I don't have another one handy, but there's a picture on page four of the PDF (document page 103) in Chapter 6 of the GM LSJ build book.

http://tunersource.gmblogs.com/Racer...LSJ_Chap_6.pdf


There's nothing wrong with a cast aluminum piston. Both the pistons in the LSJ and LNF are made by Mahle who is a major OEM supplier with pistons used many other high performance applications (such as the cast pistons in the Porsche 911 and those in the BMW M3/M5 and most of the Mercedes AMG models to name a few). The decision to use a cast design more than likely came down to cost and ease of manufacturing but that's not to say it's a "weak" or "bad" choice from a material standpoint. The piston crown on the LNF is a very complex shape and was required to direct the fuel spray from the direct injection injector. A forged unit would require a lot of machining and would significantly increase cost.

On an aftermarket high performance build it might be worthwhile to use a forged piston, but for a stock production engine it wouldn't make sense. You have to be realistic and realize that GM more than likely didn't do durability testing at 400+ hp levels; they needed a piston that would stand up to 260 hp. That's not to say the stock LNF piston won't hold up to much more than that but that it wasn't a requirement and they weren't going to add cost if it wasn't needed.

If you control detonation, the higher silicon content in a eutectic cast piston makes them very strong. They also don't expand as much as a forged aluminum piston (which has a lower silicon content alloy and is "softer") and therefore don't have the piston slap that can be common on a forged piston and might be unacceptable from a NVH (noise, vibration, harshness) standpoint in a production engine. On pistons in forced induction applications Mahle also likes to reinforce the crown area with a steel insert and given ring height requirements with having to keep them placed as high as possible to minimize fuel from being trapped on the side of the piston (which would increase hydrocarbon emissions), they usually anodize the ring lands to increase strength. Then you have their GRAFAL anti-friction coating on the skirt, the lighter weight afforded by the cast material compared to a forged piston, and a fully floating piston pin and you start to have a very advance piston that is reasonable to produce from a cost perspective.

If you're building a high horsepower monster the stock pistons very well might stand up quite well. The cast eutectic Mahle pistons used in the Neon SRT-4 have held up to over 500 whp, for example, when the tuning is spot-on. And that's going to be one of the big issues for the LNF just like it is in any other application. Many times forged pistons are a better choice because they can withstand minor knock/detonation better than a cast piston due to the softer alloy. The aftermarket forged pistons also don't have to be engineered to reduce emissions (which is a big factor in OEM pistons) and can have stronger ring lands placed further down on the piston.

The direct injection on the LNF affords the ability to run a higher compression ratio, but a higher CR also makes an engine more susceptible to detonation due to the higher peak cylinder pressures and how they ramp up on the compression stroke. If you increase the air mass (i.e. more boost) with a larger turbocharger and significantly increase power then your tuning needs to be perfect to prevent knock. If you can do that I bet the stock pistons will stand up quite well in mild builds. But a set of aftermarket forged units will give a larger safety margin and if you're spending a lot of money on machining and other lower end work, might make a wise investment.


As to the question where to get a set of forged pistons made, any number of piston suppliers should be able to fulfill your needs. Send them a factory unit and they can copy the piston crown and strengthen it as needed such as deeper ring lands. Karo mentioned having Wiseco produce a set, but if you're really interested any number of companies such as JE, Diamond, Arias, Venolia, CP, etc. will make you a custom piston.

 

Just another option. They're in my LS1/Fiero.

http://www.driveshaftshop.com/http:/...shaftshop.com/

 

We have been following and keeping up to date with Wiseco on this for MONTHS (started last October 2008)... They had to make a new forging die setup specifically for the LNF. Due to the nature of how the stock setup works (DI) the only compression ratio offered will be the stock one.

They will have a stock bore, .020" over bore, and a .080" over bore. All the same compression ratio.

They will come with pistons/rings/whrist pins/locks (and *possibly* come standard with moly skirt coatings)


We have not posted any of this information, as we always like to wait until they are available, so we don't get anyones hopes up to quickly, as the tooling process took months...

We will have more information soon on these new Wiseco forged LNF pistons.

 

hm... sounds like a winner, yeah i just hope this stuff comes out soon, i'm in need of a rebuild just to be able to handle the power safely, i'm wanting to do it from the pistons to valve cover...

 

My friend works at Wiseco here in the NorthEast Ohio area, and he told me this past Friday that they were finished and in the process of putting them out to market.

I dont need them, but its nice to know I'll be able to get his employee discount if I ever do, 60% off the retail price, and no we didnt discuss price, sorry.

 

i will be patiently waiting for those pistons... any word on other internals?? like,,, valvetrain?? everything? are some of the performance internals for the LSJ compatible with the LNF ??? may seem dumb,, but i had to ask... i'm just waiting to get the word so i can finish my first project of many to my car.... piston up rebuild.....