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#1 |
EJ207
Join Date: Dec 2002
Location: 39n53, 119w90
Posts: 2,698
Car: RX-8
Class: CS maybe
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A friend shared the following about motor oil. Some of you already know most of this, but I thought it would be a handy reference for the Tech Chat forum. Disclaimer, this does not represent opinion or sanction of SOA:
============================================ I am a mechanical engineer , so I know something of lubrication technology and bearings. I am, however, not an automotive engineer, so what I say has to be taken with a grain of salt. The logic goes like this: Wrist pin, crank, and rod bearings bear the load from combustion chamber pressures and intertial forces. The combustion forces go up linearly (at least) with engine output. There is a term, BMEP (Brake Mean Effective Pressure) which expresses the average cylinder pressure required to produce the horsepower and torque. Double the torque at a given rpm, and the BMEP has to be doubled at that point. Double the torque at a given rpm, and the horsepower doubles. The WRX produces around 113 hp/L (very high) and hence very high BMEP; consequently high bearing loads. Bearing loads due to inertial forces increase with the SQUARE of the rpm. If you run consistently at high rpms, average bearing forces are higher. So driving your WRX hard, making a lot of power (high BMEP) at high rpms really loads the bearings. Loads on the bearings are resisted by the oil film (which is partly hydrostatic (from oil pressure) and hydrodynamic (like hydroplaning on a wet road). Low viscosity oil has thinner films and less ability to resist these bearing loads. Oil temperature also goes up during hard usage, further reducing viscosity and oil film thickness. So higher viscosity is good in hard usage situations. There are also reasons not to use high viscosity: - Worse fuel economy - Poor flow during cold start (when a lot of wear occurs) - Small oil passages in some modern engines may not allow enough flow and cooling with high viscosity oils. - Some hydraulic valve lifters are designed for low viscosity oils and don't operate properly with high viscosity Given that Subaru states in the manual that up to 20W50 is permissable in the case of trailer towing and other hard use, we don't have to worry about warranty issues or the oil passages being too small for higer viscosity oils. The WRX has solid lifters, so no problem there. Sythetic oils flow much better at low tempertures than equivalent viscosity mineral oils, so cold start lubrication should be OK. And the synthetics are MUCH better at resisting high temperature breakdown (as in the turbo bearing at high sustained boost). Net is, I conclude that M1 15W50 is a good choice for someone that actually uses their WRX. Mobil says that it is OK to switch grades seasonally, as the basic oil chemistry is the same across their viscosity grades. So one could use 5W30, 10W30, or 0W40 for normal use in very cold weather (like parked outside at 0-10 degrees, etc.). |
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#2 |
Nightwalker
Real Name: Austin Join Date: Dec 2002
Location: Oshkosh, WI
Posts: 4,063
Car: '13 WRX
YGBSM
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Sounds pretty much inline with what I learned in my IC engines class in school.
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#3 | |
The Doink
Real Name: Scott Join Date: Nov 2002
Location: Portland, OR
Posts: 20,335
Car: '09 OBXT, '02 WRX, '96 Miata
Class: PDX/TT-6
The way out is through
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And yes, the 15W50 does effect the cold start. I gotta crank it a bit on cold mornings, and the idle is a little sloppy... 'course, I think the under-drive pulley adds some slop to the idle.
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#4 | ||
R.I.P.
Join Date: Apr 2003
Location: reno, nv
Posts: 599
Car: 09 335xi silva
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#5 |
The Doink
Real Name: Scott Join Date: Nov 2002
Location: Portland, OR
Posts: 20,335
Car: '09 OBXT, '02 WRX, '96 Miata
Class: PDX/TT-6
The way out is through
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I was under the impression that 15W50 is withing the specs in the manual.
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#6 |
n00b
Join Date: May 2003
Posts: 3
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There should be no warranty problem with 15W50. The generic Impreza owners manual recommends 5W30, 10W30 for higher minimum temperatures. These are to display the Energy Saving logo. Severe duty (such a trailer towing) allows for up to 20W50. Mobil 1 15W50 fits within the viscosity/temperature ranges listed in the manual. It is, however, not an energy saving oil. (As if staying in boost is energy saving!!).
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#7 | |
EJ205
Join Date: Dec 2002
Location: Reno, Nevada
Posts: 2,076
Car: 12+tires
Class: Spec.
without hard work, nothing grows but weeds
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Is this a warantee issue ??
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#8 |
The Doink
Real Name: Scott Join Date: Nov 2002
Location: Portland, OR
Posts: 20,335
Car: '09 OBXT, '02 WRX, '96 Miata
Class: PDX/TT-6
The way out is through
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Most people recommend running conventional "Dinosaur" oil for the first 5000 miles (or at least until the break-in period is over) since it'll help break-in the motor. Then switch to Synthetic.
I used dino-oil for the 1st 4000 miles, then switched to Mobile 1 during my 2nd oil change. (Changed it at 1000 miles when the break-in was over, then again 3000 miles later)
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#9 |
EJ251
Join Date: May 2003
Location: Ocala, FL
Posts: 894
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I have been using 5W30 since 3000 miles... would it be better to convert to 15w50 now with my hard driving habits?
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STIwish |
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#10 |
The Doink
Real Name: Scott Join Date: Nov 2002
Location: Portland, OR
Posts: 20,335
Car: '09 OBXT, '02 WRX, '96 Miata
Class: PDX/TT-6
The way out is through
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15w50's a little thick in the winter... I run it when I'm autocrossing for a little added protection at high rpms. But if I know I'm going to be driving long distances, I'll put in the 0w30 (i think that's the normal Mobil 1 stuff)
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#11 | |
EJ207
Join Date: Dec 2002
Location: Reno, NV
Posts: 3,707
Car: 2011 Chevy Battle Wagon
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#12 |
EJ22
Join Date: Aug 2003
Posts: 172
Never let the driver work on the car...
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The tech guys at SOA headquarters in New Jersey suggested that we use 20W50 in our race engine (which is internally stock). We only use Redline 20W50.
I don't think I would ever use a 0W30 unless I expected to be rebuilding much more often. Gary Sheehan Motor Racing www.teamSMR.com |
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#13 |
EJ251
Join Date: May 2003
Location: Ocala, FL
Posts: 894
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Where can you pick that up Gary?
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STIwish |
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#14 |
EJ22
Join Date: Aug 2003
Posts: 172
Never let the driver work on the car...
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I get it at my local Monument Car Parts.
I don't think I would use it in a street car under warranty, though. As far as I know, it is not API certified and may void your warranty. Yes, even though it is far superior than certified dino lube. Gary Sheehan Motor Racing www.teamSMR.com |
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#15 |
The Doink
Real Name: Scott Join Date: Nov 2002
Location: Portland, OR
Posts: 20,335
Car: '09 OBXT, '02 WRX, '96 Miata
Class: PDX/TT-6
The way out is through
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The Mobil 1 0w30 (or it might be 5w30 I'm drawing a blank) is what Mobil 1 recommends for motors that are supposed to have 10w30. I guess their formula is supposed to protect so well that you can get away with a lighter oil, and still have good protection.
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