You're in trouble if I need to tell you which is the stock and which is the forgie (forged piston). Pistons are from Ross Racing Pistons and are forged from 2618 billet bar stock and have been heat treated and aged to T61 condition (2618-T61). Per Ross, 2618-T61 is the material that is used when fatigue resistance and durability are of prime importance by all premium racing piston manufacturers. This material has less than 1% silicon particulate content. High silicon content pistons (eg. forged from 4032 or MS75) will not stand up to the most extreme stress placed on many racing pistons. If a crack starts to form in a high silicon piston it will continue until the piston experiences a catastrophic failure. This is because adding sand to the aluminium only makes it more brittle. A crack on a 2618-T61 piston will continue to an area where the stress is not that great and will then stop.
As usual with engineering there are compromises. High silicon content pistons have a lower coefficient of thermal expansion allowing the engine builder to specify much tighter tolerance between the piston and the cylinder wall to ensure excellent ring seal and minimize cylinder bore wear. The larger piston to cylinder wall tolerances on a low silicon content piston will lead to characteristics such as a "rattly" sound when the engine is cold and has not warmed up. While this sound is undesirable to the majority of people it is no issue for what I'm building the Slowtra's engine for. I'll take the much improved piston strength over some additional noise any time any day. Care will also need to be taken to ensure during engine warm up to minimize wear. Even with bone stock QGs one should not rev and thrash the engine when it is cold anyway.
Compression is Toby GT Auto spec which is a tad under 9.0:1. Should result in a very responsive engine. Rings are the excellent Total Seal according to Toby. Now please excuse me while I go clean my pants. More details and pictures to follow...
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