[email protected] (Dave Williams)
fordnatics  27 May 1996
- -> To be fair, I don't think a forged piston would fare much better
  -> under those circumstances.  In fact I know it wouldn't.
- If you're running lean, aluminum melts at about 1100F, doesn't matter
  if it's forged or cast.
- As for strength... back before I had a hydraulic press, I tried to get
  some pistons off some rods so I could melt the aluminum for casting.
  Knocking the skirts off was easy.  Breaking anything else took the big
  sledge.  However, it didn't make much difference if the pistons were
  cast or forged.  Modern forgings have so much silicon they're about as
  brittle as castings.  Back in the old days, "California" forgings were
  usually made from 2618 alloy, which is a ductile aluminum-copper blend.
  You could beat one of those and it'd just bend.  New TRW and Speed Pro
  forgings use essentially the same alloy as the hypereutectics.  The
  aluminum-silicon has less expansion when heated than 2618, so you can
  fit the pistons closer to the bores, which gives you advantages in less
  noise and better ring seal.  The disadvantage is they're more sensitive
  to detonation now.
- If you order a custom set of pistons you'll get 2618 or similar alloy
  forgings, because that's what the custom piston vendors are set up to
  make.  You'll also be paying about $600 a set.
- Of course, this sort of information is sourced from the piston vendors,
  SAE papers, and my engineering library, so it's just hearsay, as opposed
  to the real scoop you get from the bench racers down at the speed shop...

[email protected] (Dave Williams)
Ford  2 May 1998
- -> I have not seen any info on copper being added to the aluminum in
  -> piston use.  I am not being sarcastic, but where did you see this?
- 2xxx series aluminum, used for some forged pistons, uses copper as its 
  primary alloying additive. 
- -> Forging is the compressive casting of molten aluminum.
- No.  Forging is the shaping of a metal heated until it is ductile, ie, can 
  be forged into the desired shape without cracking.  Think of those old Play-
  Doh toys that formed little cars, toy soldiers, etc. 
- What you just described is a process commonly known as "squeeze casting."  
  Though a few piston makers used to describe it as forging, it's actually a 
  casting process.  Squeeze castings tend to have less porosity than ordinary 
  castings, and are often cast at lower temperatures than ordinary castings 
  since the squeezing (usually a moveable section of the mold) will force the 
  metal into all the required areas. 
- -> This explains why simple "forged pistons" have greater oil
  -> consumption and blow by until warmed up.  The diameter increases as
  -> it warms to engine
- The major problem with older-style forgings is that the internal shape of 
  the piston is bad.  The piston makers used simple one-piece bottom dies 
  which prevented the type of internal contours used in castings to control 
  skirt distortion.  Newer pistons make use of FEA or multipiece dies to 
  achieve shapes less likely to distort. 
- -> In the case of aluminum pistons, silicon is added to assist in the
  -> commercial viability of forgings.
- ?
- -> molten aluminum.  12% silicon is the eutectic.  A eutectic is the
  -> composition which when it melts it doesn't change composition.  It
  -> melts into a liquid of the same composition.  Typical additions are
  -> 4-12% to improve forging yields.  Hypereutectic pistons means more
  -> than 12% is added. Keith Black Silvolite adds between 16 and 18%.
  -> Hypoeutectic means less than 12% silicon is added.
- Uh, no.  Nonononoooooooooo..........