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What is ZDDP?

Zinc dialkyldithiophosphate (ZDDP) is the most commonly used anti-wear additive in motor oils. It contains both zinc and phosphorus components that work together to provide anti-wear protection and minimize lubricant breakdown. ZDDP also exhibits mild extreme-pressure protection. Some motorists think that increased zinc content equals increased wear protection. Although somewhat true, the statement can be misleading. First, the mere presence of zinc doesn’t mean it is in the form of ZDDP. In other forms, zinc offers additional oxidation protection but little wear protection. Second, other factors influence the oil’s ability to control wear, such as its viscometrics and base stocks. While zinc is important in protecting against wear, the greatest concern should be in using a well-balanced oil that is designed for its intended application.

How ZDDP Works

As temperatures rise and surfaces come closer together, ZDDP decomposes, and the resulting chemistry protects critical metal surfaces. When parts move during operation, any sliding or rolling motion takes place on top of or within the ZDDP anti-wear film, which reduces metal-to-metal contact. This is especially important in modified engines with flat-tappet camshafts because the engine is creating more horsepower than it was designed for, which puts more stress on the engine. High-tension valve springs, often used in racing applications, also increase the potential for cam wear and require additional ZDDP.

Negative Effects of ZDDP

ZDDPSince all engines benefit from oils with superior anti-wear properties, it seems obvious to formulate all motor oils with high levels of ZDDP. Generally, high levels of ZDDP result in volatile phosphorus being transferred from the combustion chamber to the catalytic converter. Phosphorus can blind over the catalytic reaction sites in the converter, making it less efficient in turning carbon monoxide (CO) into carbon dioxide (CO2). The EPA mandates that catalytic converters operate as designed for more than 100,000 miles. As a result, phosphorus is limited for newer motor oil specifications.
When the American Petroleum Institute (API) and the International Lubricants Standardization and Approval Committee (ILSAC) established phosphorus limits at 0.10 percent weight in 1996, motorists and enthusiasts wondered if it would negatively affect wear protection. The move to reduce phosphorus may not have taken into consideration high-performance and modified engines or engines that had yet to be broken in. The debate intensified in 2004 when the API and ILSAC further limited phosphorus to 0.08 percent, where it remains today.


When it comes to older engines – particularly those equipped with flat-tappet cams – and engines modified for increased performance, the challenges to delivering adequate wear protection become more pronounced.
Flat-Tappet Cams

The design of flat-tappet cams makes them especially vulnerable to wear. As the name indicates, the tappet – or lifter – is flat. During operation the surface of the cam lobe slides rapidly over the surface of the tappet, producing high friction and temperatures. The camshaft and lifters are responsible for triggering the precisely tuned movements of the valvetrain

Without the protective film barrier provided by ZDDP, the cams and lifters wear from the force of operation, negatively affecting cam and valve operation. Because most V-8 engines of the muscle car era came standard with flat-tappet cams, the problem is especially prevalent to classic-car and hot-rod owners.