Originally Posted By: Gokhan
Originally Posted By: CATERHAM
Originally Posted By: Gokhan
Your argument regarding oil pressure is seriously flawed. You will go into the bypass mode with any viscosity in high RPMs. You are also thinking about hydrostatic lubrication, not hydrodynamic lubrication, which is usually the case in internal-combustion engines. Oil pressure is not the only thing that protects the
hydrodynamically lubricated engine parts, such as bearings. You need a thick enough (strong enough) oil film, which might not be the case with a smaller HTHS viscosity.
Statements like this tell me you really don't have much if any practical understanding or knowledge of most IC engines.
At normal operating temp's NO engine that I know of operates with the oil pump in by-pass mode at high rpm's when using the specified oil grade. Furthermore, the minimum (and optimum) oil pressure spec' that engine manufactures provide is well below the by-pass point. But if you think about it for a second it would make no sense to design an engine with the oil pump in by-pass mode at maximum rev's, thereby reducing oil flow when the engine needs it the most to cool the bearings.
Additionally your statement "oil pressure is not the only thing that protects the engine" implies a misunderstanding of hydrodynamic lubrication because oil pressure in itself doesn't protect an engine at ALL; in the bearings or elsewhere.
The oil pressure provided by the oil pump is just to deliver the oil to every part of the engine that needs it and no more. It isn't that oil presuure that keeps the bearings apart, they do that all on their own in a running engine as long as sufficient oil is being supplied.
When anyone talks about oil pressure as 92Saturn12 has, one is really talking about oil back-pressure which is what an oil pressure gauge actual reads. Lower oil back pressure is actual better because it indicates higher oil flow, and maximizing oil flow is what you want. Since HTHS viscosity correlates directly with oil pressure (kinematic viscosity does not) ideally you want the lowest HTHS vis that still provides the minimum oil backpressure as specified by the engine manufacturer.
You Sir by making the assertion that you want the "highest HTHS viscosity possible for maximum protection" are making the classic mistake of disasociating kinematic viscosity from HTHS viscosity when in fact they are inextricably linked. That's why if you control the oil temperatures you can maintain normal operating oil pressure when running very low HTHS viscosity oils with no increased engine wear. And by low I mean 5wt oil (HTHS 1.7cP) or lower.
That's why in racing they can use 5wt qualifying oils, because oil temps are low. The engine doesn't know what the HTHS viscosity rating of the oil is all it knows is what the operational viscosity is and that is 100% temperature related.
I explained in greater detail the relationship between HTHS viscosity and kinematic viscosity in the following post:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2001169&page=1