|Watts Shop Performance
| Virtually all automotive pistons are aluminum, but the quality and
manufacturing processes vary. At the bottom of the pecking order is the
common cast piston. Performance HYPEREUTECTIC cast pistons are
strengthened with additional silicon content in the aluminum brew. This leads
to formation of aluminum-silicon particles as the alloy freezes.
While more costly to manufacture, forged pistons have inherent advantages
over castings in terms of density, real-world strength and durability. Forging
eliminates metal porosity, improves ductility and generally allows the piston to
run cooler than a casting. Within reason, forgings can be lightened without
adversely affecting structural integrity. However, forged pistons expand and
contract more under changing tempertures, so they traditionally require
greater piston-to-wall clearance than cast pistons. In recent years CNC
manufacturing has permitted greater sophistication in skirt design, reducing
wear even as piston-to -wall clearances are tightened up.
One of most important advantages of forged pistons is what happens at
the point of piston failure. Under extreme conditions- like detonation- forgings
tend to "go plastic" and fail gradually. There's generally time to replace them
before the entire engine is toast. Hypereutetics, although relatively strong in
terms of ultimate tensile strength, have less ductility and are prone to failure
when theirs limits are exceeded. On the other hand, custom forgings are
generally more costly.
Forged pistons are generally made from one of several different aluminum
alloys, with each offering different benefits depending on the application. The
two most popular alloys are 4032 and 2618. Speed-Pro typically uses VMS-75,
which is fairly close to 4032- both contain about 11% silicon, which helps ring
groove and skirt durability. These are the best choice for applications
expected to have decent longevity, such as street vehicles and entry- level
bracket racing and oval track combos. Although 2618 has better high-
temperature charcteristics, it contanins virtually no silicon; 2618 expands and
contracts more, so greater bore clearances are needed to prevent scuffing.
Pistons using 2618 are best suited to nitrous, blowers or high- end race
applications where frequent inspection and replacement are not a problem.
A recent innovation is "Ultralloy", A patented ceramic- aluminum alloy
presently available. Ultralloy's composite products have unprecedented
uniformity in terms of thier size, shape and dispersion in the aluminum martix.
The new alloy's strength is on par with titanium, but parts made from it cost
less and weigh less.