What are the differences between DOT3 and DOT4?
2 Answers
DOT3 and DOT4 have different compositions: 1. DOT3 is generally glycol-ether based: The chemical components of glycol-ether type are polyethylene glycol or propylene glycol. Polyethylene glycol or propylene glycol has strong hydrophilicity, so its water content will gradually increase during use or storage. Since the boiling point of brake fluid decreases as the water content increases, its braking performance will decline accordingly. 2. DOT4 is generally ester-based: The ester type adds a large amount of borate ester on the basis of the glycol-ether type. Borate ester is formed by the esterification reaction of polyethylene glycol or propylene glycol with boric acid. The boiling point of borate ester is higher than that of polyethylene glycol or propylene glycol, so its braking performance is better.
As a long-time car repair enthusiast, I must say the main differences between DOT3 and DOT4 lie in boiling points and moisture absorption. DOT4 has a higher dry boiling point, around 230 to 260°C, and its wet boiling point also increases to approximately 155°C, while DOT3 only reaches about 205°C and 140°C respectively. This means DOT4 is less prone to boiling under high temperatures, making it safer during scenarios like summer hill climbs or emergency braking. Additionally, DOT4 absorbs moisture faster due to its polyglycol ether-based composition, offering slightly better moisture resistance, but it requires more frequent changes—check every 1 to 2 years. In contrast, DOT3 absorbs moisture slower, allowing maintenance intervals up to over 2 years. Compatibility-wise, DOT4 can be used in DOT3 systems, but never mix it with mineral oil-based fluids to avoid foaming risks. Overall, for high-performance cars or mountain driving, DOT4 is the safer choice; for daily commuting, DOT3 offers better cost-effectiveness. Always remember to test moisture content regularly—safety comes first.
