What’s the Difference Between DOT 3 and DOT 4 Brake Fluid?

John Baker|Feb 27, 2018 10:12 AM

The primary difference between Dot 3 and Dot 4 is their respective boiling points.

I suspect I know your next question.

But first, some background. The U.S. Department of Transportation classifies brake fluid into four main categories:

DOT 3
DOT 4
DOT 5
DOT 5.1

Their primary differences are their wet & dry boiling points and their composition.

DOT 3 is the most common type used in cars and trucks today. DOT 4, however, is gaining popularity due to widespread use of anti-lock braking systems and traction control, which benefit from DOT 4 fluid’s lower viscosity.

DOT 4 is compatible with DOT 3, but features a higher boiling point. DOT 5 is silicone, meaning it doesn’t absorb water. It’s not compatible with the other brake fluids and is used mostly in classic cars that remain in storage for long periods and need a brake fluid that doesn’t absorb water. DOT 5.1, meanwhile, is used in high-performance and heavy-duty applications due to its high boiling point.

Bring it to a boiling point

So now we’re back to boiling point. What does it mean? After all, we’re not cooking this stuff.

Well, in the right operating and ambient conditions, you are cooking it.

Aggressive braking can lead to intense heat.

Braking generates intense heat between the brake pads and rotors. Maybe you’ve seen a race on TV where the producers stick a GoPro under the car to show the brakes literally glowing red when the driver depresses the pedal. The intense heat can vaporize the brake fluid, causing it to become compressible, which leads to a spongy feeling when you apply the brakes.

Braking also places the fluid under intense pressure, potentially causing the fluid to boil. That leaves gas in the lines, which is compressible, leading to a soft pedal. In racing and performance-driving circles, this is known as brake fade, and it’s something drivers actively want to avoid. To drive as effectively and safely as possible, the driver must be confident that the brakes will perform on lap 10 as they did on lap one.

Brake fade can also come from the brake pad/rotor interface. The pads release gasses as well, which reduces contact between the pads and rotors. That’s why high-end rotors are slotted and drilled – to release gasses quicker, limiting fade.

Brake fade isn’t just for racers

Brake fade can affect nearly anyone. Descending a steep hill, especially when hauling a heavy load or towing a trailer, can generate tremendous heat if you ride or pump the brakes.

PRO TIP: Next time, downshift into a lower gear before descending a steep incline.

By the time you reach the bottom, your pedal may go nearly to the floor, making your heart rate go nearly through the roof.

If you like to toss your vehicle around a curvy country road for a little therapy, standing on the brakes going into corners can create sufficient heat to cause brake fade, too. If you get a little too zealous, you may end up going right through a corner and into the woods.

The fluid’s boiling point indicates the temperature at which the brake fluid vaporizes. The higher the DOT classification, the higher the boiling point, thus the better the fluid is at resisting heat. That’s why racers use DOT 4, not DOT 3, brake fluid.

Boiling point is separated into dry & wet boiling points

The dry boiling point is determined using fresh fluid straight from a new container. The fluid’s wet boiling point is determined using fluid that’s been contaminated with 3.7 percent water, thus it’s always lower than the dry boiling point. Why would test administrators contaminate good fluid? Because it’s a reflection of what happens in the real world.

Brake fluid is hygroscopic, meaning it absorbs water (except silicone-based DOT 5 brake fluid). DOT 3 fluid, for example, can absorb up to two percent water every year. Moisture can enter the system when you remove the reservoir cap to add fluid, through worn seals and even through the rubber brake lines themselves. Thus the fluid’s wet boiling point is the number that more accurately represents what’s really going on in your vehicle.

Which makes it important to periodically flush the brake system and replace the fluid to remove moisture. Otherwise, not only will your brakes become spongy and unsafe, the moisture will slowly corrode metal components.

A good rule of thumb is to change the brake fluid every other year in passenger vehicles, and at least every year in racing vehicles. The AAA says 88 percent of motorists overlook brake maintenance, so you’re not alone if you haven’t changed brake fluid in awhile, like since you bought your vehicle.

It’s not too late to start, though. And when you do, check out our line of brake fluids for your vehicle. The easiest way to determine the correct brake fluid for you vehicle is to use our Product Guide.

Find AMSOIL Brake Fluid for my Vehicle

Sioux Falls Webmaster note: AMSOIL’s Dot 4 outperforms it’s new 5.1 because the 4 is designed purely as a racing brake fluid where as the 5.1 id the latest for all uses yet still beyond the minimum standard.

Why You Should Be Wary About Using Break In Oil in Powersports Equipment

John Baker|Sep 25, 2017 1:32 PM

Marco Navarro asks on our Facebook page about break in oil, with attention paid to powersports engines. (Break in oil importance, drain interval on it, and applications. To include motorcycles and ATVs since life of engine is shorter and rebuilding occurs more often.)
Thanks for the question, Marco.

Let’s get to it.

Maintaining an engine is a constant fight against wear. Over time, wear not only results in expensive damage, it reduces compression, robbing your engine of power.

