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dknv · 2003-05-09, 08:41 AM

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.).

2003-05-09, 08:41 AM	#1
dknv EJ207 Join Date: Dec 2002 Location: 39n53, 119w90 Posts: 2,698 Car: RX-8 Class: CS maybe	Engine oil 15w50 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.).