low viscosity." Liqui Moly 10W-60 and Castrol 10W-60 have nearly identical specs, as if they copied from each other. I doubt you would see any difference between the two.