This posting came from Doug's recent posting in PriusChat.com:

[quote name='tochatihu' date='Sep 18 2006, 09:58 AM' post='321131']Frank and Bob did not mention Toyota type WS transmission as backward-compatible with the 2001-2003 Prius model. I was assured that it was by a top-notch mechanic, and have used it in my 2001 Prius now with no issues for almost 4000 miles.

Actually there is one issue: the fuel economy has increased by 8-10% due (we think) to the lower viscosity of this fluid. Nothing to complain about there though.
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This 8-10% improvement is supported by a Dept. of Energy Prius transaxle study, the GreenHybrid.com database and references in a Toyota SAE paper. My studies have also given hints about the potential improvement.

The GreenHybrid.com database shows the NHW11 vehicles getting an average of 45 MPG and the NHW20 getting 48 MPG, a 6% improvement. Although many improvements were made between the NHW11 and NHW20, transaxle efficiency has a major effect since all power must flow through that part.

The Toyota paper "Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy" (SAE 2004-01-0064) by Kiochiro Mutaq, Makoto Yamazaki and Junji Tokieda discusses the various changes and improvements between the NHW11 and NHW20. It includes a graph of torque loss that shows a substantial reduction at low speeds, pp. 10. They go on to say, ". . . The main changes are the adoption of the ball bearings at the reduction gear, and low friction oil. As a result, friction loss is reduced approximately by 30% (Fig. 28)." That low friction oil is the Type WS.

The Dept. of Energy report "Report on Toyota Prius Motor Thermal Management" (ORNL/TM-2005/33) by J.S. Hsu, S.C. Nelson, P.A. Jallouk, C.W. Ayers, R.H. Wiles, S.L. Campbell, C.L. Coomer, K.T. Lowe and T.A. Burress shows a substantial improvement in transaxle energy loss as the oil warms up. From 35C to 75C, the losses went down by 20%, pp. 21. Heating transaxle oil reduces its viscosity substantially, ~78%. The Type WS oil viscosity, 24.6/5.5 cSt, has 30% less viscosity than the original Type T, 35.3/7.4 cSt. A proportional savings, heating energy savings times viscosity change (20%*30%) will be a net 6%.

When looking at used transaxle oils, my studies indicate the Type T oil suffers a rapid viscosity loss, ~15% around 15,000 miles, and a high particulate and wear metal counts. However, the one Type WS sample had gone 61,000 miles with a 14% viscosity loss and half the particulate count of Type T at 53,000 miles. This makes sense if the Type T oil is too viscous.

Excessively viscous oil suffers higher shear stress and heating. The shear stresses are against the bearing surfaces. This heat and stress leads to local, point wear of the oil and some loss of bearing material. But there is one other aspect, silicon/aluminum contamination that suggests some dirt/grit ingestion. Even so, particulate matter in the oil, regardless of the source, makes the oil into a lapping compound and punches through the lubricating film.

My hill rolling tests showed clean, Type T oil had an 11% lower rolling resistance than the used particulate laden Type T with a lower viscosity. This is one reason why the transaxle oil filter experiments are so important. I'm also looking at the transaxle vent as a potential source.

Doug's reported improvement, 8-10%, is entirely consistent with clean (no particulate), lower viscosity Type WS oil. We do not know the long-term, Type WS oil wear patterns in an NHW11, the data has yet to be collected. But IMHO, it has to be better than the 15,000 miles shown by Type T and without an oil filter, less than the 60,000 miles of the Type WS in an NHW20. I suspect that with an effective oil filter to capture the particulate matter, it would easily exceed 100,000 miles.

Bob Wilson

ps. Neither Doug nor I are representatives of Toyota. If you choose to change your NHW11, 01-03 Prius or Prius Classic transaxle oil for Type WS, you are voluntarily joining an experiment Doug started and I will soon be joining. However, I would be very interested in working with you to understand the impact.

pps. This has no effect on the NHW20, 04-current Prius, which currently uses Type WS oil.

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.

One point on MPG effects. The approximately 6% improvement in average MPG of the NHW20 over the earlier NHW11 is in spite of increased size/weight/power. I "fear" that the earlier design using Type WS will allow the early adopters to gain some bragging rights over us latecomers to these marvelous vehicles.

JeffD

LOL! More like parity. The NHW20 still has many advantages we'll never see in the NHW11.

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.

I am concerned about the condition of the Prius transmission fluids that have been analyzed, even though mine is not known to have suffered from this (my first fluid change at 70-ish K miles was not analyzed). If any of the small number of transmission failures were caused by fluid problems, we really need to start developing a precautionary fluid change schedule. This, of course, is exactly what Bob is working on.

When I got the 2001 Prius fluid changed to WS, I anticipated that it would simply be another periodic maintenance expense. Quite surprised to see the magnitude of fuel efficiency increase; such that fuel cost savings will be comparable to the cost of fluid changes henceforth.

Ha! An aspect of Prius ownership where the phrase "pays for itself" really seems appropriate.

DAS