Hypereutectic Pistons

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 piston alloys.


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 application.


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. Even Wikpedia acknowledges poorer resistance to knock with Hypers.


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, longevity, 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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