There's a lot of talk about Hypereutectic pistons
and their applications in performance engines, and
we get asked about them a lot. I put this page
together to share our view on them.
A hypereutectic piston is a cast piston that
contains a silicon content above 12%. Silicon levels
above 12% do not fully dissolve in the aluminum and
thus create hard particles, contributing to
increased hardness and wear resistance of the alloy. The
harder material offers some
opportunity for weight savings versus other cast
Similar to the factory cast pistons, a hyper's
coefficient of expansion is lower than that of a
pure forging, and as a result, they can be fit
tighter than a forging. Typical piston to cylinder clearance for a
cast piston, whether hyper or not, is .0006 to .0017.
By comparison, forgings are typically
fit anywhere from .0020 to .0030 in a Harley engine, depending on the
The combination of a harder material that can be fit
tighter is touted as an advantage, because in theory
fitting a piston tighter promotes
good ring seal, particularly when cold, and to do
this with a harder material simultaneously provides
good wear resistance and long service life. Furthermore,
a cast piston generally costs less to manufacture
than a forging, once the molds are paid for. This is
why the vast majority of OEM high volume production
pistons are cast rather than forged.
So why don't we use hyper pistons? Well, despite the
aforementioned advantages, there are
some characteristics of hyper pistons that we don't
think work well in air cooled Harley motors. In
particular, the hyper material doesn't shed heat
well. This forces certain compromises when used in
an air cooled motor and we don't think the
compromises are worth it.
Because the hyper material holds heat so badly, the
rings try to get really hot. The hyper piston makers
try to address this by asking tuners to take timing
out of the motors, and also moving the ring pack
down the piston, farther from the combustion
chamber. They also generally specify larger ring gaps to
try to avoid ring butting, but unfortunately, ring
butting and the subsequent damage is not nearly as
rare as it should be when using hypers in a Harley
motor. When a ring butts hard, it sticks the piston
in the bore. It's not at all unheard of for the
piston to literally rip the pin boss out of the
bottom of the piston when this happens.
The above was assembled by
an outstanding mechanic and fellow salt flats racer with a great deal of
experience with hypers. This is a guy who's anal
about building motors; he checks everything.
he's both a fierce competitor and one of the very
few people I would personally trust to assemble a
motor for me if I were inclined to do such a thing.
He told me point blank that
he would not be installing any more of them after he
saw this. This is the most popular brand of hypers
for Harley motors, too.
An even wider ring gap may have avoided this, but
this negates one of the supposed advantages of a
hyper, good ring seal during warm up.
The other problem we've seen with hypers is that
they're just not tolerant of detonation
the way a forging is. The material is hard but also more brittle,
and breakage between the bottom of the valve pocket
and the top ring groove isn't nearly as rare as it
should be, despite the lower ring pack. So for those
of us who like to push the envelope, whether
normally aspirated or boosted or using nitrous, hypers are not a good choice at all.
We're not the only ones who've seen this, either.
Wikpedia acknowledges poorer resistance to knock
Finally, hypers are often more difficult for the
do-it-yourselfer to install, because the lowering of
the ring pack may put the wrist pin into the oil
ring groove, depending on the specific application. The simple method of installing pistons
by first preassembling them into the cylinders and
then putting the cylinder in place and sliding the
wrist pin through won't work on these type of
pistons. You must use a ring compressor to slide the
cylinder down over the supported piston while the
piston is already attached to the rod.
Bottom line, at first glance the hyper seems to
offer some advantages in the way of quiet operation,
and low cost. But in the real world, in high
performance applications, we've found the
disadvantages to outweigh the advantages. When I've
got a street motor that's built on the verge of
being able to use pump gas, and I get caught in slow
traffic on a hot day, I'm a whole lot more
comfortable knowing I've got forged pistons banging
around in my motor. Your mileage may vary of course,
and we have no doubt that there are many builders
who disagree with us, but this is where we stand.
If you're determined to use hypers in your project,
we can certainly provide them for you, and we can
precisely bore and hone your cylinders for the
correct fit. We highly recommend, though, that you
get the domes coated with a quality thermal barrier
coating. That'll give the pistons a better chance of
survival. Also make sure you tune the bike right,
as all our data indicates this type of piston leaves less margin for error.
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