Why hyper piston?

[JVMopar]

Besides the cost factor, why is everyone running hypereutectic piston? I've heard horror stories about exploding pistons because of detonation. Why not just run a cast piston or forged piston?

[451Mopar]

Most off the lowcost cast or forged pistons are designed for service replacement parts, so if your working on a 318, 360 or 400 block your usually stuck with low compression replacement pistons. The 340, 383, and 440 are a bit better in piston selection becuase they came stock with higher compression in some applications. Because most are for stock replacement, the are designed to weigh the same as stock so the engine wouldn't need to be re-ballanced.

Now consider most of the hypereutectic are designed more for a performance engine, but at a reasonably low cost (compared to forged performance pistons, they are about 1/2 the cost.)
Some benifets of the new hypereutectic pistons is that most have compression heights that will give a zero deck height for more compression. Many even have domes for higher compression ratios. Some have "Quench" domes that make the open cylinder head design more efficent (if everything is built right.)
They also usually have large valve notches so you can run a bigger cam without bouncing it off the top of the piston.
Usually they are lighter than stock which is good for performance and reducing stress on the rods. The hypereutectic pistons don't expand much so they can be ran with tight cylinder wall clearances which reduces noise. Forged pistons usually are louder because of the larger cylinder wall clearances.
The hypereutectic pistons are also stronger than cast pistons and work well for many performance applications. Forged pistons are stronger, but they cost more and are noisier as mentioned above.

Keith Black also added an accumulator? between the first and second rings which is suposed to makes the rings seal better.
They also moved he top ring closer to the top of the piston which is supposed to be more efficent, but also caused the top ring to expand more which can cause end-butting if the top ring is not gaped correctly.

[mtrv8n]

hypertuetic pistons expand less, so they can be fit tighter for less blowby, therefore yielding more power.

This is theory, as some builders get power by fitting really loose and using vacuum scavenging to seal the rings.

[gthomas]

Heres a pretty good reason not to run "hyper" pistons in MOPARS with open chamber heads:

http://www.moparchat.com/forums/atta...?postid=247718

Click on the pic I included and see what happened on a hot summer day and a quick run up a freeway ramp did to my pistons.

These were TRW "hypers". If you do a search on the net for "hyper" pistons you'll come up with quite a few negative comments. One of the cautions mentioned in the literature when buying "hyper pistons" is NEVER allow the engine to experience predetonation. At the time I used these, they were supposed to be a new technology that were beneficial to performance. Yeah benefical to building another engine. I didn't use the "hypers" to save money, because money wasn't ever the deciding factor when I built this engine. I wouldn't take a set of them for free because they won't work on the street with 10:1 compression and warm SoCal weather.

[amxauto-x]

You also have to be VERY carefull of the quality of gas when using these pistons. I can't remember what all was said about them. But when traveling be very carefull that the premium fuel you get at an country station is really hi test! And always carry some additive.

[gthomas]

Even additives didn't help this time, I was running at least 10 times the recommended dosage and was running fuel from my usual source. Problem with most additive boosters is the recommended dosage improves you only .1 octane so you generally have to add quite a lot to (in theory) boost it by one whole point.

With my new engine (forged pistons of course) I run 5 gallons of 101 racing fuel mixed with 11 gallons of 91 octane, combined at I believe a little over 94 octane.

[hedgehog]

Hypereutectic pistons are stronger than eutectic and hypoeutectic pistons due to the increased silicone content. However, they are also are more sensitive to detonation. What happens is the hypereutectic pistons reflect more heat back into the chamber increasing the combustion temperature and effectively increasing the octane requirements for the engine over other pistons with the same compression ratio. So in this instance that could make the difference between one engine detonating with hypereutectic and the other engine not detonating with forged pistons, both with the same static compression ratios. The relief between the first and second ring groove reduces gas pressures between the rings to reduce or prevent ring flutter at higher rpms. However, this can be done by running a wider gap on the second ring.