- Apr 27, 2003
- South Florida, USA
My car is an Audi 1.8T and the appropriate oil to satisfy the lubrication needs of the turbo must meet VW502. A key requirement within VW502 is ACEA-A3. Hope you will find the following useful and comments are welcome: HTHS specification ensures sufficient lubricant film strength for engine parts where the lubricant (oil) may experience a shear rate of approximately 1 million reciprocal seconds (106 s-1). The HTHS specification value will vary with an engine design and its permissible safe operating envelope. The shear rate of an oil film between two surfaces is the relative velocity between the two surfaces divided by the distance between them, hence the unit “reciprocal second”. Some authorities on the impact of shear rate (velocity gradient) on oil viscosity believe that if there is a reduction in viscosity it will not be felt until the shear rate is approximately 106 s-1. An understanding the meaning of 106 s-1 may be garnered by realizing that a bearing 2 in. in diameter with a radial clearance of 0.001 in. and spinning at 9550 RPM will reach a velocity gradient (shear rate) of 106 s-1. ACEA-A3 spec stipulates HTHS viscosity > 3.5 at 150 deg. F. and 106 s-1. This will ensure adequate oil film strength for turbocharger bearings that in general spin at very high RPM. Oils that physically shear apart (due to say VI) will negatively impact their HTHS spec because of the reduction in base viscosity. In conclusion, the HTHS viscosity if called out in an OEM oil specification may be regarded as a function of the engine design and the selected oil should at a minimum satisfy that requirement.