That’s why it can be tough to accept that “controlled wear” during a new or rebuilt engine’s break in period is critical to maximizing its power and longevity.

Take a seat

One of the primary reasons to break in an engine is to seat the piston rings, and that means allowing the rings and piston skirt to carefully wear down the peaks (called asperities) on the cylinder wall.

The images show what we mean.

Although a new or freshly honed cylinder appears smooth to the naked eye, it contains microscopic peaks and valleys. If the valleys are too deep, they collect excess oil, which burns during combustion and leads to oil consumption.

The sharp peaks, meanwhile, provide insufficient area to allow the rings to seat tightly. That means highly pressurized combustion gases can blow past the rings and into the crankcase, contaminating the oil and taking potential horsepower with it.

Breaking in the engine wears the cylinder-wall asperities, providing increased surface area for the rings to seat tightly. The result is maximum compression (i.e. power) and minimum oil consumption.

Getting the shaft

That brings us to the other primary reason to break-in an engine: to season, or harden, the flat-tappet cam. Flat-tappet cams can wear out faster than their roller-cam cousins, especially in engines modified with high-tension valve springs.

And cam wear is bad. Really bad. Worn lobes or tappets affect valve lift and duration, which reduces engine power and efficiency. In extreme cases, increased pressure can remove material from the lobes and deposit it in the oil, where it circulates through the engine and causes damage. Break-in helps harden the metal so it’s more resilient to wear.

That raises a critical question: How do we simultaneously allow controlled wear to the cylinder wall/piston rings while protecting the cam against wear? Those two tasks seem mutually exclusive.

In a word, oil

The solution is to use a properly formulated break in oil that allows controlled wear at the cylinder wall/piston interface, but that also protects the cam lobes and tappets from wear.

How do we accomplish this black magic?

Most break in oils, including AMSOIL Break-In Oil, use conventional base oils. Compared to their higher-quality synthetic counterparts, conventional base oils result in a thinner, less durable protective oil film on engine parts. The thinner fluid film allows controlled wear at the cylinder wall/ring interface.

But what about the cam? Won’t it wear, too?

ZDDP

That’s where anti-wear additives come into play. ZDDP anti-wear additives are heat-activated, meaning they provide wear protection in areas of increased friction. In this case, it’s at the cam lobe/tappet interface. The additives form a sacrificial layer on the surface of parts, which absorbs contact and helps prevent cam and tappet wear.

As a rule of thumb, a good break in oil should be formulated with at least 1,000 ppm ZDDP. At AMSOIL, we take it a few steps further; our Break-In Oil contains 2,200 ppm zinc and 2,000 ppm phosphorus.

How long does break in require?

Another rule of thumb states you should season a flat-tappet cam by running the engine above 2,500 rpm for 15 minutes.

As for seating the rings, our testing has shown it can take as little as seven dyno passes. That time varies depending on the engine, ring tension, cylinder hone and other factors.

If you don’t have access to a dyno, follow the engine builder’s or manufacturer’s recommendations. If none are provided, consult the recommendations on the break in oil label. In general, run the engine under light-to-moderate loads for about 500 miles. Again, that duration is a rule of thumb, but break in shouldn’t exceed 1,000 miles. Then, drain the break-in oil, install the synthetic oil of your choice and commence driving.

An engine dyno provides the best method of determining exactly when the rings are seated. You’ll notice a boost in horsepower as the rings seat. Eventually, horsepower will stabilize once the rings are seated.

Check out 5 Ways to Boost Horsepower for Under $500

You can also perform a leak-down test. Another, albeit more time-consuming, method is to remove the exhaust headers and check for oil residue in the exhaust ports. Presence of oil shows the engine burning oil, meaning the rings aren’t completely seated. Once the oil residue is gone, the rings are seated.

What about powersports engines?

Ask yourself a few questions about your motorcycle, ATV or other powersports application before using a break in oil:

Does it have a wet clutch? If so, the break in oil may not be formulated for wet-clutch compatibility, leading to reduced performance.
Does it use a shared sump with the transmission? Many motorcycles use one oil to lubricate the engine, transmission and primary chaincase. The churning action of transmission gears, especially in high-rpm applications, can tear apart – or shear – the oil if it’s not formulated to handle the stress. Using a break in oil not designed to handle high-shear applications can lead to damage.
Does it have a dry sump? Some motorcycles store motor oil in a tank separate from the engine. Residual break in oil can collect in the system following the break in period and contaminate the service-fill oil. In this case, run the engine long enough to circulate the oil throughout the system and change it a second time to ensure the break in oil is completely removed.

Given the above challenges, we recommend breaking in a rebuilt powersports engine using the motor oil you’ve always used. Run it according to the original equipment manufacturer’s (OEM) new-engine recommendation, then change the oil. In short, treat it like a new engine from the factory.

For new engines, just follow the OEM guidelines. Typically they recommend a shorter interval for the first oil change to remove wear particles and contaminants from the factory. Then, change to the AMSOIL synthetic motor oil that’s recommended for your application and commence riding.

Category: Maintenance