chaos.lrk.ar.us!dave.williams (Dave Williams)
hotrod  23 Jun 1993
- -> Now there is alot of hype about the hyperuetectic (sp?) or high
  -> silcon steel pistons.  What's the deal here?  Are these a "Good
  -> Thing" or a "Bad Thing"? Failing that generalization ... are they
  -> good for some applications and bad for others?
- They're stronger than conventional castings, not as strong as forgings.
  The high silicon content adds strength and resistance to heat.
  (silicon, BTW, is the primary incredient of ordinary dirt, and when the
  chunks are big enough, you call it sand.  High-silicon aluminum alloys
  can take the edge off carbide cutting tools astoundingly fast!)
- TRW publishes plenty of hate material about hyper-eutectic pistons, but
  they're coming out with a line of their own.  For a sprint car, Pro
  Stocker, or NASCAR racer, I'd still want forgings.  I don't have any
  problem with the HE pistons on the street.  Heck, I seldom have trouble
  with plain old cast pistons.
- -> Anywa, I saw some of the pistons ... one was actually missing pieces
  -> of the outer edge and the others all showed some degree of cracking
  -> or breakage.
- That type of damage is typical of detonation.  As a S.W.A.G., I'd
  figure the HE pistons might be a little more brittle than conventional
  castings.  From what I remember proper heat treating would take care of
  this under normal conditions, but "heat treat" and "piston" aren't
  really complementary terms.
- High-alloy aluminum really isn't an alloy.  Technically, it's a
  solution, particularly with silicon and zinc alloys.  Given enough time
  the additives will precipitate out into pockets, which will weaken the
  structure.
- Still, it sounds more like a detonation problem than an actual piston problem.
- -> machine shop set them up to"normal" clearances, somewhere around
  -> 0.020".   (In fact, NO literature of any kind was in any of the boxes
- HOLY CRIMINY!  You should be looking at somewhere around .0025-.0045
  for clearance.  +.020 is first overbore if you can get pistons that
  size.  I guess it wouldn't blow too much oil if the rings were the right
  size, but the piston was probably rattling around there like a marble in
  a Cuisinart.  Big clearances went out of style thirty years ago, but I
  guess some shops haven't been clued in yet.
- .010 sounds awfully wide, too.  Ford recommends .0025-.004 for cast
  pistons, a couple thou more for forgings.
- My Federal-Mogul HE pistons didn't come with directions either, so I
  set things up to standard Ford clearance.  My brother bought a set of
  the Silv-O-Lites for his drag motor.  In all fairness, the Federals are
  pretty cobby looking.  They look like they were cast in used kitty
  litter.  The Silv-O-Lites by comparison are machined all over and
  apparently used permament molds for the inside.  It's a good-looking
  part.  I would have got them instead, only the Federals are specials
  with Cleveland comes and Windsor pins.

[email protected] (Dave Williams)
mustangs  12 Dec 1993
- -> Hypereutectic aluminum could in fact be difficult to forge.  The
  -> hypereutectic nature would cause large amounts of virtually pure
- The idea behind hypereutectic pistons is to *not* be forged, yet be stronger 
  than ordinary castings.  Forged pistons tend not to be dimensionally stable.  
  The forging process leaves residual stresses which don't seem to be affected 
  much by subsequent heat treats.  When forged pistons get hot, they warp.  
  Forgings thus must be installed with wider running clearances, which cause 
  ring sealing problems (piston cock) and oiling problems (clearance between 
  piston and bore). 
- Castings are much more stable, right up to the point where they melt. 
  Unfortunately forgings are physically stronger and better able to take 
  occasional past-redline excursions and the odd bit of detonation. 
- Hypereutectic castings are about midway between average forgings and average 
  castings in terms of durability.  For a generic hotrod motor, if you're not 
  running into piston-related problems due to detonation you can probably use 
  hypereutectics anywhere you would use forgings.  The hypereutectics will 
  seal better, last longer, and make slightly more power than forgings.  
  They're also a lot cheaper.  If you're running a big nitrous system or a 
  blower and rattle the engine a bit every now and then, you'd be better off 
  with forgings.  If you're building a 7500-RPM screamer that needs the 
  lightest piston you can get, you need forgings. 
- You'll be better off with hypereuctectics anywhere you don't *need*
  forgings.