bufmatmuslepants

iTrader: (3)

There was an article in HOTROD last month where they were talking about Vipers, and they interviewed the design engineer on why he used cast pistons for so long on all the viper motors. He said that cast pistons typically give 10 HP over forged because you can do a more intricate mold for cast as opposed to your limits on how to make a forged piston, it was not about cost since the difference is negligible in a $100,000 car. The 08-12 vipers had cast pistons and 600hp, ls7 had titanium rods and a forged crank and cast pistons at 500hp and I know head/cam ls7s are over 600 no issues, for an NA build, has anyone purposely selected cast pistons over forged with this in mind? How did you select which ones?

GSAWYERS

I've used cast pistons in almost all the NA street motors I've built. Generally, you can run a tighter piston to wall clearance and spend less on the pistons. In an NA street motor, I don't see an advantage to using a forged piston. I've built several 4 cyl engines that reliably turned 10k+ rpm and a couple v8's that turned 8400rpm with cast pistons. Mahle makes some very light and pretty tough cast pistons. I've done everything the experts tell you not to do and have always gotten away with it. If you spray or boost a cast piston motor you'll find they just take less spray/boost than a forged piston and your margin of error is smaller. I've done things to cast piston engines that would make an engine builder cringe, maybe I'm just lucky

speedtigger

iTrader: (16)

Agree 100%. I see a lot of people use forged pistons in applications where they would do better to use a cast piston.

ZMX

Cast pistons are made with higher silicon content.

More silicon means less thermal expansion, which allows tighter piston-to-wall clearance. High-silicon pistons are also more scuff resistant, and tend to last longer - especially when cold revved.

Tighter piston-to-wall clearance is huge for the LS7, because it means reduced blowby-emissions; the LS7 needed all the help it could get to pass emissions, since it's so big and runs a more aggressive cam. Tighter PtW clearance also means increased power/efficiency.

The downside to higher silicon content is that it makes pistons brittle - more liable to shatter under knocking. They aren't as good for boost.

Typical cast piston - "Hypereutectic" - 16-19% silicon
4032 Alloy - "Eutectic" - 11% silicon.
2618 Alloy - "Hypoeutectic" - less than 2% silicon

The LS7 uses titanium connecting rods, but cast pistons. Mean piston speed is 23.7m/s.

As far as I can tell, no one makes a hypereutectic forged piston. Probably because if they're going through the expense of forging the piston, they want all the strength they can get.

chuntington101

It's interesting that a lot if OEM turbo engines us cast pistons also whereas the aftermarket this is a no no. As said above there is no reason a cast piston, that uses a different metal for a forged piston that expands less, can not be used in a high output environment. I think the reason forged pistons are so popular on the aftermarket scene is they give you a little more resilience as most 'tuning' is pretty poor in terms of reliability / durability of components.

Ari G

iTrader: (25)

Cast pistons dont get affected by heat cycles like aluminum and dose not asorb the heat so it in fact makes more heat in cyllender for power

On a n/a build i prefer cast but the cost of aftermarket cast pistons is high,so sometimes the customer dose not want to get it as for 200$ more he can have a solid piston and ping package to take nitrous when he wants to

JGalt

Whats stopping a forged piston from reaching the same operating clearance as a cast piston, even if it starts out with a larger clearance when the engine is started up from cold?

This would be observed when piston slap noise at startup goes away after the engine warms up. Clearance starts big and gets smaller.

Not specifically related to a production car where piston slap noise at any time is probably a no no. But when we're talking about modified engines, then would operating clearance really be different?

speedtigger

iTrader: (16)

That is a good thought, but in practical application it does not prove to be true. If this engine started up and ran non stop at a steady speed, load and temperature the forged piston might maintain similar clearances. But, in the real world they do not. The daily driver engine endures multiple heat cycles and a wide variety of load conditions every day.

A daily driver engine is started from cold multiple times per day. It endures conditions ranging from extended idle time to constant load of climbing a hill to a WOT blast. These different conditions put different levels of heat into the piston causing various levels of expansion and to some degree dimensional distortion. So, if the material expands more, it needs more room to accommodate all of this expansion.

To add to this, the high silicon cast pistons are not only more rigid, they wear better too. And to pile on even more, the tighter tolerances allowed by the more thermally stable material allows the piston to dissipate the heat into the cylinder walls better which improves the pistons dimensional stability too.