My message is too long for a PM (n fact, it's too long for 3 of them, damnit!
Originally Posted by DodgeRida67
Thanks. I'd appreciate it if you'd give me your thoughts on this thread...
Ok, here's what I'd tell them: there are guys in the thread that run KBs with no problems, and guys that hate them. The guys that HAVE them have more proof than the guys who simply SAY they will break. Simply put, at room temps, the forged are stronger than the hypers, but at elevated temps, it's the other way around. Ask the guys sayig the KBs are "glass pistons" for some proof to back up their claims, OTHER than math.
The most common piston found in production car engines, is constructed from a Eutectic alloy. The term Eutectic mean that the piston allow contains around 12.5% silicon. This is just about the point of total dissolved silicon saturation. With older piston designs which have conventional ring lands, there is little need to have any more than this simple, reliable material which has served the industry so well for so long. In fact all the worldwide passenger car engine manufactures we know of still use the 12.5 per cent alloy.
Hypereutectic alloy is pretty damn similar, but has a much higher degree of silicon in its makeup, something around the 16-18 per cent mark. What this actually achieves in the piston-manufacturing process is a high degree of free (undissolved) silicon in the end piston. The silicon/aluminium ration affect the metal's character. The higher silicon content in the Hypereutectic alloy lends itself to improved scuff resistance and, importantly, a relatively low expansion rate. The Hypereutectic alloy not only expands 15% less than a forged alloy, it insulates the skirts from combustion chamber heat. If the skirt stays cool piston expansion is drastically reduced. Running close clearances is beneficial to piston ring seal and ring life. A small short term HP improvement can be had by running additional piston clearance because friction is reduced.
With Hypereutectic pistons, the primary reason for having all of this free silicon is to reduce piston ring groove wear. This allows piston designers to move the top compression ring much farther up the side of the piston (where combustion temperatures are much hotter), and run much smaller, thinner piston ring lands (the metal section separating the ring grooves). Their main advantages being reduced chance of ring/grove welding and reduced piston ring grove wear. If higher piston strength is needed, then generally, a piston manufacturer will add more copper and nickel to the alloy to gain extra high temperature strength.
The only real disadvantage of a cast piston (in high output situations) is in the case of a piston failure, a cast items is more likely to shatter and damage the engine, as a whole, more than a forged piston. So, as i said, if you drop a valve or a bolt, a forged piston won't try to destroy the rest of the engine (but the engine's still ****ed..)
A big advantage with forged pistons is they generally result in a more ductile material, with the effect being the piston can take a higher level of detonation before failing. As far as I am concerned, this is not such a huge bonus as you engine should be tuned not to detonate in the first place. In extremely high rpm/high horsepower applications, the great strength of the forged piston can add reliability, with most builders recommending they be used once power levels rise past about 90hp per litre of engine capacity.
Silvolite actually likes to use hyper pistons in nitrous engines, as the material withstands heat and detonation better than forgings (though a poorly set up nitrous system can melt ANY piston). Designing a maximum performance nitrous engine is more of an exercise in heat management than it is in engine building.
Now, the guys over there are right about the stock forged rods. They are plenty strong, with some good ARP rod bolts.