||02-20-2010 07:11 PM
nice little read about it here:
Zinc Oil Additive
The benefits of oils with a higher zinc content, or using a zinc additive.
By Marlan Davis
Zinc Oil Additive
Question: I thoroughly enjoyed the tech article "When Good Cams Go Bad" (June '06), but I have a couple of questions: If you have an older car without a catalytic converter but don't have flat tappets, is there any advantage to using the oils with higher zinc content? Also, I have a '56 Chevy with a Bill Mitchell 427 small-block. It has the aluminum block and heads (my version of a ZL1). Is there any adverse reaction between aluminum and zinc?
Answer: Quaker State engineer Mark Ferner said zinc's extreme pressure qualities also aid in reducing piston ring-to-cylinder bore wear at BDC and TDC, where the rotating assembly sees an abrupt change in velocity and direction. Besides its extreme pressure protection, multifunctional zinc also functions as an antioxidant. That is, things exposed to heat and oxygen are subject to oxidation. In the case of a ferrous metal like iron, oxidation causes corrosion, which we see as rust. As applied to motor oil, Ferner said as the "heat accelerates oxygen, the oil starts to oxidize and thicken, changing into other molecular combinations. As the oil thickens, it forms deposits or varnish. In motor oil, the antioxidant additives first sacrifice themselves to prevent this oxidation from occurring, but if they get overwhelmed, the oil eventually turns into a hard, crusty sludge."
In its antioxidant role, zinc fights heat buildup in various hot spots like the piston-to-piston ring interface and the underside of the piston itself. Oil can accumulate in the honing crosshatch marks in the cylinder bore where it is exposed to combustion temperatures as high as 400-450 degrees F, a prime candidate for oxidation if not protected. Today's modern reduced-zinc passenger-car street oils contain other dedicated antioxidants to make up for the loss of zinc.
The potential zinc downside is that in theory, an engine with excessive blow-by could accumulate zinc deposits in the combustion chamber, potentially becoming a detonation magnet. But Ferner maintained, "You would need a fairly good amount of blow-by to really be bad for the chamber. There used to be 1,400-1,500 zinc ppm in [passenger-car] motor oils, about where [Shell] Rotella [diesel-truck] oil is now. We ran those high levels for years and years and never had a problem with chamber deposits building up. Zinc accumulation mainly affects oxygen sensors and catalytic converters, which are very sensitive to the ash that may accumulate on them."
Regarding a potential reaction between aluminum and zinc, again, in theory, when dissimilar metals come in contact with one another, a phenomenon known as galvanic corrosion can occur. When this happens, the least-noble metal acts as an anode and will corrode preferentially compared with the other, less active, cathodic metal. A common example we've all seen is using stainless steel bolts and washers to retain an aluminum intake manifold. After long-term use, if you remove the bolts and washers, the aluminum surface underneath those washers will be corroded because aluminum is less noble than stain-less steel. A solution would be to use zinc- or cadmium-plated washers instead of stainless washers. The plated steel washers then become the least noble material of the three in contact, sacrificing themselves to protect the critical parts. (Of course, they'll end up looking ugly and need to be replaced, but that's better than ruining an expensive intake.) In the case of zinc additives versus your aluminum block, Ferner reported "no adverse reaction between aluminum and zinc at the levels we are discussing. Fourteen-hundred ppm would be 0.14 percent by weight-it's insignificant." Actually it's even less than that, since "the zinc additive is not free metallic zinc or powder; it's part of a zinc/phosphorous/sulphur compound, so really it's only about 0.06-0.08 percent by weight" of the motor-oil brew.