| The What and Why of Hypereutectic Pistons |
|
|
|
 Spend much time talking about engines and eventually the subject of pistons will come up. Often referred to as "hyperpathetic" pistons, hypereutectic pistons are found in virtually every car on the road but what are they, why do people seem to dislike them for performance applications, and where did they get that strange name? First things first. What does that name mean? Many people think that it is descriptive of the shape of the piston (much in the same way that "parabolic hyperboloid" is descriptive of the shape of a Pringle) but it is nothing more than a metalurgist's term that means "over saturated." In this particular case aluminum is heated up and silicon is added to the mixture until, somewhere around 12 percent, no more silicon can be mixed in and it starts floating around in the resulting alloy as chunks of actual, granular silicon. The easiest way to think of this is sweet tea: You can add lots of sugar to tea but there comes a point where the tea will no longer accept any sugar, no matter how vigorously you stir it. It just floats around in the tea and is clearly visible if you hold the tea up to a light source. It should be noted that in my opinion this is almost sweet enough. The “4032” performance piston alloy has an approximate Silicon content of 11%. This means that it expands from heat less than a piston with no Silicon, but since its eutectic level of Silicon is fully alloyed on a molecular level, this alloy is less brittle and more flexible than a stock Hypereutectic “smog” piston. These pistons can survive mild detonation with less damage than stock pistons.  The reason silicon is added at all is because aluminum with a high concentrate of silicon doesn't expand at the 600 degree temperatures that your car engine tends to generate inside its cylinders. This allows car manufacturers to design engines with extremely close tolerances, thereby increasing performance and economy while decreasing emissions. Overall it is a very good compromise.When someone discusses hypereutectic pistons usually they are talking about factory standard cast pistons. Many people aren't aware of this but there are forged high performance hypereutectic pistons as well. So probably a short discussion of the differences between forged and cast pistons is in order. In the most basic sense a cast piston is one where the alloy is heated until it is a liquid and then is poured into a cast and allowed to cool. A forged piston is left as a solid and then literally hammered into shape. A forged piston is stronger but only very rarely will you find them as installed factory equipment. In fact you will usually only see them in cars that are factory turbocharged or supercharged. The reason for this is that a cast hypereutectic piston generally does not survive a condition called detonation. Detonation is when the fuel in an engine's cylinder ignites prematurely, and is also called "engine ping." There are a number of things that can cause detonation such as bad gas or timing being set too far forward but in a performance application detonation is a danger that is usually related to forced induction, such as turbocharging and supercharging. Forcing air into an engine's cylinders usually heats that air as it is compressed making for a much more volatile and easy to ignite fuel air mixture and lends itself to detonation if the timing is not retarded sufficiently to take this into account. Hypereutectic pistons are actually very strong, but that strength is gained at the cost of flexiblity. As such, when a cylinder detonates into a hypereutectic piston it tends to either crack the piston (rarely) or cause the piston to twist slightly, transferring damage to the rings and the connecting rods.  The situations where detonation is most likely to occur are: low octane fuel; too far advanced timing; high cylinder temperatures; "lugging" the engine by giving it too much pedal in too high of a gear.Overall hypereutectic pistons are a very good choice for modern passenger cars, and they also provide sterling service in normally aspirated performance applications as well. Caution must be observed when these types of pistons are used in a forced induction motor (nitrous charged motors as well) to ensure that the danger of detonation is reduced as much as possible or these pistons will generally self-destruct. The best ways to avoid detonation are to run a higher octane fuel, add an intercooler, run "colder" spark plugs, and pay attention to what gear you are in to ensure that the engine is "free revving" if it is under boost. And that's where the bad reputation comes from. If you take a stock motor and slap a nitrous kit on it set at the kit's "big shot" without paying attention to your timing, cylinder temperatures, fuel and engine conditions you are likely to find yourself with a motor that is starting to blow blue smoke after a few runs. Then it starts to use lots of oil, then it starts to knock and finally the motor self destructs. If someone takes the time to pull the heads they will find cylinder damage and then the blame gets put on "hyperpathetic pistons."             Comments (0)
 Write comment
|
| < Prev | Next > |
|---|
