Whats the difference between a forged and Hypereutic(however its spelled) Piston?

Forged pistons are pressed into a mold with extremely high force while hypereutectic (I don't know how to spell it either) pistons are made with less force like cast molding. The alloy of the hypereutectics have higher silicon which makes them harder and expand less when heated. Keith Black Pistons catalog has a great discussion of there hypereutectic pistons. The forged pistons are lighter.

Joseph

I looked it up in my dictionary/thesaurus one time, it said something about "two different metals one being greater in amount than the other". don't hold me to that definition, I seem to suffer from CRS most of the time. I think it has something to do with the amount of silicon in them. don't know for sure.

Pro,
Here is a good link http://www.kb-silvolite.com/

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

Hypereutectic is the correct spelling. The composite is higher in silicon, like 13 percent. Supposed to make them stronger. They don't expand like a forged piston, so you can't throw them in a loose bore, or they'll rattle. Most new cars use these types of pistons, and most of them have piston slap when they're cold. KB pistons take a bigger top ring gap than any other hyper or forged piston.

If they don't expand much and you install them with tight tolerances why would they slap?


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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

My understanding is that they are nearly as strong as forged, but with the expansion characteristics of cast, that is, you run them tighter than forged pistons and they don't slap. Forged pistons slap when they're cold.

Jim - I agree. I'm running hypereutectics in my new 400 and we set them up nice and tight. No slap so I'm curious about the comment from Dave.



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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

Ever driven a new Camaro or Mudstang???? They rattle like a BB in a boxcar. Even my new LS1 style motor in my truck has a bit of rattle. Got 2 wrecked '98 Z's in the garage that rattle.
I bought a '97 30th anniversary Z28. I got pictures of the motor in a bazillion pieces at 400 miles cause the thing made so much noise it sounded like the rod bearings were left out. It was sent to a local machine shop where they honed it to fit hi-limit (.005) oversize pistons. Set the piston to bore clearance at .0018 and it STILL rattled when it was cold, but at least then it would go away when it warmed up. I'm not real impressed with hypereutectics. It may have been different if not for the fact that the pistons in the LT1 motors are very short on the skirts. I think this allows the piston more opportunity to rock in the bore and cause the slap.
GM even states in their service manual "Any piston noise lasting less than 1.5 minutes is to be considered normal"

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Dave - sounds like I'm fortunate that my pistons don't make a lot of noise - of course, maybe I just can't hear them over the cam and exhaust!

What I was really interested in was the technical reason for slap with these pistons. I understand that forged pistons expand a lot and therefore start out loose in the bores and make noise until they expand and fit right. But hypreuetectics don't expand near as much and therefore theoretically should not have room to move around and make noise.

Do you have any idea why they would slap anyway? Maybe the GM production units have less silicon or are built with more forged piston characteristics? Or they build in more clearance for safety margin (doesn't explain your after rebuild problem..)

Just one of those things that makes me think and try to figure out.

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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

Adding silicon to a piston definitely makes it harder, but there is such a thing as too hard. Forged pistons are tough, yet maintain their ductility, therefore they are the ULTIMATE for pistons. Much like 4340 is considered the ULTIMATE for crankshafts. There are HARDER steels out there, but a crankshaft must maintain some sort of ductility or it will break easily. 4340 is the perfect mix of strength and ductility for crankshafts. Pistons are the same way. A hypereutectic piston is stronger, but if you ever reach its yeild point, expect total disaster. I have used the KB pistons, and have set them up for about 0.002" clearance. I have not noticed any piston noise (of course this was in a drag car with open headers), but I did notice they tend to scuff easily. I think the tighter tolerances do not allow oil to lubricate the skirts well enough. I think for the street cast pistons are fine. I have put cast pistons through hell (sounded like someone playing the xylephone) and they have come through unscaved. For racing, though, I think forged are the only way to go. I have used TRW forged pistons in my street/strip car (ran low 11's), and never had any complaints. Yeah, they were a little noisy, but who cares, you couldn't hardly hear them at 8000 rpm anyway!!!! Just my 2 cents.

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69 SSRS Frame-off Resto
81 Z-28 377ci Drag Car

For 69SS, yes, no question forged pistons are the strongest and most durable, but for street use hyperetics (sic) have a lot of advantages. For Dave, I don't know about piston slapping Camaros, but the LS1 in my '98 Vette is quiet as a church.

I recently built a 305 for the s-10 and choose the HYPER pistons for it. I think the key here is take into consideration the piston to be used and machine accordinly, cause mine makes no noise.


71 Camairo

The aftermatket Hypers may be quieter, but the GM pistons make noise, IMO, due to the fact that the skirt on the side is very short. I was surprised to see how short the pistons were. This probably allows them to rock at TDC making the noise. My 99 Chevy truck with the 5.3 even has some slap in it. It doesn't seem to affect the durability, as I've seen Camaros with 50-60,000 miles that still go, and still rattle.
As a side note, it was also interesting to see that the new powdered metal rods used in the LT1's had a pink paint mark on them. http://www.camaros.net/forum/smile.gif