sixtentouge
Totally Hooked
- Aug 28, 2023
- 206
- 832
TLDR: Don't put standard DOT-4 ISO 4925 CLASS-4 brake fluid in your Yaris. It's worse for brake performance, and has a lower boiling point than what's in there from the factory.
I did a search on here and didn't see anything on it (surprising), so I'll post if for anyone looking.
The major difference between standard DOT-4 (CLASS-4) and DOT-4 CLASS-6 is the viscosity.
DOT-4 ISO 4924 CLASS-4 fluid has a viscosity of 1,800 cSt, a dry boil temp of 230°C and a wet boil temp of 155°C.
DOT-4 ISO 4925 CLASS-6 fluid has a viscosity of 750 cSt, a dry boil temp of 250°C and a wet boil temp of 156°C.
Why does the viscosity matter?
In cars with ABS and ADAS systems like EHB, standard DOT-4 CLASS-4 is the worst fluid you can use. I've read tests on brake actuation where with CLASS-6 fluid, brake response is 120ms, but over 350ms with standard DOT-4.
The 200ms disparity in automatic emergency braking should be obvious. From a performance standpoint, a lot of us push the car into ABS. Calipers can apply and release 15 times per second. CLASS-4 fluid can't meet that kind of response time, and performance is degraded, lengthening stopping distance under ABS. I don't know about other markets, but the JDM RZ High Performance, when in "Expert" mode uses the brakes for torque vectoring when the system senses tire slip. e.g. when exiting a corner on power and the inside tire lifts and starts to slip, making the Torsen differential essentially an open differential, the system will apply brake to the tire with loss of traction, sending power to the outside tire. A 200ms delay in brake application might as well be a lifetime.
DOT-5.1 ISO 4925 CLASS-5.1 viscosity is 900 cSt, a dry boiling point of 260°C and a wet boiling point of 180°C. IMO, this is the only fluid upgrade for the system, as the viscosity is close to CLASS-6 and boiling point is a bit higher. Also note that CLASS-6 fluid has a higher boiling point than standard DOT-4 CLASS-4. DOT-5 ISO 4925 CLASS-5 (silicone) fluid has the same viscosity and boiling point as DOT-5.1, so really no point.
I also see a standard for ISO 4925 CLASS-7 fluid, which has the boiling point of DOT-5.1, but the viscosity of CLASS-6. However, I haven't seen it for sale yet.
Cheers.
I did a search on here and didn't see anything on it (surprising), so I'll post if for anyone looking.
The major difference between standard DOT-4 (CLASS-4) and DOT-4 CLASS-6 is the viscosity.
DOT-4 ISO 4924 CLASS-4 fluid has a viscosity of 1,800 cSt, a dry boil temp of 230°C and a wet boil temp of 155°C.
DOT-4 ISO 4925 CLASS-6 fluid has a viscosity of 750 cSt, a dry boil temp of 250°C and a wet boil temp of 156°C.
Why does the viscosity matter?
In cars with ABS and ADAS systems like EHB, standard DOT-4 CLASS-4 is the worst fluid you can use. I've read tests on brake actuation where with CLASS-6 fluid, brake response is 120ms, but over 350ms with standard DOT-4.
The 200ms disparity in automatic emergency braking should be obvious. From a performance standpoint, a lot of us push the car into ABS. Calipers can apply and release 15 times per second. CLASS-4 fluid can't meet that kind of response time, and performance is degraded, lengthening stopping distance under ABS. I don't know about other markets, but the JDM RZ High Performance, when in "Expert" mode uses the brakes for torque vectoring when the system senses tire slip. e.g. when exiting a corner on power and the inside tire lifts and starts to slip, making the Torsen differential essentially an open differential, the system will apply brake to the tire with loss of traction, sending power to the outside tire. A 200ms delay in brake application might as well be a lifetime.
DOT-5.1 ISO 4925 CLASS-5.1 viscosity is 900 cSt, a dry boiling point of 260°C and a wet boiling point of 180°C. IMO, this is the only fluid upgrade for the system, as the viscosity is close to CLASS-6 and boiling point is a bit higher. Also note that CLASS-6 fluid has a higher boiling point than standard DOT-4 CLASS-4. DOT-5 ISO 4925 CLASS-5 (silicone) fluid has the same viscosity and boiling point as DOT-5.1, so really no point.
I also see a standard for ISO 4925 CLASS-7 fluid, which has the boiling point of DOT-5.1, but the viscosity of CLASS-6. However, I haven't seen it for sale yet.
Cheers.
