What's your opinion of forged pistons in the LS6?

[jub jub]

Just curious if anyone here has opinions, both good or bad of high quality forged slugs for the gen III and up aluminum engine?

[vettex2]

I can't think of a reason why forged wouldn't be a good choice.
BUT
I don't build LSx motors.

[c4c5specialist]

HI there,
Forged pistons are usually needed for racing applications or forced induction, like turbo charging and supercharging.
Stability is really the biggest reason.
Thermal stability from the stresses of that type of application.
The ONLY down side is that you have to give more piston to bore clearance when building the engine. Refer to your piston supplier for installation instructions on tolerancing.
That will translate to a slightly noisier engine.
That is all.
Allthebest,

[vettex2]

Originally Posted by c4c5specialist

Thermal stability from the stresses of that type of application.
The ONLY down side is that you have to give more piston to bore clearance when building the engine. Refer to your piston supplier for installation instructions on tolerancing.
That will translate to a slightly noisier engine.

So forged expand more than cast ?
Or is it an aluminum block issue ?

[c4c5specialist]

HI there,
Standard high silicon content cast pistons are the standard in the industry for production vehicles at this point in time in Generation 3 and 4 engine.
The thermal stability allows piston to bore clearancing of .0005 on these engine when the piston is equipped with the polymer skirts.
Normally, forged aluminum pistons do expand slightly MORE then stock pistons, so the clearancing of .001-.003 is usually the normal tolerance.
We all remember that the cold piston slap on these engines are due to carbon buildup on the top piston ring land causing a slight rocking motion when cold, till the piston skirt heats up and expands. The polymer coating and tight tolerancing is what has helped this become a non issue in the newer engines.
When installing a forged piston, this clearance increases and therefore, the noise may increase slightly.
Allthebest,

[jub jub]

Originally Posted by c4c5specialist

When installing a forged piston, this clearance increases and therefore, the noise may increase slightly.
Allthebest,

Does the noise persist even after the engine and pistons come up to normal operating temp?

[BillF6531]

Originally Posted by jub jub

Does the noise persist even after the engine and pistons come up to normal operating temp?

The noise quits when the piston expands to better fill the bore. An alternative is to use stock pistons, at the appropriate compression ratio, and have the tops thermally coated to contain the increased heat in the combustion chamber.

[jub jub]

Thanks for the replies.

I would also imagine that forged pistons would rob a little HP as well due to increased weight. I this a correct assumption? I'm just trying to weigh some options here.

[motorman]

Originally Posted by jub jub

Thanks for the replies.

I would also imagine that forged pistons would rob a little HP as well due to increased weight. I this a correct assumption? I'm just trying to weigh some options here.

custom made forged pistons from JE can be run as close as .0015/.002 because of the shape if the skirts. GM bought JE a computer controled machine for them to make the marine 502 pistons and this is the reason for their close fit. i would bet they are even lighter than the stock pistons in a LS series engine. i bet the people who complained about the piston noise inthe LS engines never owned a 60s 427 hi per BBC with forged pistons.

[BillF6531]

Originally Posted by motorman

i bet the people who complained about the piston noise inthe LS engines never owned a 60s 427 hi per BBC with forged pistons.

Boy you got that right! It sounded like both its top and bottom ends were going to puke onto the ground in the next few seconds, until you put your foot in it, and then the exhaust and squealing tires drowned out everything else. You had to own one to understand. Bystanders couldn't grasp it.