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Old 06-04-2011, 09:55 AM   #1 (permalink)
Engineering first
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NHW11 transaxle oil testing - Phase I



Transaxle oil testing has shown the NHW11, 2001-03 Prius, transaxle oil, Type T-IV, will shear-down to nearly the Type WS viscosity range in just under 40,000 miles. Early testing with Amsoil ATF as a substitute revealed excessive copper wear and its use is strongly discouraged. Switching to Type WS, the same oil as the NHW20, reduced the copper wear but still about 4x the original Type T-IV and Fe and Al were 2x compared to Type T-IV. Based upon oil analysis from half a dozen vehicles and our NHW03, we recommend:
NHW11: 30,000 mile oil change interval - which is half the Toyota recommended 60,000 miles.
Early reports of contamination based upon Si levels were traced to the sealant leaching.


Transaxle friction losses along with tire rolling resistance are the primary sources of rolling losses. The transaxle loss is primarily a function of the lubricant that needs to balance viscosity 'stirring' losses versus friction and wear loss.

Viscosity Shear-Down

Toyota has two transaxle oils for the Prius and these are the measured, viscosity wear rates:
  • 40C / 100C - Type T-IV
  • 35.3 cTs / 7.4 cTs - 0 mile Type T-IV
  • 30.0 cTs / 6.3 cTs - 15% viscosity loss - oil needs replacement
  • 26.5 cTs / 5.1 cTs - 40k mile Type T-IV
  • 26.1 cTs / 5.2 cTs - 53k mile Type T-IV
  • 40C / 100C - Type WS
  • 24.5 cTs / 5.5 cTs - 0 mile Type WS
  • 21.3 cTs / 4.7 cTs - 61k mile Type WS
  • 20.1 cTs / 4.6 cTs - 15% viscosity loss - oil needs replacement
Oil testing services recommend that a 15% viscosity loss means the oil should be changed, absent any other metrics.

The NHW11 transaxle was redesigned in the 2004 with the NHW20 to replace some bearings with roller bearings. Using the Type WS was cited as a friction reduction strategy.


Foreign material, dirt and grit, are usually identified by high Si and Al rates. The first samples had high Si with higher rates found in more rural, agricultural and dusty areas. But testing with cured sealant in a sample container with virgin transaxle oil revealed rapid Si leaching from the sealant. Since leaching is temperature related, hotter climates may have accelerated the observed correlation to agricultural and dusty areas.

Oil testing of a new, ZVW30 at 5,000 miles revealed early wear material including small bits of sealant. Furthermore, an NHW20 that has gone over 180,000 miles revealed the Si and other wear rates were substantially lower suggesting a 90,000 mile change rate in the future.

I will be backing out my transaxle oil breather modification and replace the one-way valve:

Material Wear Rates

Testing Type WS in the NHW11 transaxle designed to use the Type T-IV revealed:
  • 2x - Fe and Al wear rates
  • 4x - Cu wear rate
Like the thinning of brake shoes, loss of iron and aluminum are associated with mechanical removal. This is an indirect measure of friction removing material. Type WS in an NHW11 transaxle saves viscosity losses but at the expense of higher mechanical friction and wear losses.

Viscosity Temperature

It takes about 30 minutes to bring the transaxle oil temperature to operating temperature. Measured warm-up:
  • 88F/31C - starting temperature
  • 125F/52C - warmed temperature 30 minutes later
Notice these temperature nearly bracket the 40C viscosity reading.

What this means is oil viscosity testing at 40C is all that is really needed to understand the operating range of transaxle oil. There will be point sources with higher temperatures between the bearing and contact surfaces but these are a small fraction of the total oil.

Oil viscosity is a strong function of oil temperature:

Given the effect of cold temperatures, we can observe viscosity effects in freezing weather with a hill roll-down test:

The longer rolling distances were as a result of an oil change that also removed a substantial amount of wear material in spite of the lower viscosity of the used oil.


This initial study took from 2005 to 2011 but by no means is the last answer.

  1. Early transaxle oil change - a new car should have the transaxle oil changed early with the first engine oil change. This will flush the early wear material and loss sealant bits.
  2. NHW11 - should use Type T-IV over Type WS as the wear patterns suggest the lower viscosity losses may be more than offset by higher wear, friction losses. NEVER USE AMSOIL ATF DUE TO EXCESSIVE COPPER WEAR. Do an early change and then every 30k miles.
  3. NHW20 transaxle oil should be changed early, at 60k miles, and then 90k miles thereafter.
  4. Use of a transaxle heater to bring the oil closer to operating temperature should receive equal weight to an engine block heater in colder weather.

Future Studies
  • Blending Type T-IV and WS may find a transaxle oil with optimum viscosity and wear losses. However, it is difficult to measure viscosity losses apart from friction. We can see friction losses in the wear material. We need a better model to figure out viscosity losses.
  • Boron CLS - oil analysis suggests the commercial product does not introduce material not already found in Type T-IV and WS. This may provide a mechanism for post-treatment to reduce friction wear losses. However, there is an ongoing engine friction analysis and this is a risk only experimenters should assume.

Bob Wilson

2019 Std. Range Plus Model 3 - 134 MPG3 || 2014 BMW i3-REx - 117 MPGe, 39 MPG
JuiceBox 40 Pro (240 VAC, 40 A), KHONS portable (120-240 VAC, 12-32 A)
Retired engineer, Huntsville, AL (five times AutoPilot saved.)
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Old 06-04-2011, 02:16 PM   #2 (permalink)
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Great info, thanks Bob!

Current project: Adding regenerative braking to the Mirage
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