DOT 3/4/5/6?

[Brad]

I am getting new slotted rotors soon and I was offered DOT5+ fluid, however I got to wondering, what is the real-life benefit of these when I am only using a road/race pad?

A lot of people talk about running DOT4+ etc in their cars as it's better than DOT3, however what is the actual physical propery which makes it better? I have heard that they have increased boiling points.

That said, what about DOT5/6etc, would there be any tangible benefit of these. By this I mean, would DOT4 be give stronger braking than DOT5/6 in a normal situation or do you have to really get the brakes steaming to the point where DOT4 is at its boiling point for DOT5/6 to be worth it?

 

[Cant_get_enough_of_peugeots]

Lets look at what the DOT ratings mean.
The list below shows the MINIMUM wet boiling points for DOT 2, 3, 4, and 5 brake fluid in degrees fahrenheit.

Dot 2 248 degrees f
Dot 3 284
Dot 4 311
dot 5 356

The DOT 2 spec is for drum brakes and is
obsolete. If you have any DOT 2 in your garage, throw it away! DOT 5 is for silicone brake fluid. Silicone brake fluid (DOT 5) should be avoided because it is not compatible with regular brake fluid, it is hard to pour without introducing bubbles and thus results in soft pedal feel, and moisture still gets into your system and will pool in low areas like your calipers and encourage rapid corrosion. STAY AWAY!

That leaves DOT 3 and DOT 4 fluids. These fluids are compatible with each other and may be interchanged or mixed with no ill effects.

Let's look at some popular brake fluids and their boiling points:

Fluid DRY WET
Castrol LMA DOT 3/4 (446/311)
Ford Heavy Duty DOT 3 (550/290)
ATE Super Blue Racing (536/392)
ATE TYP 200 (536/392)
Motul Racing 600 (585/421)
Castrol SRF (590/518)
Performance Friction (550/284)

The amount of moisture in brake fluid definitely affects its performance. The big problem is it is absorbs moisture quickly. Over a relatively short period of time brake fluid will absorb moisture from the air. SAE field tests have shown that the average one year old car has 2% moisture in the fluid. A random test of vehicles in the U.S. showed an average water content of 2.6% for vehicles with an average age of 8 years. And 25% of these vehicles had water content greater than 4%.

As water content in brake fluid increases over time, the boiling point decreases. Fluid with a reduced boiling point (or high water content) can create vapor by boiling in the caliper, or wheel cylinder. The result is sudden brake failure. And water in the brake fluid can contribute to corrosion of parts such as steel pistons and ABS modulators.

The end result is even though DOT 3 fluid is "rated" at greater than 401oF, in the typical 3 to 4 year old car with 3 to 4% moisture content, it could boil under 300oF. And if it has got more than 4% moisture, you may as well be running straight water!

Moral: Flush your brake fluid every year or so. But only if you would like it to work well scarcely an inch away from those toasty 500oF rotors on your SHO during a couple of hard stops! Or would you rather have a squishy pedal?

Regards
Adrian Colman

 

[tekkie]

Brad

more reading about <a href="http://www.shotimes.com/brakes/part5.html" target="_blank">Glycol vs Silicone fluids.</a>

<a href="http://www.minimania.com/MM/BRAKES___Silicone_brake_fluid__to_use_or_not_to_use___that_is_the_question__785.htm" target="_blank">More here</a>
"In conclusion then, just because silicone brake fluid is relatively expensive in comparison to its glycol-based counterpart - like fully synthetic high quality engine oils are in comparison to mineral oils - it does not mean that a higher performance will be achieved, as is the case for the oil. And just to underline this, neither of the major brake specialist companies in England produces silicone-based brake fluid. In fact they are most emphatic in their condemnation of it."

From what Paul Vassalo says Dot4+ fluid is pretty much what you need for day to day driving with occasional race track days. Dot5 and silicone fluids require constant maintenance.

I hope it helps, the performance forums is your link to more information.

cya
Voytek

 

[CHRI'S16]

Real life benifits are only that Dot5 and higher requires more service/flushing as it also tends to absorb water/moisture quicker too. Thats what ive been led to beleive, i can get 2-3 race meets from the EBC DOT5, and thats 2-3 whole days, abit like 4-5 hours of constant Old Pac Runs lol... Xq

 

[Pug307]

Don't most car manufacturers use DOT4+ these days?

 

[XTC]

Pug307:
Don't most car manufacturers use DOT4+ these days?

Just about everything I've seen is DOT4, the DOT5 comments are spot on, brake bleeds were a regular thing on the rally car, and ensuring all air was removed required a lot of patience or a special power pump (it literally sucks everything through the system).

XTC206

 

[Pugnut403]

As I understand, the higher the DOT rating, the higher the boiling point, but also the faster the fluid absorbs water. Sililcone fluid will not absorb water, hence the pooling and subsequent corrosion.